九乡新闻网错版通宝:Internetworking Basics互联网络联网基础(含自译，自注及自评)
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一、Internetworking Basics互联网络联网基础知识 P.7
二、Introduction to LAN ProtocolsLAN协议简介 P.19
三、Introduction to WAN Technologies广域网技术简介 P.22
四、Bridging and Switching Basics桥接和交换基础 P.26
五、Routing Basics路由基础知识 P.
六、Network Management Basics网络管理基础知识 P.
七、Open System Interconnection Protocols开放系统互连各协议 P.35
http://docwiki.cisco.com/wiki/Internetworking_Technology_Handbook
This page was last modified on 17 December 2009, at 21:53
Internetworking Technology Handbook 互联网络联网技术手册概述
http://docwiki.cisco.com/wiki/Internetworking_Technology_Handbook
This page was last modified on 17 December 2009, at 21:53.
Internetworking Technology Handbook
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This article provides guidance for understanding internetworking technology. Different components of internetwork and the protocols used are described.
本文指导你理解互联网络联网技术。叙述了互联网络的各个组成部分和所使用的协议。
联网技术手册 指导目录
Part1 Internetworking Basics 互联网络联网基本原理
Part2 LAN Technologies 局域网技术
part3 WAN Technologies 广域网技术
part4 Internet Protocols 互联网协议
part5 Bridging and Switching 桥接和交换
part6 Routing 路由
part7 Network Management 网络管理
part8 Voice/Data Integration Technologies 声音/数据集成技术
part9 Wireless Technologies 无线技术
part10 Cable Access Technologies 有线接入技术
part11 Dial-up Technology 拨号技术
part12 Security Technologies 安全技术
part13 Quality of Service Networking 联网服务质量
part14 Network Caching Technologies 网络缓存技术
part15 IBM Network Management IBM网络管理
part16 Multiservice Access Technologies 多业务接入技术
PART 1. Internetworking Basics
An internetwork is a collection of individual networks, connected by intermediate networking devices, that functions as a single large network. Internetworking refers to the industry, products, and procedures that meet the challenge of creating and administering internetworks.
PART 1.互联网络联网基础知识
互联网络是众独立网络的一个集合，通过中间联网设备s连接起来，其功能就像单一的大型网络。互联网络联网指的是,满足创建和管理互联网络所挑战的,工业,产品,和例程。
The following articles provide information about internetworking basics:
一、 Internetworking Basics
二、 Introduction to LAN Protocols
三、 Introduction to WAN Technologies
四、 Bridging and Switching Basics
五、 Routing Basics
六、 Network Management Basics
七、 Open System Interconnection Protocols
下面各篇文章提供了互联网络联网基础知识的有关资料：
一、 互联网络联网基础知识
二、 LAN协议简介
三、 广域网技术简介
四、 桥接和交换基础
五、 路由基础知识
六、 网络管理基础知识
七、 开放系统互连各协议
PART 2. LAN Technologies 局域网技术
A LAN is a high-speed data network that covers a relatively small geographic area. It typically connects workstations, personal computers, printers, servers, and other devices. LANs offer computer users many advantages, including shared access to devices and applications, file exchange between connected users, and communication between users via electronic mail and other applications.
局域网是一种高速数据网络，覆盖了相对较小的地理区域。它通常连接工作站,个人电脑,打印机,服务器和其他设备。对于计算机用户s而言,LANs提供了很多优点：包括对设备s和应用s的共享访问，已连接用户间交换文件，用户间通过电子邮件进行通信，以及其他应用。
The following articles provide information different LAN technologies:
Ethernet Technologies
Token Ring/IEEE 802.5
下列文章提供了不同局域网技术的信息：
以太网技术
令牌环网/IEEE 802.5
Fiber Distributed Data Interface(FDDI)（见第六篇：‘路由’）
PART 3. WAN Technologies
A WAN is a data communications network that covers a relatively broad geographic area and that often uses transmission facilities provided by common carriers, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer.
PART 3.广域网技术
广域网是一个数据通信网络，覆盖了相对广阔的地理区域，通常使用由公共承运商,如电话公司,提供的传输设施。广域网技术一般作用在OSI参考模型低三层：物理层，数据链路层和网络层。
The following articles provide information about the various protocols and technologies used in WAN environments:
Frame Relay
High-Speed Serial Interface
Integrated Services Digital Network
Point-to-Point Protocol
Switched Multimegabit Data Service
Synchronous Data Link Control and Derivatives
X.25
Digital Subscriber Line
下列文章提供了用于广域网环境下的各种协议的有关资料：
帧中继
高速串行接口
综合业务数字网
点至点协议
交换式多兆位数据服务
同步数据链路控制 及其派生协议s
X.25
数字用户线路
PART 4. Internet Protocols Internet协议组
The Internet protocols are the world's most popular open-system (nonproprietary) protocol suite because they can be used to communicate across any set of interconnected networks and are equally well suited for LAN and WAN communications. The Internet protocols consist of a suite of communication protocols, of which the two best known are the Transmission Control Protocol (TCP) and the Internet Protocol (IP). The Internet protocol suite not only includes lower-layer protocols (such as TCP and IP), but it also specifies common applications such as electronic mail, terminal emulation, and file transfer. This article provides a broad introduction to specifications that comprise the Internet protocols. Discussions include IP addressing and key upper-layer protocols used in the Internet. Specific routing protocols are addressed individually later in this document.
互联网协议是世界上最流行的开放式系统（非专利）协议栈，因为它们可在遍及任意组已互联的网络上用于通信,而且也同样能很好地适合于LAN和WAN的通信。互联网协议s由一套通信协议组成，其中两个最有名协议是TCP（传输控制协议）和IP（互联网协议）。互联网协议栈不仅包括较低层协议s(如TCP和IP),而且也指定了,如电子邮件,终端仿真,以及文件传输等常见的应用协议。本文对含盖互联网协议栈的各种规范,提供一个广泛的介绍。讨论包括IP寻址,以及用于互联网的关键上层协议s。特定的路由协议是在本文档稍后单独讲解。
The following articles provide information about different IOS IP technologies:
Internet Protocols
AppleTalk
Banyan VINES
IBM Systems Network Architecture Protocols
DECnet
Simple Multicast Routing Protocol
Internet Protocol Multicast
IPv6
下面的文章提供了有关各种OSI IP技术的有关资料：
Internet协议
AppleTalk
Banyan VINES
IBM系统网络体系结构协议
DECnet
简单组播路由协议
互联网协议组播
IPv6
PART 5. Bridging and Switching
Bridges and switches are data communication devices that operate principally at Layer 2 of the OSI reference model. As such, they are widely referred to as data link layer devices. Several kinds of bridging have proven important as internetworking devices. Transparent bridging is found primarily in Ethernet environments, while source-route bridging occurs primarily in Token Ring environments. Translational bridging provides translation between the formats and transit principles of different media types (usually Ethernet and Token Ring). Finally, source-route transparent bridging combines the algorithms of transparent bridging and source-route bridging to enable communication in mixed Ethernet/Token Ring environments. Today, switching technology has emerged as the evolutionary heir to bridging-based internetworking solutions. Switching implementations now dominate applications in which bridging technologies were implemented in prior network designs. Superior throughput performance, higher port density, lower per-port cost, and greater flexibility have contributed to the emergence of switches as replacement technology for bridges and as complements to routing technology.
PART 5. Bridging and Switching 桥接和交换
网桥和交换机是数据通信设备，主要操作在OSI参考模型的第二层。因此，它们被广泛称为数据链路层设备。若干种桥接已经证实了它们作为互联网络联网设备的重要性。透明桥接主要出现在以太网环境中，而源路由桥接主要发生在令牌环环境。桥接转换提供了不同媒体类型（通常是以太网和令牌环）的格式和传输原理之间的转换。最后，源路由透明桥接结合了透明桥接和源路由桥接两者的算法,以使混合的以太网/令牌环环境能实现通信。今天，交换技术已经出现,革命性地继承了基于桥接的互联网络联网的解决方案。以前曾经是在网络设计中采用桥接技术的地方, 现在,已是交换实现占统治地位的主要应用领域。卓越的吞吐性能，更高的端口密度，每端口成本的降低，和更大的灵活性,交换机的出现为替代网桥技术以及作为路由技术的补充手段作出了贡献。
The following articles provide information about the technologies employed in devices loosely referred to as bridges and switches:
Transparent Bridging
Mixed-Media Bridging
Source-Route Bridging
Asynchronous Transfer Mode Switching
LAN Switching and VLANs
MPLS/Tag Switching
Data-Link Switching
Tag Switching
下面的文章所提供的,是关于通常被泛称为网桥和交换机的设备,所采用的技术,的有关资料：
透明桥接
混合媒体桥接
源路由桥接
异步传输模式交换
局域网交换和虚拟局域网
MPLS /标记交换
数据链路交换
标签交换
PART 6. Routing
Routing is the act of moving information across an internetwork from a source to a destination. Along the way, at least one intermediate node typically is encountered. Routing is often contrasted with bridging, which might seem to accomplish precisely the same thing to the casual observer. The primary difference between the two is that bridging occurs at Layer 2 (the link layer) of the OSI reference model, whereas routing occurs at Layer 3 (the network layer). This distinction provides routing and bridging with different information to use in the process of moving information from source to destination, so the two functions accomplish their tasks in different ways.
PART 6. Routing 路由
路由是一种活动,将信息从信源,通过互联网络,传送到信宿。沿此通路,通常至少遇到一个中间节点。泛泛一看,对比路由与桥接,似乎这两者要完成的是同样的事情。两者的主要区别在于,桥接发生在OSI参考模型第二层（链路层），而路由发生在第3层（网络层）。这一区别,使得路由和桥接,从源到目的地的过程中所要移动的信息,是不同的；因此，完成他们任务的方法也是不同的。
The following articles provide information different routing technologies:
Fiber Distributed Data Interface
IBM Systems Network Architecture Routing
NetWare Link-Services Protocol
Open System Interconnection Routing Protocol
Open Shortest Path First
Routing Information Protocol
Border Gateway Protocol
Interior Gateway Routing Protocol
Enhanced Interior Gateway Routing Protocol
Xerox Network Systems
下面的文章提供了不同路由技术的有关资料：
光纤分布式数据接口(FDDI)
IBM系统s的网络体系结 构的路由
NetWare的链路服务协议
开放系统互连路由协议
开放最短路径优先
路由信息协议
边界网关协议
内部网关路由协议
增强式内部网关路由协议
Xerox网络系统
PART 7. Network Management 网络管理
Network management means different things to different people. In some cases, it involves a solitary network consultant monitoring network activity with an outdated protocol analyzer. In other cases, network management involves a distributed database, auto polling of network devices, and high-end workstations generating real-time graphical views of network topology changes and traffic. In general, network management is a service that employs a variety of tools, applications, and devices to assist human network managers in monitoring and maintaining networks.
网络管理对于不同的人们意味着不同的事情。在某些情况下，它包含一个单独的网络，使用无用协议分析器,以协助监视网络活动。在其他情况下，网络管理包括一个分布式数据库,网络设备的自动轮询,以及能生成网络拓扑变化和信流的实时图形视图的高端工作站s。一般来说，网络管理是一种服务，通过使用各种工具,应用程序和设备，协助网络管理人员监视和维护网络。
The following articles provide information different network management technologies:
Virtual Private Networks
Directory-Enabled Networking
Remote Monitoring
Simple Network Management Protocol
下面的文章提供不同网络管理技术的有关资料：
虚拟专用网(VPN)
启用目录联网
远程监控
简单网络管理协议
PART 8.Voice/Data Integration Technologies 语音/数据集成技术
Voice/data integration is important to network designers of both service providers and enterprise. Service providers are attracted by the lower-cost model-the cost of packet voice is currently estimated to be only 20 to 50 percent of the cost of a traditional circuit-based voice network. Likewise, enterprise network designers are interested in direct cost savings associated with toll-bypass and tandem switching. Both are also interested in so-called "soft savings" associated with reduced maintenance costs and more efficient network control and management. Finally, packet-based voice systems offer access to newly enhanced services such as Unified Messaging and application control. These, in turn, promise to increase the productivity of users and differentiate services.
语音/数据集成,对于服务提供商和企业这两种网络设计者,是很重要的。吸引服务提供者的是低成本模型---分组语音的成本,目前估计只有传统的基于电路的语音网络成本的百分之20至50。.同样，企业网络设计者的兴趣在于,与收费的旁路和汇接交换相比,节约了直接成本。两者也有兴趣在降低维护成本和更有效的网络控制和管理的同时,还有所谓的“软节约”。最后，基于分组的语音系统能提供访问新的增强服务，如统一发布消息和统一控制应用。从而能承诺提高用户的工作效率和增进差异化服务。
Integration of voice and data technologies has accelerated rapidly in recent years because of both supply- and demand-side interactions. On the demand side, customers are leveraging investment in network infrastructure to take advantage of integrated applications such as voice applications. On the supply side, vendors have been able to take advantage of breakthroughs in many areas, including standards, technology, and network performance.
语音和数据的集成技术,由于供应和需求双方的互动,近年来进展非常迅速。在需求方面，客户正促进着网络基础设施投资的转化，以取得集成应用的优点,如语音应用。在供给方面，厂商已经能够在许多领域，包括标准,技术,和网络性能等方面，取得突破性的优势。
The following article provides information about Voice/Data Integration Technologies:
Voice/Data Integration Technologies
下面的文章提供了有关语音/数据集成技术的有关资料：
语音/数据集成技术
PART 9. Wireless Technologies 无线技术
Wireless communication is the transfer of information over a distance without the use of electrical conductors or "wires".[1] The distances involved may be short (a few meters as in television remote control) or long (thousands or millions of kilometers for radio communications). When the context is clear, the term is often shortened to "wireless". Wireless communication is generally considered to be a branch of telecommunications.
无线通信是一种,在某种距离下,不使用电导体或“电线”,就能实现信息传送。 [1]，距离可短（如电视遥控,短至数米）可长（数千或数百万公里的无线电通讯）。在上下文是明确的情况下，这个术语经常简称为“无线”。无线通讯通常认为是电信的一个分支。
It encompasses various types of fixed, mobile, and portable two way radios, cellular telephones, personal digital assistants (PDAs), and wireless networking. Other examples of wireless technology include GPS units, garage door openers and or garage doors, wireless computer mice, keyboards and headsets, satellite television and cordless telephones.
它包括各种类型的固定,移动和便携式双向无线电通讯设备,蜂窝电话,个人数字助理PDA(掌上电脑),和无线联网。无线技术的其他例子包括GPS设备，车库门开门闭,无线鼠标，键盘和耳机，卫星电视和无绳电话等等。
The following article provides information about Wireless Technologies:
Wireless Technologies
下面的文章提供了有关无线技术的资料：
无线技术
PART 10. Cable Access Technologies 有线接入技术
Historically, CATV has been a unidirectional medium designed to carry broadcast analog video channels to the maximum number of customers at the lowest possible cost. Since the introduction of CATV more than 50 years ago, little has changed beyond increasing the number of channels supported. The technology to provide high-margin, two-way services remained elusive to the operator.
历史上，有线电视是一种单向媒介,设计用来以最低的成本,承载模拟视频广播频道,传播给最大数量的客户。由于有线电视的引入是在50多年前，除了增加可支持频道的数量外并没有多大改变。提供高利润,双向服务的技术，对于经营者而言,仍然难以捉摸。
Cable television (CATV) is a unidirectional medium carrying broadcast analog video channels to the most customers possible at the lowest possible cost to the CATV service provider. Since the introduction of CATV more than 50 years ago, little has changed beyond increasing the number of channels supported.
The following article provides information about Cable Access Technologies:
下面的文章提供了有关有线接入技术的资料：
有线接入技术
Cable Access Technologies
PART 11. Dial-up Technology 拨号技术
Dialup is simply the application of the Public Switched Telephone Network (PSTN) to carry data on behalf of the end user. It involves customer premises equipment (CPE) device sending the telephone switch a phone number to direct a connection to. The AS3600, AS5200, AS5300, and AS5800 are all examples of routers that have the capability to run a PRI along with banks of digital modems. The AS2511, on the other hand, is an example of a router that communicates with external modems.
拨号是公共交换式电话网（PSTN）的简单应用,承载着代表最终用户利益的数据。它需要客户附属装备（CPE）设备发送一个指向要连接处的电话号码给电话交换机。AS3600，AS5200，AS5300和AS5800是路由器的所有实例,它们数字调制解调器堆一起,有能力运行一个PRI。另一方面，AS2511是一个与外部调制解调器通信的,路由器实例。
Since the time of Internetworking Technologies Handbook, 2nd edition, the carrier market has continued to grow, and there have been demands for higher modem densities. The answer to this need was a higher degree of interoperation with the telco equipment and the refinement of the digital modem: a modem capable of direct digital access to the PSTN. This has allowed the development of faster CPE modems that take advantage of the clarity of signal that the digital modems enjoy. The fact that the digital modems connecting into the PSTN through a PRI or a BRI can transmit data at more than 53 K using the V.90 communication standard attests to the success of the idea.
自从互联网络技术手册第二版以来，承运器市场持续增长，出现了对更高密度调制解调器的需求。对于这种需求的回答是一种高度互操作的电信设备和改进的数字调制解调器：一种调制解调器,有能力对PSTN进行直接数字访问。这就允许开发更快的,能取得清晰信号的CPE调制解调器,而这正是数字调制解调器所需要的。事实上，数字调制解调器通过PRI或BRI连接到PSTN,使用V.90通信标准，可以传输数据的速率超过53k波特率。证实了想法的成功。
The following article provides information about Dial-up Technology:
Dial-up Technology
下面的文章提供了有关拨号技术的资料：
拨号技术
PART 12. Security Technologies 安全技术
With the rapid growth of interest in the Internet, network security has become a major concern to companies throughout the world. The fact that the information and tools needed to penetrate the security of corporate networks are widely available has increased that concern.
随着对互联网的兴趣迅速增长，网络安全已成为整个世界各公司的主要关注。事实是,渗透到企业网络,破坏其安全性所用的信息和工具,已被广泛使用；增加了人们的担心。
Because of this increased focus on network security, network administrators often spend more effort protecting their networks than on actual network setup and administration. Tools that probe for system vulnerabilities, such as the Security Administrator Tool for Analyzing Networks (SATAN), and some of the newly available scanning and intrusion detection packages and appliances, assist in these efforts, but these tools only point out areas of weakness and may not provide a means to protect networks from all possible attacks. Thus, as a network administrator, you must constantly try to keep abreast of the large number of security issues confronting you in today's world. This article describes many of the security issues that arise when connecting a private network to the Internet.
由于这种网络安全的日益重视，网络管理员除了日常网络建立和网络管理外,常常要花费更多的精力来保护他们的网络。能探测系统薄弱环节的工具，如SATAN(用于分析网络的安全管理员工具),和一些新的用于扫描和入侵检测的软件包和装置，有助于管理员的努力效果；但这些工具只能指出的薄弱范围，还没有可能提供一种方法来保护网络免于所有可能的攻击。因此，作为一个网络管理员，你必须不懈地保持努力,以面对各种当今世界出现的大量安全问题。本文(指安全技术)将介绍许多出现在,当专用网络与互联网相连时,的安全问题。
The following article provides information about Security Technologies:
Security Technologies
下面的文章提供了有关安全技术的资料：
安全技术
PART 13. Quality of Service Networking 网络服务质量
Quality of Service (QoS) refers to the capability of a network to provide better service to selected network traffic over various technologies, including Frame Relay, Asynchronous Transfer Mode (ATM), Ethernet and 802.1 networks, SONET, and IP-routed networks that may use any or all of these underlying technologies. The primary goal of QoS is to provide priority including dedicated bandwidth, controlled jitter and latency (required by some real-time and interactive traffic), and improved loss characteristics. Also important is making sure that providing priority for one or more flows does not make other flows fail. QoS technologies provide the elemental building blocks that will be used for future business applications in campus, WAN, and service provider networks. This article outlines the features and benefits of the QoS provided by the Cisco IOS QoS.
服务质量（QoS）是指一种网络服务能力，对于建立在不同技术上的网络信流,包括帧中继,异步传输模式（ATM）,以太网和802.1网络,SONET,以及IP路由网络---其能使用任何或所有这些基础技术---，来为信流选择提供更好的服务。 QoS的主要目标是提供优先权，包括专用带宽,控制抖动和延迟（为某些实时性和交互性信流所要求的）,并改善损耗特性。.同样重要的是,确保为一个或多个流量提供优先权,而不会使其它流量失效。QoS技术提供的基本积木块，它们将在校园网,广域网,和服务提供商网络s中用于将来的业务应用。本文概述了Cisco IOS QoS 所具有的QoS的特点和效益。
The following articles provide information about Quality of Service:
Quality of Service Networking
Resource Reservation Protocol
下面的文章提供了有关服务质量的资料：
网络服务质量
资源预留协议
PART 14. Network Caching Technologies 网络缓存技术
Although the volume of Web traffic on the Internet is staggering, a large percentage of that traffic is redundant-multiple users at any given site request much of the same content. This means that a significant percentage of the WAN infrastructure carries the identical content (and identical requests for it) day after day. Eliminating a significant amount of recurring telecommunications charges offers an enormous savings opportunity for enterprise and service provider customers.
虽然在互联网上网站信流量大得惊人，信流中占很大的百分比是冗余的--在任意地点的多个用户,其申请的大部分内容是相同的。这意味着,广域网基础设施,日复一日地承载的内容,其相同的部分(以及相同的申请)占很大的比例。取消大量重复电信的资费,为企业和服务提供商客户,提供了巨大的节约机会。
Web caching performs the local storage of Web content to serve these redundant user requests more quickly, without sending the requests and the resulting content over the WAN.
网站缓存执行网站内容的本地存储,使这些冗余的用户申请更迅速,而不必再在广域网上发送申请及其内容。
The following article provides information about Network Caching Technologies:
Network Caching Technologies
下面的文章提供了有关网络缓存技术的资料：
网络缓存技术
PART 15.IBM Network Management IBM网络管理
IBM network management refers to any architecture used to manage IBM Systems Network Architecture (SNA) networks or Advanced Peer-to-Peer Networking (APPN) networks. IBM network management is part of the IBM Open-Network Architecture (ONA) and is performed centrally by using management platforms such as NetView and others. It is divided into five functions that are similar to the network management functions specified under the Open System Interconnection (OSI) model. This article summarizes the IBM network management functional areas, ONA network management architecture, and management platforms.
IBM网络管理是指用于管理IBM系统网络体系结构（SNA）网络或高级对等联网（APPN）网络上的任何体系结构。IBM网络管理是IBM的开放式网络架构（ONA）的一部分,是通过使用管理平台,诸如NetView和其他的,执行集中化。它分为五个功能,类似于指定在开放系统互连（OSI）模型下的网络管理功能。本文总结了IBM网络管理功能区s,ONA网络管理体系结构,和管理平台s。
下面的文章提供了有关IBM网络管理资源：
IBM网络管理
The following article provides information about IBM Network Management:
IBM Network Management
PART 16. Multiservice Access Technologies 多业务接入技术
Multiservice networking is emerging as a strategically important issue for enterprise and public service provider infrastructures alike. The proposition of multiservice networking is the combination of all types of communications, all types of data, voice, and video over a single packet-cell-based infrastructure. The benefits of multiservice networking are reduced operational costs, higher performance, greater flexibility, integration and control, and faster new application and service deployment.
多业务联网,在为企业和公共服务提供商安排基础设施方面,正作为一种战略上的重要议题而出现。多业务联网的命题是结合所有类型的通信，所有类型的数据,语音,和视频，承载在单一的基于分组包—信元的基础设施上。多业务联网的好处是降低运营成本，增强更高的性能,提高更大的灵活性,集成和控制，以及更快部署新的应用和服务。
The following article provides information about Multiservice Access Technologies:
Multiservice Access Technologies
下面的文章提供了有关多业务接入技术资料：
多业务接入技术
Retrieved from "http://docwiki.cisco.com/wiki/Internetworking_Technology_Handbook"
Category:IOS Technology Handbook
分类 ：IOS技术手册
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互联网络技术手册
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本文指导你理解互联网络联网技术。叙述了互联网络的各个组成部分和所使用的协议s。
Contents
[hide]
Part1 Internetworking Basics 互联网络联网基本原理
Part2 LAN Technologies局域网技术
part3 WAN Technologies 广域网技术
part4 Internet Protocols 互联网协议
part5 Bridging and Switching 桥接和交换
part6 Routing 路由
part7 Network Management 网络管理
part8 Voice/Data Integration Technologies 声音/数据集成技术
part9 Wireless Technologies 无线技术
part10 Cable Access Technologies 有线接入技术
part11 Dial-up Technology 拨号技术
part12 Security Technologies 安全技术
part13 Quality of Service Networking 联网服务质量
part14 Network Caching Technologies 网络缓存技术
part15 IBM Network Management IBM网络管理
part16 Multiservice Access Technologies 多业务接入技术
第一部分 互联网络联网基础知识
互联网络是众独立网络的一个集合，通过中间联网设备s连接起来，其功能就像单一的大型网络。互联网络联网指的是,满足创建和管理互联网络所挑战的,工业,产品s,和例程s。
下面各篇文章提供了互联网络联网基础知识的有关资料：
互联网络联网基础知识
LAN协议简介
广域网技术简介
桥接和交换基础
路由基础知识
网络管理基础知识
开放系统互连各协议
第二部分 局域网技术
局域网是一种高速数据网络，覆盖了相对较小的地理区域。它通常连接工作站,个人电脑,打印机,服务器和其他设备。对于计算机用户s而言,LANs提供了很多优点：包括对设备s和应用s的共享访问，已连接用户间交换文件，用户间通过电子邮件进行通信，以及其他应用。
下列文章提供了不同局域网技术的信息：
以太网技术
令牌环网/IEEE 802.5
Fiber Distributed Data Interface(FDDI)（见第六篇：‘路由’）
第三部分 广域网技术
广域网是一个数据通信网络，覆盖了相对广阔的地理区域，通常使用由公共承运商,如电话公司,提供的传输设施。广域网技术一般运行在OSI参考模型的低三层：物理层，数据链路层和网络层。
下列文章提供了用于广域网环境下的各种协议的有关资料：
帧中继
高速串行接口
综合业务数字网
点至点协议
交换式多兆位数据服务
同步数据链路控制 及其派生协议s
X.25
数字用户线路
第四部分 Internet各协议
互联网协议是世界上最流行的开放式系统（非专利）协议栈，因为它们可在遍及任意组已互联的网络上用于通信,而且也同样能很好地适合于LAN和WAN的通信。互联网协议s由一套通信协议组成，其中两个最有名协议是TCP（传输控制协议）和IP（互联网协议）。互联网协议栈不仅包括较低层协议s(如TCP和IP),而且也指定了,如电子邮件,终端仿真,以及文件传输等常见的应用协议。本文对含盖互联网协议栈的各种规范,提供一个广泛的介绍。讨论包括IP寻址,以及用于互联网的关键上层协议s。特定的路由协议是在本文档稍后单独讲解。
下面的文章提供了有关各种OSI IP技术的有关资料：
Internet协议
AppleTalk
Banyan VINES
IBM系统网络体系结构协议
DECnet
简单组播路由协议
互联网协议组播
IPv6
第五部分 Bridging and Switching 桥接和交换
网桥和交换机是数据通信设备，主要操作在OSI参考模型的第二层。因此，它们被广泛称为数据链路层设备。若干种桥接已经证明了它们作为互联网络联网设备的重要性。透明桥接主要出现在以太网环境中，而源路由桥接主要发生在令牌环环境。桥接转换提供了不同媒体类型（通常是以太网和令牌环）的格式和传输原理之间的转换。最后，源路由透明桥接结合了透明桥接和源路由桥接两者的算法,以使混合的以太网/令牌环环境能实现通信。今天，交换技术已经出现,革命性地继承了基于桥接的互联网络联网的解决方案。现在,交换实现占统治地位的主要应用领域，以前曾经是在网络设计中采用桥接技术的地方。卓越的吞吐性能，更高的端口密度，降低每端口的成本，和更大的灵活性,交换机的出现为替代网桥技术以及作为路由技术的补充手段作出了贡献。
下面的文章所提供的,是关于通常被泛称为网桥和交换机的设备,所采用的技术,的有关资料：
透明桥接
混合媒体桥接
源路由桥接
异步传输模式交换
局域网交换和虚拟局域网
MPLS /标记交换
数据链路交换
标签交换
第六部分 Routing 路由
路由是一种活动,将信息从信源,通过互联网络,传送到信宿。沿此通路,通常至少遇到一个中间节点。泛泛一看,对比路由与桥接,似乎这两者要完成的是同样的事情。两者的主要区别在于,桥接发生在OSI参考模型第二层（链路层），而路由发生在第3层（网络层）。这一区别,使得路由和桥接,从源到目的地的过程中所要移动的信息,是不同的；因此，完成他们任务的方法也是不同的。
下面的文章提供了不同路由技术的有关资料：
光纤分布式数据接口(FDDI)
IBM系统s的网络体系结 构的路由
NetWare的链路服务协议
开放系统互连路由协议
开放最短路径优先
路由信息协议
边界网关协议
内部网关路由协议
增强式内部网关路由协议
Xerox网络系统
第七部分 Network Management 网络管理
.网络管理对于不同的人们意味着不同的事情。在某些情况下，它包含一个单独的网络，使用无用协议分析器,以协助监视网络活动。在其他情况下，网络管理包括一个分布式数据库,网络设备的自动轮询,以及能生成网络拓扑变化和信流的实时图形视图的高端工作站s。一般来说，网络管理是一种服务，通过使用各种工具,应用程序和设备，协助网络管理人员监视和维护网络。
下面的文章提供不同网络管理技术的有关资料：
虚拟专用网(VPN)
启用目录联网
远程监控
简单网络管理协议
第八部分 Voice/Data Integration Technologies 语音/数据集成技术
语音/数据集成,对于服务提供商和企业这两种网络设计者,是很重要的。吸引服务提供者的是低成本模型---分组语音的成本,目前估计只有传统的基于电路的语音网络成本的百分之20至50。.同样，企业网络设计者的兴趣在于,与收费的旁路和汇接交换相比,节约了直接成本。两者也有兴趣在降低维护成本和更有效的网络控制和管理的同时,还有所谓的“软节约”。最后，基于分组的语音系统提供访问新的增强服务，如消息统一发布和统一控制应用。这些，从而能承诺提高用户的工作效率和差异化服务。
语音和数据的集成技术,由于供应和需求双方的互动,近年来进展非常迅速。在需求方面，客户正促进着网络基础设施投资的转化，以取得集成应用的优点,如语音应用。在供给方面，厂商已经能够在许多领域，包括标准，技术和网络性能等方面,取得突破的优势。
下面的文章提供了有关语音/数据集成技术的有关资料：
语音/数据集成技术
第九部分 无线技术
无线通信是一种,在某种距离下,不使用电导体或“电线”,实现信息传送。[1]使用[1]，距离可短（如电视遥控,短至数米）可长（数千或数百万公里的无线电通讯）。.在上下文是明确的情况下，这个术语经常简称为“无线”。无线通讯通常认为是电信的一个分支。
它包括各种类型的固定,移动和便携式双向无线电通讯设备,蜂窝电话,个人数字助理PDA(掌上电脑),和无线联网。无线技术的其他例子包括GPS设备，车库门开闭合,无线鼠标，键盘和耳机，卫星电视和无绳电话等等。
下面的文章提供了有关无线技术的资料：
无线技术
第十部分 有线接入技术
历史上，有线电视是一种单向媒介,设计用来以最低的成本,承载模拟视频广播频道,传播给最大数量的客户。由于有线电视的引入是在50多年前，除了增加可支持频道的数量外并没有多大改变。提供高利润,双向服务的技术，对于经营者而言,仍然难以捉摸。
Cable television (CATV) is a unidirectional medium carrying broadcast analog video channels to the most customers possible at the lowest possible cost to the CATV service provider.有线电视（CATV）的执行是一个单向的媒介，以最低的成本向有线电视服务提供商播出模拟视频通道，以最可能的客户。 Since the introduction of CATV more than 50 years ago, little has changed beyond increasing the number of channels supported.由于有线电视超过50年前推出，并没有多大改变之外增加支持的通道数量。
下面的文章提供了有关有线接入技术的资料：
有线接入技术
第十一部分 拨号技术
拨号是公共交换式电话网（PSTN）的简单应用,承载着代表最终用户利益的数据。它需要客户附属装备（CPE）设备发送一个指向要连接处的电话号码给电话交换机。AS3600，AS5200，AS5300和AS5800是路由器的所有实例,它们数字调制解调器堆一起,有能力运行一个PRI。另一方面，AS2511是一个与外部调制解调器通信的,路由器实例。
自从互联网络技术手册第二版以来，承运器市场持续增长，出现了对更高密度调制解调器的需求。对于这种需求的回答是一种高度互操作的电信设备和改进的数字调制解调器：一种调制解调器,有能力对PSTN进行直接数字访问。这就允许开发更快的,能取得清晰信号的CPE调制解调器,而这正是数字调制解调器所需要的。事实上，数字调制解调器通过PRI或BRI连接到PSTN,使用V.90通信标准，可以传输数据的速率超过53k波特率,成功地证实了想法。
下面的文章提供了有关拨号技术的资料：
拨号技术
第十二部分 安全技术
随着对互联网的兴趣迅速增长，网络安全已成为整个世界各公司的主要关注。事实是,渗透到企业网络,破坏其安全性所用的信息和工具,已被广泛使用；增加了人们的担心。
由于这种网络安全的日益重视，网络管理员除了日常网络建立和网络管理外,常常要花费更多的精力来保护他们的网络。探测系统薄弱能力的工具，如SATAN(用于分析网络的安全管理员工具),和一些新的可用的扫描和入侵检测的软件包和装置，有助于管理员的努力，但这些工具只能指出的薄弱范围，还没有可能提供一种方法来保护网络免于所有可能的攻击。因此，作为一个网络管理员，你必须不懈地努力保持,同时面临在当今的世界里出现的,大量安全问题。本文(指安全技术)介绍了许多出现在专用网络连接到互联网时的安全问题。
下面的文章提供了有关安全技术的资料：
安全技术
第十三部分 网络服务质量
服务质量（QoS）是指一种网络服务能力，对于建立在不同技术上的网络信流,包括帧中继,异步传输模式（ATM）,以太网和802.1网络,SONET,以及IP路由网络---其能使用任何或所有这些基础技术---，来为信流选择提供更好的服务。 QoS的主要目标是提供优先权，包括专用带宽,控制抖动和延迟（为某些实时性和交互性信流所要求的）,并改善损耗特性。.同样重要的是,确保为一个或多个流量提供优先权,而不会使其它流量失效。QoS技术提供的基本积木块，它们将在校园网,广域网,和服务提供商网络s中用于将来的业务应用。本文概述了Cisco IOS QoS 所具有的QoS的特点和效益。
下面的文章提供了有关服务质量的资料：
网络服务质量
资源预留协议
第十四部分 网络缓存技术
虽然在互联网上网站信流量大得惊人，信流中占很大的百分比是冗余的---在任何所给位置的多个用户申请的大多数内容是相同的。这意味着,广域网基础设施,日复一日地,承载的内容,其相同的部分(以及相同的申请)占很大的比例。取消大量的重复电信资费,为企业和服务提供商客户,提供了巨大的节约机会。
网站缓存执行网站内容的本地存储,使这些冗余的用户申请更迅速,而不必再在广域网上发送申请和最终的内容。
下面的文章提供了有关网络缓存技术的资料：
网络缓存技术
第十五部分 IBM网络管理
IBM网络管理是指用于管理IBM系统网络体系结构（SNA）网络或高级对等联网（APPN）网络上的任何体系结构。IBM网络管理是IBM的开放式网络架构（ONA）的一部分,是通过使用管理平台,诸如NetView和其他的,执行集中化。它分为五个功能,类似于指定在开放系统互连（OSI）模型下的网络管理功能。本文总结了IBM网络管理功能区s,ONA网络管理体系结构,和管理平台s。
下面的文章提供了有关IBM网络管理资源：
IBM网络管理
第十六部分 多业务接入技术
多业务联网,在为企业和公共服务提供商的战略基础设施方面,正作为一种战略上的重要议题出现。多服务网络的命题是结合所有类型的通信，所有类型的数据,语音,和视频，承载在单一的基于分组包—信元的基础设施上。多业务联网的好处是降低运营成本，增强更高的性能,提高更大的灵活性,集成和控制，以及更快部署新的应用和服务。
下面的文章提供了有关多业务接入技术资料：
多业务接入技术
取自
“http://docwiki.cisco.com/wiki/Internetworking_Technology_Handbook”
分类 ：IOS技术手册
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PART1 Internetworking Basics 互联网络联网的基本原理
联网技术手册 指导目录
Part1 Internetworking Basics 互联网络联网基本原理
Part2 LAN Technologies 局域网技术
part3 WAN Technologies 广域网技术
part4 Internet Protocols 互联网协议
part5 Bridging and Switching 桥接和交换
part6 Routing 路由
part7 Network Management 网络管理
part8 Voice/Data Integration Technologies 声音/数据集成技术
part9 Wireless Technologies 无线技术
part10 Cable Access Technologies 有线接入技术
part11 Dial-up Technology 拨号技术
part12 Security Technologies 安全技术
part13 Quality of Service Networking 联网服务质量
part14 Network Caching Technologies 网络缓存技术
part15 IBM Network Management IBM网络管理
part16 Multiservice Access Technologies 多业务接入技术
PART1 Internetworking Basics 互联网络联网基本原理
PART 1. Internetworking Basics
An internetwork is a collection of individual networks, connected by intermediate networking devices, that functions as a single large network. Internetworking refers to the industry, products, and procedures that meet the challenge of creating and administering internetworks.
1.互联网络联网基础知识
互联网络是众独立网络的一个集合，通过中间联网设备s连接起来，其功能就像单一的大型网络。互联网络联网指的是,满足创建和管理互联网络所挑战需要的工业,产品s,和例程s。
The following articles provide information about internetworking basics:
一． Internetworking Basics互联网络联网基础知识
二． Introduction to LAN ProtocolsLAN协议简介
三． Introduction to WAN Technologies广域网技术简介
四． Bridging and Switching Basics桥接和交换基础
五． Routing Basics路由基础知识
六． Network Management Basics网络管理基础知识
七． Open System Interconnection Protocols开放系统互连各协议
下面各篇文章提供了互联网络联网基础知识的有关资料：
一、 互联网络联网基础知识
二、 LAN协议简介
三、 广域网技术简介
四、 桥接和交换基础
五、 路由基础知识
六、 网络管理基础知识
七、 开放系统互连各协议
一Internetworking Basics 互联网络联网基础知识
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This article along with the next six articles acts as a foundation for the technology discussions that follow. In this article, some fundamental concepts and terms used in the evolving language of internetworking are addressed. In the same way that this book provides a foundation for understanding modern networking, this article summarizes some common themes presented throughout the remainder of this book. Topics include flow control, error checking, and multiplexing, but this article focuses mainly on mapping the Open System Interconnection (OSI) model to networking/internetworking functions, and also summarizing the general nature of addressing schemes within the context of the OSI model. The OSI model represents the building blocks for internetworks. Understanding the conceptual model helps you understand the complex pieces that make up an internetwork.
本文与下面的六篇文章一起(组成第一部分)用于讨论联网手册后继部分介绍的技术s的一个基础。在本文中,可以找到用于涉及互联网络联网语言的某些基本概念和术语。本手册提供了一种用于理解现代联网的基本方法。用此方法,本文概述了某些出现在通篇本手册其余部分的一般原理。论题包括流量控制，错误检验，和复用，但本文主要关注于,将OSI模型映射到网络互联/互联网络联网的功能，同时,也概述了OSI模型上下文内寻址策略的通性。OSI模型展示了用于互联网络的积木块。理解概念模型有助于理解构成一个互联网络的复杂条块。
Contents
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1 What Is an Internetwork? 什么是互联网络？
1.1 Figure: Different Network Technologies Can Be Connected to Create an Internetwork 图：不同的网络技术可以连接起来构成一个互联网络
1.2 History of Internetworking 互联网络联网的历史
1.3 Internetworking Challenges 互联网络联网的挑战
2 Open System Interconnection Reference Model 开放系统互联参考模型
2.1 Figure: The OSI Reference Model Contains Seven Independent Layers 图：OSI参考模型包括七个独立层
2.2 Characteristics of the OSI Layers OSI各层的特性
2.2.1 Figure: Two Sets of Layers Make Up the OSI Layers 图：两组层次构成OSI层次
2.3 Protocols 协议s
2.4 OSI Model and Communication Between Systems 系统间的OSI模型和通信
2.4.1 Interaction Between OSI Model Layers OSI模型各层间的相互作用
2.4.2 OSI Model Layers and Information Exchange OSI模型层次和信息交换
2.5 OSI Model Physical Layer OSI模型物理层
2.5.1 Figure: Physical Layer Implementations Can Be LAN or WAN Specifications 图：可能是LAN或WAN规范的物理层实现
2.6 OSI Model Data Link Layer OSI模型数据链路层
2.6.1 Figure: The Data Link Layer Contains Two Sublayers 图：数据链路层包含两个子层
2.7 OSI Model Network Layer OSI模型网络层
2.8 OSI Model Transport Layer OSI模型传输层
2.9 OSI Model Session Layer OSI模型会晤层
2.10 OSI Model Presentation Layer OSI模型表示层
2.11 OSI Model Application Layer OSI模型应用层
3 Information Formats 信息格式s
3.1 Figure: Data from Upper-Layer Entities Makes Up the Data Link Layer Frame 图：来自上层实体的数据构成数据链路层的帧
3.2 Figure: Three Basic Components Make Up a Network Layer Packet 图：三个基本组件构成一个网络层数据包
3.3 Figure: Two Components Make Up a Typical Cell 图：两个组件构成一个典型的信元
4 ISO Hierarchy of Networks 网络s的ISO分层结构
4.1 Figure: A Hierarchical Network Contains Numerous Components 图：一个分层网络包含许多组件
5 Connection-Oriented and Connectionless Network Services 面向连接和无连接的网络服务
6 Internetwork Addressing 互联网络寻址
6.1 Data Link Layer Addresses 数据链路层地址
6.1.1 Figure: Each Interface on a Device Is Uniquely Identified by a Data-Link Address 图：一个设备上的每个接口是用数据链路地址作唯一识别
6.2 MAC Addresses MAC地址
6.2.1 Figure: MAC Addresses, Data-Link Addresses, and the IEEE Sublayers of the Data Link Layer Are All Related 图：数据链路层的MAC地址,数据链路地址,和IEEE子层都是有关联的,
6.2.2 Figure: The MAC Address Contains a Unique Format of Hexadecimal Digits 图：MAC地址包含一个唯一的十六进制数的格式
6.3 Mapping Addresses 映射地址
6.4 Network Layer Addresses 网络层地址
6.4.1 Figure: Each Network Interface Must Be Assigned a Network Address for Each Protocol Supported 图：每个网络接口必须指定一个网络地址以用于每个所支持的协议
6.5 Hierarchical Versus Flat Address Space 对比分层和扁平地址
6.5.1 Figure: Hierarchical and Flat Address Spaces Differ in Comparison Operations 图：分层和扁平地址空间s在比较操作中的不同
6.6 Address Assignments 地址指定
6.7 Addresses Versus Names 对比地址s和名字s
7 Flow Control Basics 流量控制基本原理
8 Error-Checking Basics 错误检验基本原理
9 Multiplexing Basics 复用基本原理
9.1 Figure: Multiple Applications Can Be Multiplexed into a Single Lower-Layer Data Packet 图：多个应用可以复用到单一低层数据包中
9.2 Figure: Multiple Devices Can Be Multiplexed into a Single Physical Channel 图：多个设备可以复用到单一物理通道中
10 Standards Organizations 标准化组织
11 Summary 总结
12 Review Questions 复习问答
13 For More Information 更多的参考信息
1. What Is an Internetwork? 什么是互联网络？
An internetwork is a collection of individual networks, connected by intermediate networking devices, that functions as a single large network. Internetworking refers to the industry, products, and procedures that meet the challenge of creating and administering internetworks.
一个互联网络是独立网络的一种集合，用中间联网设备连接起来，能起到单一大型网络的作用。互联网络联网指的是,能满足创建和实施互联网络挑战的工业，产品s和规程s。
Figure 1.1: Different Network Technologies Can Be Connected to Create an Internetwork illustrates some different kinds of network technologies that can be interconnected by routers and other networking devices to create an internetwork.
图1-1举例说明了,某些不同种类的网络技术,可以通过路由器和其他联网设备互联起来，以创建一个互联网络。
1.1 Figure 1.1: Different Network Technologies Can Be Connected to Create an Internetwork
各种网络技术可以连接起来创建一个互联网络
1.2 History of Internetworking 互联网络联网的历史
The first networks were time-sharing networks that used mainframes and attached terminals. Such environments were implemented by both IBM's Systems Network Architecture (SNA) and Digital's network architecture.
首批网络是时分网络，使用主机s和附加的终端s。这样的环境是由IBM的SNA(系统网络结构)和DEC网络结构两者实现的。
Local-area networks (LANs) evolved around the PC revolution. LANs enabled multiple users in a relatively small geographical area to exchange files and messages, as well as access shared resources such as file servers and printers. LANs(局域网)是围绕着PC的变革演变而来。LANs使多个用户能在一个相对小的地理区域范围内交换文件和消息,以及共享资源，如文件服务器和打印机。
Wide-area networks (WANs) interconnect LANs with geographically dispersed users to create connectivity. Some of the technologies used for connecting LANs include T1, T3, ATM, ISDN, ADSL, Frame Relay, radio links, and others. New methods of connecting dispersed LANs are appearing everyday.
WANs(广域网)将LANs互联起来，为地理上分散的用户创建了可连接性。用于连接LANs的一些技术，包括有T1, T3, ATM, ISDN, ADSL, 帧中继, 无线链路, 以及其它。把分散的LANs连接起来的新方法每天都在出现。
Today, high-speed LANs and switched internetworks are becoming widely used, largely because they operate at very high speeds and support such high-bandwidth applications as multimedia and videoconferencing.
今天，高速LANs和交换式的互联网络能被广泛地使用，很大程度上是由于它们能以极高速度进行操作,并能支持高带宽的应用,如多媒体和视频会议。
Internetworking evolved as a solution to three key problems: isolated LANs, duplication of resources, and a lack of network management. Isolated LANs made electronic communication between different offices or departments impossible. Duplication of resources meant that the same hardware and software had to be supplied to each office or department, as did separate support staff. This lack of network management meant that no centralized method of managing and troubleshooting networks existed.
演变中的互联网络联网,已经成为三大关键问题的解决方案：相互隔离的LANs孤岛，资源重复，和网络缺乏管理。隔离的LANs孤岛使不同办公室和部门间不可能通信；资源重复意味着同样的硬件和软件必须给每个办公室和部门重复提供，就像以前分别支持各成员那样；网络缺乏管理就是没有集中化的方法来管理和排障现有的网络。
1.3 Internetworking Challenges互联网络联网的挑战
Implementing a functional internetwork is no simple task. Many challenges must be faced, especially in the areas of connectivity, reliability, network management, and flexibility. Each area is key in establishing an efficient and effective internetwork. 实现一个功能上实用的互联网络并不是简单的任务。必须面对很多挑战，特别是在连接性，可靠性，网络管理，和灵活性上。要建立一个效能高及实用性好的互联网络，每个方面都是关键。
The challenge when connecting various systems is to support communication among disparate technologies. Different sites, for example, may use different types of media operating at varying speeds, or may even include different types of systems that need to communicate.
当连接各种系统时，其挑战在于,要求处于根本不同的技术环境下的各种媒体和系统,都能支持相互通信。例如,不同地点可能使用类型不同,以不同速度运行的介质，或者,甚至可能包括不同类型的系统，却需要互相通信。
Because companies rely heavily on data communication, internetworks must provide a certain level of reliability. This is an unpredictable world, so many large internetworks include redundancy to allow for communication even when problems occur.
由于公司s非常依赖于数据通信,互联网络必须提供某种程度的可靠性。然而，这是一个不可预测的世界，所以许多大型互联网络包含了冗余性，甚至当问题发生时仍容许保持通信。
Furthermore, network management must provide centralized support and troubleshooting capabilities in an internetwork. Configuration, security, performance, and other issues must be adequately addressed for the internetwork to function smoothly. Security within an internetwork is essential. Many people think of network security from the perspective of protecting the private network from outside attacks. However, it is just as important to protect the network from internal attacks, especially because most security breaches come from inside. Networks must also be secured so that the internal network cannot be used as a tool to attack other external sites.
此外，在一个互联网络中，网络管理必须提供集中化的支持和排障能力。配置,安全,性能,和其他问题都必须适当地解决以使互联网络平稳地发挥作用。在互联网络中,安全是必须的。许多人认为网络安全是从保护私有网络的观点免于受到外部的攻击，然而更为重要的是,保护网络免于受到内部攻击，尤其是,因为大多数安全漏洞来自内部。网络s也必须是安全的,以使内部网络不能被用作攻击其他外部网点的工具。
Early in the year 2000, many major web sites were the victims of distributed denial of service (DDOS) attacks. These attacks were possible because a great number of private networks currently connected with the Internet were not properly secured. These private networks were used as tools for the attackers.
在2000年早期,许多主要网站曾是DDOS (分布式拒接服务)的受害者。这些攻击之所以能发生,是因为当前与互联网连接的大量私有网络并不安全。这些私有网络常被用作攻击者的工具。
Because nothing in this world is stagnant, internetworks must be flexible enough to change with new demands.
由于在这个世界上一切都在发展，互联网络s必须具有足够的灵活性以适应新的需求。(尤其是当前就急需适应对上层网络结构动态变化快速响应的迫切需求)
2.Open System Interconnection Reference Model开放系统互连参考模型
The Open System Interconnection (OSI) reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer. The OSI reference model is a conceptual model composed of seven layers, each specifying particular network functions. The model was developed by the International Organization for Standardization (ISO) in 1984, and it is now considered the primary architectural model for intercomputer communications. The OSI model divides the tasks involved with moving information between networked computers into seven smaller, more manageable task groups. A task or group of tasks is then assigned to each of the seven OSI layers. Each layer is reasonably self-contained so that the tasks assigned to each layer can be implemented independently. This enables the solutions offered by one layer to be updated without adversely affecting the other layers. The following list details the seven layers of the Open System Interconnection (OSI) reference model:
OSI参考模型叙述了信息是如何,从一台计算机中的应用软件,通过一个网络介质,移动到另一台计算机的应用软件中。OSI参考模型是一个概念模型,由七层组成，每一层指定了具体的网络功能。该模型由ISO于1984年开发，现被认为是用于计算机间通信的主要结构模型。OSI模型将联网计算机间移动信息的任务s,分成七个较小的,更易管理的任务组。一个任务或任务组于是被指定给七层OSI层的每一层。每一层合理地自我包含以使指定给每一层的任务可以独立完成。这就解决了当某层升级时不会反过来影响其他层。下面的列表详细列出了OSI参考模型的七层：(表示层中有些部分似乎应在会晤层之下(外层),例如,尤其是加密部分;否则会给会晤造成困难。或者,会晤宜采用统一的对等一致的方式,以独立于表示层。)
Layer 7-Application
Layer 6-Presentation
Layer 5-Session
Layer 4-Transport
Layer 3-Network
Layer 2-Data link
Layer 1-Physical
Note:
A handy way to remember the seven layers is the sentence "All people seem to need data processing." The beginning letter of each word corresponds to a layer.
有一种方便的方法用来记忆七层。即：英文句“All People Seem To Need Data Processing.”(“所有的人看来都需要数据处理”)中,每字第一个字母就是七层层名的第一个字母。
All--------Application layer
People-----Presentation layer
Seem-------Session layer
To---------Transport layer
Need-------Network layer
Data-------Data link layer
Processing-Physical layer
Figure 2.1: The OSI Reference Model Contains Seven Independent Layers illustrates the seven-layer OSI reference model.
图2-1 举例说明了七层OSI参考模型
2.1 Figure: The OSI Reference Model Contains Seven Independent Layers
OSI参考模型包含了七个独立层
2.2 Characteristics of the OSI Layers OSI层次特性
The seven layers of the OSI reference model can be divided into two categories: upper layers and lower layers.
OSI参考模型的七层可以分成两大类：上三层和低四层。
The upper layers of the OSI model deal with application issues and generally are implemented only in software. The highest layer, the application layer, is closest to the end user. Both users and application layer processes interact with software applications that contain a communications component. The term upper layer is sometimes used to refer to any layer above another layer in the OSI model.
上三层处理应用问题(整体处理有关应用的通信问题)，通常只用软件执行。最高的一层,应用层,最接近最终用户。用户s和应用层进程s,同时与包含有一个通信组件的应用软件相互作用。术语‘较高层’有时指的是OSI模型中另一层之上的任何层。
The lower layers of the OSI model handle data transport issues. The physical layer and the data link layer are implemented in hardware and software. The lowest layer, the physical layer, is closest to the physical network medium (the network cabling, for example) and is responsible for actually placing information on the medium.
下四层处理数据传输问题。物理层和数据链路层是用硬件和软件实现的。最低层,物理层,最接近物理网络介质(例如,网络布缆),其职责是将信息放到实际介质上去。
Figure: Two Sets of Layers Make Up the OSI Layers illustrates the division between the upper and lower OSI layers.
图2.2.1说明了上三层和下四层的区分。
2.2.1 Figure: Two Sets of Layers Make Up the OSI Layers
两组层次构成了OSI层次
2.3 Protocols 协议
The OSI model provides a conceptual framework for communication between computers, but the model itself is not a method of communication. Actual communication is made possible by using communication protocols. In the context of data networking, a protocol is a formal set of rules and conventions that governs how computers exchange information over a network medium. A protocol implements the functions of one or more of the OSI layers.
OSI模型提供了一个概念性的框架用于计算机间的通信，模型本身并不是一种通信的方法。实际通信可以使用通信协议s来完成。在数据联网的上下文中,一个协议是一组规则和约定的一种形式,管理计算机如何在一个网络介质上交换信息。一个协议执行OSI的一层或更多层的功能。
A wide variety of communication protocols exist. Some of these protocols include LAN protocols, WAN protocols, network protocols, and routing protocols. LAN protocols operate at the physical and data link layers of the OSI model and define communication over the various LAN media. WAN protocols operate at the lowest three layers of the OSI model and define communication over the various wide-area media. Routing protocols are network layer protocols that are responsible for exchanging information between routers so that the routers can select the proper path for network traffic. Finally, network protocols are the various upper-layer protocols that exist in a given protocol suite. Many protocols rely on others for operation. For example, many routing protocols use network protocols to exchange information between routers. This concept of building upon the layers already in existence is the foundation of the OSI model.
OSI有一大类范围很广的协议s。其中包括有LAN协议s,WAN协议s,网络协议s,和路由协议s。LAN协议操作在OSI模型的物理层和数据链路层，并定义了在各种LAN介质上的通信。WAN协议操作在最低三层，它们定义了在各种广域网介质上的通信。路由协议是网络层协议s，其职责是用于在路由器之间交换信息，以使路由器可以选择用于网络信流的适当通路。最后,网络协议栈是各种上层协议s，它们存在于所给的一组协议族中。许多协议依赖其它协议才可操作。例如,许多路由协议使用网络协议s来交换路由器之间的信息。这种建立在已经存在的层上(的执行结果上,所谓’服务’)的概念已经成为OSI模型的基础。
2.4 OSI Model and Communication Between Systems
Information being transferred from a software application in one computer system to a software application in another must pass through the OSI layers. For example, if a software application in System A has information to transmit to a software application in System B, the application program in System A will pass its information to the application layer (Layer 7) of System A. The application layer then passes the information to the presentation layer (Layer 6), which relays the data to the session layer (Layer 5), and so on down to the physical layer (Layer 1). At the physical layer, the information is placed on the physical network medium and is sent across the medium to System B. The physical layer of System B removes the information from the physical medium, and then its physical layer passes the information up to the data link layer (Layer 2), which passes it to the network layer (Layer 3), and so on, until it reaches the application layer (Layer 7) of System B. Finally, the application layer of System B passes the information to the recipient application program to complete the communication process.
被传输的信息,从一个计算机系统的应用软件到另一个计算机系统的应用软件,必须通过OSI层次······
2.4.1 Interaction Between OSI Model Layers OSI模型各层间的互相作用
A given layer in the OSI model generally communicates with three other OSI layers: the layer directly above it, the layer directly below it, and its peer layer in other networked computer systems. The data link layer in System A, for example, communicates with the network layer of System A, the physical layer of System A, and the data link layer in System B.
OSI模型中所给的一层,通常与其它三层相通信：直接在它上面的一层，直接在它下面的一层，以及它的对等层---它处在已联网的其它计算机系统中。例如,系统A的数据链路层与系统A的网络层,系统A的物理层,以及系统B的数据链路层相通信。
Figure2.4.1.1: OSI Model Layers Communicate with Other Layers illustrates this example. 图2.4.1.1举例说明了这一例子。
2.4.1.1 Figure: OSI Model Layers Communicate with Other Layers
OSI模型层次与其它OSI层次的通信
2.4.1.2 OSI Layer Services OSI层服务s
One OSI layer communicates with another layer to make use of the services provided by the second layer. The services provided by adjacent layers help a given OSI layer communicate with its peer layer in other computer systems. Three basic elements are involved in layer services: the service user, the service provider, and the service access point (SAP).
一个给定的OSI层,通过与其它层通信,来使用由次层所提供的服务。邻层提供的服务,帮助所给OSI层与它在其他计算机系统中的对等层实现通信。在层服务中有三个基本组成部分：服务用户,服务提供者,和服务访问点(SAP)(见图2.4.1.2)。
In this context, the service user is the OSI layer that requests services from an adjacent OSI layer. The service provider is the OSI layer that provides services to service users. OSI layers can provide services to multiple service users. The SAP is a conceptual location at which one OSI layer can request the services of another OSI layer.
在这种上下文关系中,服务用户是申请服务的OSI层，它申请来自相邻(下层)OSI层的服务。服务提供者则是提供服务s给服务用户s的OSI层。OSI层s可以提供服务给多个服务用户。SAP是一个概念性的位置，在那里，一个OSI层可以申请其它OSI层的服务s。(所谓概念性是否指只是原理性、理解性,而不是实体，没有如协议一类的东西可以明确具体实现？我们认为，对于OSI和SAPs而言，其实不应该仅限于此！OSI既然是一个框架，就应该有格式来支撑；也就是说，OSI框架就是体现实现的软件框架和硬件框架，这种框架的具体体现就应该在每层SAP的头部(和/或尾部)的统一格式,以及对格式的统一处理(即:协议)上。它们应该成为实体性的夹层，即所谓：As:1:P:1: :1:S:1:T:1:N:1:D:1:P，甚至还应该在最前面加上一个SAP夹层；而SAP不但要’承上---对等层前作还原服务’，而且要‘’启下---选下层功能或下层路由’，以实现双向传递。承上的控制信息可以放在头部，应在对等层释放；启下的控制信息可以放在尾部(SDU?服务数据单元)，应在本SAP中完成启下任务进入下邻层时释放。尾部的控制信息结构，可以取下邻层SAP作承上头部的格式和结构)
（话到这里，就值得继续说下去。SAP承上起下的工作，应该智能实现；更进一步，应该‘语义+智能’实现。从而隔离了两个(乃至多个)知识领域的人工交互(例如，需要专家和编程者的交互)。）
Figure: Service Users, Providers, and SAPs Interact at the Network and Data Link Layers illustrates how these three elements interact at the network and data link layers.
图2.4.1.2举例说明在网络层和数据链路层上这三个组成部分是如何相互作用的。
Figure 2.4.1.2: Service Users, Providers, and SAPs Interact at the Network and Data Link Layers
服务用户s,服务提供者s,和服务访问点s在网络层和数据链路层上的相互作用
2.4.2 OSI Model Layers and Information Exchange OSI模型层次和信息交换
The seven OSI layers use various forms of control information to communicate with their peer layers in other computer systems. This control information consists of specific requests and instructions that are exchanged between peer OSI layers. 七层OSI层,使用各种形式的控制信息,与相应的,在其它计算机系统s中的对等层s,进行通信。控制信息是由特殊的申请s和指令s组成，它们在对等的OSI层间进行交换。
Control information typically takes one of two forms: headers and trailers. Headers are prepended to data that has been passed down from upper layers. Trailers are appended to data that has been passed down from upper layers. An OSI layer is not required to attach a header or a trailer to data from upper layers.
(本层的)控制信息通常取两种形式之一：(本层的)头部和尾部。头部是由前加到来自上层的数据构成。尾部则后拼来自上层的数据。一个OSI层并不需要为来自于上层的数据加一个(上层的)头部或尾部。
Headers, trailers, and data are relative concepts, depending on the layer that analyzes the information unit. At the network layer, for example, an information unit consists of a Layer 3 header and data. At the data link layer, however, all the information passed down by the network layer (the Layer 3 header and the data) is treated as data. 头部,尾部,和数据都是相对的概念，取决于解析信息单元的层。例如,在网络层,一个信息单元是由层3的头部和数据组成。然而,在数据链路层,所有传输通过网络层(层3头部和数据)的信息都被当作数据处理。
In other words, the data portion of an information unit at a given OSI layer potentially can contain headers, trailers, and data from all the higher layers. This is known as encapsulation.
换句话说,一个所给OSI层的信息单元,都有可能包含来自上层s的头部,尾部,和数据。这称之为封装。(能够逐层把信息单元封装起来的依据,正是建立在OSI层次中,低层是高层的外围,层层包含,且各层间的功能独立,的基础上)
Figure 2.4.2.1: Headers and Data Can Be Encapsulated During Information Exchange shows how the header and data from one layer are encapsulated into the header of the next lowest layer.
图2.4.2.1显示了来自某层的头部和数据是如何被封装到邻接低层的头部 (?数据)的。
2.4.2.1 Figure: Headers and Data Can Be Encapsulated During Information Exchange 在信息交换期间头部和数据可以被封装起来
2.4.2.2 Information Exchange Process信息交换过程
The information exchange process occurs between peer OSI layers. Each layer in the source system adds control information to data, and each layer in the destination system analyzes and removes the control information from that data.
在OSI对等层之间发生信息交换。在源系统的每一层把控制信息加到数据中，而在目的系统的每一层则解析,并从数据中移去控制信息。
If System A has data from a software application to send to System B, the data is passed to the application layer. The application layer in System A then communicates any control information required by the application layer in System B by prepending a header to the data. The resulting information unit (a header and the data) is passed to the presentation layer, which prepends its own header containing control information intended for the presentation layer in System B. The information unit grows in size as each layer prepends its own header (and, in some cases, a trailer) that contains control information to be used by its peer layer in System B. At the physical layer, the entire information unit is placed onto the network medium.
若系统A······
The physical layer in System B receives the information unit and passes it to the data link layer. The data link layer in System B then reads the control information contained in the header prepended by the data link layer in System A. The header is then removed, and the remainder of the information unit is passed to the network layer. Each layer performs the same actions: The layer reads the header from its peer layer, strips it off, and passes the remaining information unit to the next highest layer. After the application layer performs these actions, the data is passed to the recipient software application in System B, in exactly the form in which it was transmitted by the application in System A.
在系统A中的物理层······
2.5 OSI Model Physical Layer OSI模型的物理层
The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems. Physical layer specifications define characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors. Physical layer implementations can be categorized as either LAN or WAN specifications.
物理层定义了电气的,机械的,过程的,和功能的规范，用于激活,维持,和去激活通信网络系统之间的物理链路。物理层规范定义了,如电压电平,电压变化的时间关系,物理数据速率,最大传输距离,以及物理连接器,等等的物理特性。(不同的物理介质有不同的物理特性。)物理层的实现,可以按LAN或WAN规范来分类。
Figure 2.5.1: Physical Layer Implementations Can Be LAN or WAN Specifications illustrates some common LAN and WAN physical layer implementations.
图2.5.1举例说明了一些通常的LAN和WAN物理层的实现。
2.5.1 Figure: Physical Layer Implementations Can Be LAN or WAN Specifications 物理层的实现可能是LAN规范，或者是WAN规范
2.6 OSI Model Data Link Layer OSI模型的数据链路层
The data link layer provides reliable transit of data across a physical network link. Different data link layer specifications define different network and protocol characteristics, including physical addressing, network topology, error notification, sequencing of frames, and flow control. Physical addressing (as opposed to network addressing) defines how devices are addressed at the data link layer. Network topology consists of the data link layer specifications that often define how devices are to be physically connected, such as in a bus or a ring topology. Error notification alerts upper-layer protocols that a transmission error has occurred, and the sequencing of data frames reorders frames that are transmitted out of sequence. Finally, flow control moderates the transmission of data so that the receiving device is not overwhelmed with more traffic than it can handle at one time.
数据链路层为通过一个物理网络链路的数据,提供数据的可靠运送。不同的数据链路规范定义不同的网络和协议特性，包括物理寻址,网络拓扑,错误通告,帧排序,和流量控制。物理寻址(不是网络寻址。LLC地址涵盖MAC地址。LLC地址不是网络地址。MAC地址与物理层紧密结合。)定义了设备在数据链路层中是如何地址址。网络拓扑(应在MAC层上)由数据链路层的规范s构成，通常定义设备s在物理上是如何连接的，如,在一个总线中或是在路环拓扑中。错误通告通知上层协议：一个传送错误已经发生,数据帧排序要重新再排,因为发送出列了。最后,流量控制平缓了数据的发送,以使接收设备不会因为信流太多,超过它能一次处理的能力而被淹没。
The Institute of Electrical and Electronics Engineers (IEEE) has subdivided the data link layer into two sublayers: Logical Link Control (LLC) and Media Access Control (MAC).
IEEE已经细分了数据链路层为两个子层：LLC(逻辑链路控制)和MAC(介质访问控制)。
Figure: The Data Link Layer Contains Two Sublayers illustrates the IEEE sublayers of the data link layer.
图2.6.1 举例说明了IEEE的数据链路层的子层。
2.6.1 Figure: The Data Link Layer Contains Two Sublayers
The Logical Link Control (LLC) sublayer of the data link layer manages communications between devices over a single link of a network. LLC is defined in the IEEE 802.2 specification and supports both connectionless and connection-oriented services used by higher-layer protocols. IEEE 802.2 defines a number of fields in data link layer frames that enable multiple higher-layer protocols to share a single physical data link. The Media Access Control (MAC) sublayer of the data link layer manages protocol access to the physical network medium. The IEEE MAC specification defines MAC addresses, which enable multiple devices to uniquely identify one another at the data link layer.
数据链路层的LLC子层管理一个网络的单一链路上的设备间的(一次完整的)通信。LLC定义在IEEE 802.2的规范中,支持用于高层协议s的无连接和面向连接服务这两者。IEEE 802.2在数据链路帧中定义了若干字段,使多个高层协议能共享单一物理数据链路。MAC子层 管理协议对物理网络介质的访问。IEEE MAC规范定义MAC地址,使多个设备能在数据链路层上被唯一地识别,以区分彼此。
2.7 OSI Model Network Layer OSI模型的网络层
The network layer defines the network address, which differs from the MAC address. Some network layer implementations, such as the Internet Protocol (IP), define network addresses in a way that route selection can be determined systematically by comparing the source network address with the destination network address and applying the subnet mask. Because this layer defines the logical network layout, routers can use this layer to determine how to forward packets. Because of this, much of the design and configuration work for internetworks happens at Layer 3, the network layer.
网络层定义了网络地址,它不同于MAC地址。某些网络层地址的实现,如IP(互联网协议),可以使用一种定义网络地址的方法,即路由选择能从系统上通过比较源网络地址和目的网络地址,并施加子网屏蔽来实现。由于该层定义了逻辑网络布置,路由器就可以使用该层以确定如何转发数据包。因此,对于互联网络的许多设计和配置,都发生在第三层(网络层)。
2.8 OSI Model Transport Layer OSI模型的传输层
The transport layer accepts data from the session layer and segments the data for transport across the network. Generally, the transport layer is responsible for making sure that the data is delivered error-free and in the proper sequence. Flow control generally occurs at the transport layer. 传输层从会晤层接收数据,并将数据分段,以便于传输通过网络。通常,传输层的职责在于保证以适当的次序交付没有错误的数据。流量控制通常发生在传输层。
Flow control manages data transmission between devices so that the transmitting device does not send more data than the receiving device can process. Multiplexing enables data from several applications to be transmitted onto a single physical link. Virtual circuits are established, maintained, and terminated by the transport layer. Error checking involves creating various mechanisms for detecting transmission errors, while error recovery involves acting, such as requesting that data be retransmitted, to resolve any errors that occur.
流量控制对设备间的数据传送进行管理，以使发送设备不会送出大于接收设备处理能力的数据。复用,使来自几个应用程序要发送的数据,能发送到单一网络链路上。通过传输层,虚电路就被建立,维持和终止。错误检查涉及产生用于检测传送错误的各种机制,而纠错恢复需要各种活动,如申请数据重新发送,分辨发生的任何错误。
The transport protocols used on the Internet are TCP and UDP.
在互联网上的传输层协议是TCP和UDP。
2.9 OSI Model Session Layer OSI模型的会晤层
The session layer establishes, manages, and terminates communication sessions. Communication sessions consist of service requests and service responses that occur between applications located in different network devices. These requests and responses are coordinated by protocols implemented at the session layer. Some examples of session-layer implementations include Zone Information Protocol (ZIP), the AppleTalk protocol that coordinates the name binding process; and Session Control Protocol (SCP), the DECnet Phase IV session layer protocol.
会晤层实现 建立,管理,和终止通信会晤。通信会晤是由服务申请和服务响应组成，它们发生在位于不同网络地点设备中的应用程序之间(即对等层之间)。这些申请和响应,由在会晤层实现的协议s来协调。会晤层实现的某些例子包括ZIP(区域信息协议), ,协调名字捆绑过程的AppleTalk协议,和作为DECnet 阶段4会晤层协议的SCP (会晤控制协议)。(会晤层的一个重要职责,是分配资源(建立),控制资源(管理),释放资源(终止))
2.10 OSI Model Presentation Layer OSI模型的表示层
The presentation layer provides a variety of coding and conversion functions that are applied to application layer data. These functions ensure that information sent from the application layer of one system would be readable by the application layer of another system. Some examples of presentation layer coding and conversion schemes include common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes. 表示层提供了各种编码和变换功能，它们用在应用层数据上。这些功能保证由一个系统的应用层送出的信息能被另一个系统的应用层读出。表示层编码和变换策略的一些例子,包括常见的数据表示格式，字符表示格式的变换,通常的数据压缩策略,以及常见的数据加密策略。
Common data representation formats, or the use of standard image, sound, and video formats, enable the interchange of application data between different types of computer systems. Conversion schemes are used to exchange information with systems by using different text and data representations, such as EBCDIC and ASCII. Standard data compression schemes enable data that is compressed at the source device to be properly decompressed at the destination. Standard data encryption schemes enable data encrypted at the source device to be properly deciphered at the destination.
使用通用的数据表示格式,或使用标准的图像,声音,和视频格式,使你能在不同类型的计算机系统间互相交换应用数据。变换策略则用于所交换的信息,使用了文本和数据格式不相同的表示系统,如EBCDIC和ASICC。标准数据压缩策略,使得在源设备处压缩的数据,在目的处能适当地解压出来。标准数据加密策略使在源设备处加密的数据在目的处能适当地解密出来。
Presentation layer implementations are not typically associated with a particular protocol stack. Some well-known standards for video include QuickTime and Motion Picture Experts Group (MPEG). QuickTime is an Apple Computer specification for video and audio, and MPEG is a standard for video compression and coding.
表示层的实现通常并不与具体的协议栈有关。一些著名的视频标准包括QuickTime和MPEG(移动图像专家组)。QuickTime是苹果公司用于视频和音频的一个规范,MPEG是一个用于视频压缩和编码的标准。
Among the well-known graphic image formats are Graphics Interchange Format (GIF), Joint Photographic Experts Group (JPEG), and Tagged Image File Format (TIFF). GIF is a standard for compressing and coding graphic images. JPEG is another compression and coding standard for graphic images, and TIFF is a standard coding format for graphic images.
著名的图形图像格式是GIF(图形交换格式),JPEG(联合照片专家组),和TIFF(标志的影像文件格式)。GIF是一个图形影像的压缩和编码标准，JPEG是另一个这样的标准，TIFF是用于图形影像的编码格式。
2.11 OSI Model Application Layer OAI模型的应用层
The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application.
应用层是最接近用户的OSI层，也就是说,OSI应用层及用户,直接与应用软件相互作用。
This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication.
该层与一种应用软件相互作用，其中软件起到一个通信组件的作用。这样的应用程序已经超出OSI模型的范围。应用层的功能通常包括识别通信伙伴s,确定资源可用性,以及同步通信。
When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer.
在识别通信伙伴s时,对于一个欲传送数据的应用而言,应用层确认通信伙伴的身份和可用性。在确定资源的可用性时，应用层必须判断,对于被申请的,已存在的通信,是否有足够的网络资源。在同步通信中,应用程序间的所有通信,需要协调,合作,这是通过应用层来管理的。
Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).
应用层实现的某些例子包括Telnet,FTP(文件传输协议),和SMTP(简单邮件传输协议)。
3. Information Formats 信息格式
The data and control information that is transmitted through internetworks takes a variety of forms. The terms used to refer to these information formats are not used consistently in the internetworking industry but sometimes are used interchangeably. Common information formats include frames, packets, datagrams, segments, messages, cells, and data units.
通过互联网络发送的数据和控制信息(源层各层建立的,用于对等的目的层的控制信息—通常用于服务--存放在该层的头部或尾部)可取多种格式。用来引用这些信息格式的术语,在互联网络联网中,并不统一,有时可以互相交换。普通信息格式包括帧,数据包,数据报,分段,消息,信元和数据单元。
A frame is an information unit whose source and destination are data link layer entities. A frame is composed of the data link layer header (and possibly a trailer) and upper-layer data. The header and trailer contain control information intended for the data link layer entity in the destination system. Data from upper-layer entities is encapsulated in the data link layer header and trailer.
术语帧是一个信息单元,其源和目的是数据链路层的实体。一帧是由数据链路层的头部(也可能是尾部)和上层数据组成。头部和尾部包含了拟用于目的系统中链路层实体的控制信息。来自上层实体的数据与本层头部或尾部封装在数据链路层中。
Figure 3.1: Data from Upper-Layer Entities Makes Up the Data Link Layer Frame illustrates the basic components of a data link layer frame.
图3.1举例说明了一个数据链路层帧的基本组成部分
3.1 Figure: Data from Upper-Layer Entities Makes Up the Data Link Layer Frame
A packet is an information unit whose source and destination are network layer entities. A packet is composed of the network layer header (and possibly a trailer) and upper-layer data. The header and trailer contain control information intended for the network layer entity in the destination system. Data from upper-layer entities is encapsulated in the network layer header and trailer.
术语分组包是一个信息单元,它的源和目的是网络层实体。一个分组包是由网络层的头部(也可能是一个尾部)和上层数据组成。头部和尾部包含控制信息,拟用于目的系统的网络层实体。来自上层实体的数据与本层头部或尾部封装在网络层中。
Figure: Three Basic Components Make Up a Network Layer Packet illustrates the basic components of a network layer packet.
图1-10说明了一个网络层分组包的三个基本组成部分。
3.2 Figure: Three Basic Components Make Up a Network Layer Packet
三个基本组成部分组成了一个网络层分组包
The term datagram usually refers to an information unit whose source and destination are network layer entities that use connectionless network service.
术语数据报通常指的是一个信息单元,其源和目的是网络层实体,它们使用无连接网络服务。
The term segment usually refers to an information unit whose source and destination are transport layer entities.
术语分段通常指的是一个信息单元,其源和目的是传输层实体。
A message is an information unit whose source and destination entities exist above the network layer (often at the application layer).
术语消息是一个信息单元,其源和目的实体存在于网络层之上(通常在应用层)。
A cell is an information unit of a fixed size whose source and destination are data link layer entities. Cells are used in switched environments, such as Asynchronous Transfer Mode (ATM) and Switched Multimegabit Data Service (SMDS) networks. A cell is composed of the header and payload. The header contains control information intended for the destination data link layer entity and is typically 5 bytes long. The payload contains upper-layer data that is encapsulated in the cell header and is typically 48 bytes long.
一个信元是一个固定尺寸的信息单元,其源和目的是数据链路层实体。信元s用于交换环境中，如ATM(异步传输模式),和SMDS(交换的多兆位数据服务,)网络。一个信元由头部和有效载荷组成。头部包含有控制信息,拟用于目的数据链路层实体,通常是5字节长。有效载荷包含上层数据,它们封装在信元头部,通常是48字节长。
The length of the header and the payload fields always are the same for each cell.
对于每一个信元,头部和有效载荷字段s的长度都是一样的。
Figure: Two Components Make Up a Typical Cell depicts the components of a typical cell.
图1-11说明了一个典型的信元的组成。
3.3 Figure: Two Components Make Up a Typical Cell
Data unit is a generic term that refers to a variety of information units. Some common data units are service data units (SDUs), protocol data units, and bridge protocol data units (BPDUs). SDUs are information units from upper-layer protocols that define a service request to a lower-layer protocol. PDU is OSI terminology for a packet. BPDUs are used by the spanning-tree algorithm as hello messages.
数据单元是一个一般性的术语,指的是各种信息单元。一些通常的数据单元是服务数据单元(SDUs),协议数据单元(PDUs),和桥协议数据单元(BPDUs)。SDUs是来自上层协议的信息单元,它们定义了一种申请一个低层协议的服务。PDU是OSI术语,用于一个数据包。BPDUs被生成树算法用作hello消息。
4. ISO Hierarchy of Networks ISO网络的体系结构
Large networks typically are organized as hierarchies. A hierarchical organization provides such advantages as ease of management, flexibility, and a reduction in unnecessary traffic. Thus, the International Organization for Standardization (ISO) has adopted a number of terminology conventions for addressing network entities. Key terms defined in this section include end system (ES), intermediate system (IS), area, and autonomous system (AS). 大型网络通常组织成体系结构。一个体系结构组织提供了这样的的优点,如易于管理,灵活性,和减少不必要的信流。于是ISO采用了若干个术语惯例,用于选择网络实体s。在本节中定义的关键术语包括ES(端系统,指端设备),IS(中间系统,指转发设备,如路由器等),和AS(自治系统)。（参见图4.1）
An ES is a network device that does not perform routing or other traffic forwarding functions. Typical ESs include such devices as terminals, personal computers, and printers. An IS is a network device that performs routing or other traffic-forwarding functions. Typical ISs include such devices as routers, switches, and bridges. Two types of IS networks exist: intradomain IS and interdomain IS. An intradomain IS communicates within a single autonomous system, while an interdomain IS communicates within and between autonomous systems. An area is a logical group of network segments and their attached devices. Areas are subdivisions of autonomous systems (AS's). An AS is a collection of networks under a common administration that share a common routing strategy. Autonomous systems are subdivided into areas, and an AS is sometimes called a domain.
一个端系统ES是一种网络设备,但它并不执行路由或其它信流转发功能s。通常ESs包括如终端,PC,和打印机这样的设备。一个中间系统IS也是一种网络设备,它执行路由或其他信流转功能。通常ISs包括如路由器,交换机,和路桥。有两类IS网络：域内IS与域间IS。域内IS在单一自治系统内通信,而域间IS则是包含在自治系统内和自治系统间通信。一个自治系统AS是一个网络集合,处于共享同一路由策略的统一管理。有时称AS为域。自治系统细分为区Area，一个区Area是网络段s及其附加设备s的逻辑组合。区是自治系统s的细分部分s。。
（将整个互联网络---包括TCP/IP,OSI,AppleTalk，乃至不断衍生和创建崭新类型的互联网络,如已经出现的并迅速往深广发展的物联网,和必将出现的星际网---能涵盖在全网一致的,层次结构化的,可物理寻址的分层结构中，是一个十分重要而迫切的科研论题。）
Figure 4.1: A Hierarchical Network Contains Numerous Components illustrates a hierarchical network and its components.
图4.1举例说明了一个体系结构的网络和组成部分。
4.1 Figure: A Hierarchical Network Contains Numerous Components
一个体系结构的网络包含许多组成部分
5. Connection-Oriented and Connectionless Network Services
面向连接和无连接网络服务
（属于通信层次的传输层,网络层,和数据链路层都有一个面向连接或无连接的问题。面向连接的物理层通常是虚电路连接。会晤层是面向连接的。）
In general, transport protocols can be characterized as being either connection-oriented or connectionless. Connection-oriented services must first establish a connection with the desired service before passing any data. A connectionless service can send the data without any need to establish a connection first. In general, connection-oriented services provide some level of delivery guarantee, whereas connectionless services do not
通常,传输协议s可以按面向连接或无连接的特性分类。面向连接的服务必须在传送任何数据前以所希望的服务首先建立起一种连接。一种无连接服务则不必建立连接就可以发送数据。一般说来,面向连接的服务s通过某种水平的交付保证,而无连接服务则没有。.
Connection-oriented service involves three phases: connection establishment, data transfer, and connection termination.
面向连接的服务涉及三个阶段: 连接建立,数据传输,和连接终止。
During connection establishment, the end nodes may reserve resources for the connection. The end nodes also may negotiate and establish certain criteria for the transfer, such as a window size used in TCP connections. This resource reservation is one of the things exploited in some denial of service (DOS) attacks. An attacking system will send many requests for establishing a connection but then will never complete the connection. The attacked computer is then left with resources allocated for many never-completed connections. Then, when an end node tries to complete an actual connection, there are not enough resources for the valid connection.
在连接建立阶段,端节点s可以保存用于连接的资源。端节点s也可以协商,建立用于传输的某些准则,如一个用于TCP连接的窗口大小。资源保存在某些'拒绝服务攻击s'中被利用的做法中的一个。一个攻击系统可以送出许多用于建立连接的资源申请,但从不完成连接。这样受攻击计算机的大量资源'被冻结',使正常连接因为没有足够的资源而无法建立.
The data transfer phase occurs when the actual data is transmitted over the connection. During data transfer, most connection-oriented services will monitor for lost packets and handle resending them. The protocol is generally also responsible for putting the packets in the right sequence before passing the data up the protocol stack.
数据传输阶段发生于当实际数据在连接上被发送时。在数据传输期间,大多数面向连接的服务s将监视数据包丢失和重发。协议通常也有责任,在传送数据前,以正确的次序把数据包放入堆栈中。
When the transfer of data is complete, the end nodes terminate the connection and release resources reserved for the connection.
连接终止阶段:当传输数据完成后,端节点s终止连接,释放用于连接所保存的资源。
Connection-oriented network services have more overhead than connectionless ones. Connection-oriented services must negotiate a connection, transfer data, and tear down the connection, whereas a connectionless transfer can simply send the data without the added overhead of creating and tearing down a connection. Each has its place in internetworks.
面向连接的网络服务s比无连接网络要有更多的开销。
6. Internetwork Addressing 互联网络寻址
Internetwork addresses identify devices separately or as members of a group. Addressing schemes vary depending on the protocol family and the OSI layer. Three types of internetwork addresses are commonly used: data link layer addresses, Media Access Control (MAC) addresses, and network layer addresses.
互联网络的地址可以用来识别单个设备,或将一组设备作为单个成员进行识别。寻址策略是可变的,取决于协议族和所处OSI层。通常使用三类互联网络地址：数据链路层地址,MAC(介质访问控制)地址,和网络层地址。
6.1 Data Link Layer Addresses 数据链路层地址
A data link layer address uniquely identifies each physical network connection of a network device. Data-link addresses sometimes are referred to as physical or hardware addresses. Data-link addresses usually exist within a flat address space and have a pre-established and typically fixed relationship to a specific device. 一个数据链路层地址唯一地识别每个物理网络连接的一个网络设备。数据链路地址有时指的是物理或硬件地址。数据链路地址通常存在于一个扁平地址空间内,与一个预建的指定设备通常有固定关系。(为什么不能把唯一对应LAN接口的MAC地址按物理寻址排列的思路分层结构化？而把唯一可识别的固定关系的扁平地址—即URI—作为物理对象的识别,留给物联网使用。)
End systems generally have only one physical network connection and thus have only one data-link address. Routers and other internetworking devices typically have multiple physical network connections and therefore have multiple data-link addresses. 端系统通常只有一个物理网络连接,于是就只有一个数据链路地址。路由器和其他互联网络联网设备通常有多个物理网络连接,因此就有多个数据链路地址。
Figure: Each Interface on a Device Is Uniquely Identified by a Data-Link Address illustrates how each interface on a device is uniquely identified by a data-link address.
图6.1.1举例说明了每个接口如何用一个数据链路地址唯一地识别。
6.1.1 Figure: Each Interface on a Device Is Uniquely Identified by a Data-Link Address
6.2 MAC Addresses MAC层地址
Media Access Control (MAC) addresses consist of a subset of data link layer addresses. MAC addresses identify network entities in LANs that implement the IEEE MAC addresses of the data link layer. As with most data-link addresses, MAC addresses are unique for each LAN interface.
MAC地址组成了数据链路层地址的一个子集。MAC地址识别LAN中的网络实体,该实体实现了数据链路层的IEEE MAC地址。如同大多数数据链路地址一样,MAC地址唯一地用于每个LAN接口。
Figure 6.2.1: MAC Addresses, Data-Link Addresses, and the IEEE Sublayers of the Data Link Layer Are All Related illustrates the relationship between MAC addresses, data-link addresses, and the IEEE sublayers of the data link layer.
图6.2.1举例说明了MAC地址,数据链路地址,和数据链路层的IEEE子层之间的关系。
6.2.1 Figure: MAC Addresses, Data-Link Addresses, and the IEEE Sublayers of the Data Link Layer Are All Related
MAC地址,数据链路地址,和IEEE的数据链路层的各子层相互间的关系
MAC addresses are 48 bits in length and are expressed as 12 hexadecimal digits. The first 6 hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor and thus comprise the Organizationally Unique Identifier (OUI). The last 6 hexadecimal digits comprise the interface serial number, or another value administered by the specific vendor. MAC addresses sometimes are called burned-in addresses (BIAs) because they are burned into read-only memory (ROM) and are copied into random-access memory (RAM) when the interface card initializes.
MAC地址是48位长度,用12位十六进制数字表示。十六进制数字的前6位是由IEEE管理的,用来识别制造商或售主,构成OUI(唯一的机构识别符)；后6位十六进制数字组成接口串号,或其它的为专门售主所管理的值。MAC地址有时叫做BIA(烙印地址),因为它们被烧制在ROM内,并在接口卡启动时被拷贝到RAM中。.
Figure 6.2.2: The MAC Address Contains a Unique Format of Hexadecimal Digits illustrates the MAC address format.
图6.2.2列举了MAC地址格式
6.2.2 Figure: The MAC Address Contains a Unique Format of Hexadecimal Digits MAC地址包含了一个唯一的十六进制数字格式
6.3 Mapping Addresses 映射地址
Because internetworks generally use network addresses to route traffic around the network, there is a need to map network addresses to MAC addresses. When the network layer has determined the destination station's network address, it must forward the information over a physical network using a MAC address. Different protocol suites use different methods to perform this mapping, but the most popular is Address Resolution Protocol (ARP).
因为互联网络通常使用网络地址来路由围绕着网络的信流,于是就有一种需要将网络地址映射到MAC地址。当网络层已经确定了目的站网络地址,就必须在物理网络上使用MAC地址转发信息。不同协议族使用不同方法来执行这种映射,但大多数流行的是ARP(地址解析协议)。
Different protocol suites use different methods for determining the MAC address of a device. The following three methods are used most often. Address Resolution Protocol (ARP) maps network addresses to MAC addresses. The Hello protocol enables network devices to learn the MAC addresses of other network devices. MAC addresses either are embedded in the network layer address or are generated by an algorithm.
不同的协议族使用不同的方法来确定一个设备的MAC地址。下面是最常使用的三种方法。ARP映射网络地址到MAC地址；Hello协议使网络设备获悉其他网络设备的MAC地址；MAC地址或被嵌入到网络层地址中,或由一种算法来产生。
Address Resolution Protocol (ARP) is the method used in the TCP/IP suite. When a network device needs to send data to another device on the same network, it knows the source and destination network addresses for the data transfer. It must somehow map the destination address to a MAC address before forwarding the data. First, the sending station will check its ARP table to see if it has already discovered this destination station's MAC address. If it has not, it will send a broadcast on the network with the destination station's IP address contained in the broadcast. Every station on the network receives the broadcast and compares the embedded IP address to its own. Only the station with the matching IP address replies to the sending station with a packet containing the MAC address for the station. The first station then adds this information to its ARP table for future reference and proceeds to transfer the data.
ARP是一种用于TCP/IP族的方法。当一个网络设备需要把数据送到同一网络的另一个设备时,它知道用于数据传输的源和目的地址。它必须在转发数据前将目的地址以某种方法映射到MAC地址。首先,发送站检查ARP表,了解是否已经发现了目的站点的MAC地址。如果还没有,就在网络上发送一个含有目的站点IP地址的广播。网络上的每个站点都会接收该广播并将广播中嵌入的IP地址与自身的地址相比较；只有地址匹配的站点,才会向原发送站返回一个包含自身MAC地址的数据包；原发送站于是就把该信息加到它自己的ARP表中,用于将来引用和实现数据传输。
When the destination device lies on a remote network, one beyond a router, the process is the same except that the sending station sends the ARP request for the MAC address of its default gateway. It then forwards the information to that device. The default gateway will then forward the information over whatever networks necessary to deliver the packet to the network on which the destination device resides. The router on the destination device's network then uses ARP to obtain the MAC of the actual destination device and delivers the packet. 如果目的设备位于远处,超过一个路由器,的网络，其处理过程相同,但除了发送站的ARP申请是针对默认网关的MAC地址。(它含在目的地址的子网掩码中)
The Hello protocol is a network layer protocol that enables network devices to identify one another and indicate that they are still functional. When a new end system powers up, for example, it broadcasts hello messages onto the network. Devices on the network then return hello replies, and hello messages are also sent at specific intervals to indicate that they are still functional. Network devices can learn the MAC addresses of other devices by examining Hello protocol packets.
Hello协议是网络层协议,它能使网络设备彼此相认,并指示它们保持联系。例如,当一个新的端设备启动时,它就在网络上广播hello消息,网络上的设备于是返回hello回答；hello消息也送往指定区间s以指明它们保持联系。网络设备s通过检查Hello协议数据包就能获悉其它设备的MAC地址。
Three protocols use predictable MAC addresses. In these protocol suites, MAC addresses are predictable because the network layer either embeds the MAC address in the network layer address or uses an algorithm to determine the MAC address. The three protocols are Xerox Network Systems (XNS), Novell Internetwork Packet Exchange (IPX), and DECnet Phase IV. 第三种方法是,有三种协议都是用来获悉可预测MAC地址。在这些协议族中,MAC地址是可预测的,因为或者在网络层地址中嵌入了MAC地址,或者使用了一种算法以确定MAC地址。三种协议是XNS(Xerox网络系统),IPX(Novell互联网络包交换),和DECnet阶段IV。
6.4 Network Layer Addresses 网络层地址
A network layer address identifies an entity at the network layer of the OSI layers. Network addresses usually exist within a hierarchical address space and sometimes are called virtual or logical addresses.
一个网络层地址识别OSI网络层上的一个实体。网络地址s通常位于一个结构地址空间内,有时称为虚地址或逻辑地址s。
The relationship between a network address and a device is logical and unfixed; it typically is based either on physical network characteristics (the device is on a particular network segment) or on groupings that have no physical basis (the device is part of an AppleTalk zone). End systems require one network layer address for each network layer protocol that they support. (This assumes that the device has only one physical network connection.) Routers and other internetworking devices require one network layer address per physical network connection for each network layer protocol supported. For example, a router with three interfaces each running AppleTalk, TCP/IP, and OSI must have three network layer addresses for each interface. The router therefore has nine network layer addresses.
一个网络地址和一个设备间的关系是逻辑的,不固定的；它或基于物理网络特性(其设备在一个具体网络段上),或基于非物理基础的归类(其设备是AppleTalk地带的一部分)。端设备需要一个网络层地址用于它们所支持的每一个网络层协议。(假定设备只连接一个物理网络。)路由器和其它互联网络联网设备,对于每一个它所支持的网络层协议的物理网络连接,需要一个网络层地址。例如,一个有三个接口的路由器,每一个都运行AppleTalk,TCP/IP,和 OSI,那么每一个接口必须有三个网络层地址,这样,路由器就有九个网络层地址。
不应该把高层的地址功能放到网络层地址上来实现。这已经违反了七层协议‘分层包含,分字段,功能独立'的原则。实际上，网络层分组包的头部已经设有‘协议’字段,就是为了把两者区分开来而设置的。于是每个接口只对应一个网络层地址
而不是三个，大大缩小了网络层的地址空间；而且两者功能各自独立,升级、修改和增添都大为方便。
Figure: Each Network Interface Must Be Assigned a Network Address for Each Protocol Supported illustrates how each network interface must be assigned a network address for each protocol supported.
图6.4.1举例说明了每一个网络接口是如何必须指定一个网络地址给所支持的每一种协议。
6.4.1 Figure: Each Network Interface Must Be Assigned a Network Address for Each Protocol Supported
每个网络接口必须赋予一个网络地址用于每个所支持的协议
6.5 Hierarchical Versus Flat Address Space
结构式地址空间对比扁平地址空间
Internetwork address space typically takes one of two forms: hierarchical address space or flat address space. A hierarchical address space is organized into numerous subgroups, each successively narrowing an address until it points to a single device (in a manner similar to street addresses). A flat address space is organized into a single group (in a manner similar to U.S. Social Security numbers).
互联网络地址空间通常取两种形式之一：分层结构式地址空间或扁平地址空间。一个结构式地址空间组织层许多(嵌套的)子组,它们逐级缩小地址范围,直至子组只指向单一设备(用一种类似于街道地址的方法)。一个扁平地址空间则组织成单一组(用一种类似于美国社会安全号码s)。
Hierarchical addressing offers certain advantages over flat-addressing schemes. Address sorting and recall is simplified using comparison operations. For example, "Ireland" in a street address eliminates any other country as a possible location.
结构式寻址提供某些比扁平寻址更好的优点。它可以使用比较操作,简化了分拣和记忆。例如,一个街道地址中的"爱尔兰岛",排除了任何其它国家再把它作为一个可能的重复位置。
Figure 6.5.1: Hierarchical and Flat Address Spaces Differ in Comparison Operations illustrates the difference between hierarchical and flat address spaces.
图6.5.1举例说明了结构式地址空间和扁平地址空间的不同。
6.5.1 Figure: Hierarchical and Flat Address Spaces Differ in Comparison Operations
6.6 Address Assignments 地址安排
Addresses are assigned to devices as one of two types: static and dynamic. Static addresses are assigned by a network administrator according to a preconceived internetwork addressing plan. A static address does not change until the network administrator manually changes it. Dynamic addresses are obtained by devices when they attach to a network, by means of some protocol-specific process. A device using a dynamic address often has a different address each time that it connects to the network. Some networks use a server to assign addresses. Server-assigned addresses are recycled for reuse as devices disconnect. A device is therefore likely to have a different address each time that it connects to the network.
安排给设备的地址可取两种类型之一：静态的和动态的。静态地址是通过网络管理员按照已拟互联网络寻址计划安排的。一个静态地址不会改变直至网络管理员手动改变它。动态地址则通过附加到网络上的一个设备根据某些协议的安排过程来获得的。使用动态地址的设备通常在每次连接网络时有一个不同的地址。有些网络使用一个服务器来安排地址。服务器安排的地址,当设备断开连接后,能回收再利用。因此,一个设备当它每次连接到网络时,可能有不同的地址。
6.7 Addresses Versus Names 地址对照名字
Internetwork devices usually have both a name and an address associated with them. Internetwork names typically are location-independent and remain associated with a device wherever that device moves (for example, from one building to another). Internetwork addresses usually are location-dependent and change when a device is moved (although MAC addresses are an exception to this rule). As with network addresses being mapped to MAC addresses, names are usually mapped to network addresses through some protocol. The Internet uses Domain Name System (DNS) to map the name of a device to its IP address. For example, it's easier for you to remember www.cisco.com instead of some IP address. Therefore, you type www.cisco.com into your browser when you want to access Cisco's web site. Your computer performs a DNS lookup of the IP address for Cisco's web server and then communicates with it using the network address.
互联网络设备通常既有一个名字,又有一个与之相关的地址。互联网络的名字通常是位置独立的,而且永随设备,不论设备移动到何处(例如,从一座建筑物到另一座)。互联网络地址则通常是位置有关的,当设备移动时是变化的(虽然MAC地址是这一规则的一个例外)。正如网络地址映射到MAC地址,名字通常通过某种协议映射到网络地址。互联网使用DNS(域名系统)来将一个设备的名字映射到它的IP地址上。例如,用www.cisco.com来代替某种它的IP地址就易于记忆。因此,当你要访问Cisco网站时,就输入www.cisco.com。计算机通过DNS查找相应IP地址,然后与之通信。
7. Flow Control Basics 流量控制基本原理
Flow control is a function that prevents network congestion by ensuring that transmitting devices do not overwhelm receiving devices with data. A high-speed computer, for example, may generate traffic faster than the network can transfer it, or faster than the destination device can receive and process it. The three commonly used methods for handling network congestion are buffering, transmitting source-quench messages, and windowing.
流量控制是一种功能,通过保证发送设备发送的数据不淹没接收设备,从而阻止网络拥塞。例如,一个高速计算机可以产生比网络传送更快的信流,或者快于目的设备能够接收和处理它的速度。三种通常用于处理网络拥挤的方法是缓冲,发送源收紧,,和变更窗口。
Buffering is used by network devices to temporarily store bursts of excess data in memory until they can be processed. Occasional data bursts are easily handled by buffering. Excess data bursts can exhaust memory, however, forcing the device to discard any additional datagrams that arrive.
缓冲是网络设备用来将过量的突发数据暂存于存储器中,直到它们能被处理。数据偶然突发是容易使用缓冲来处理的。然而过量的数据突发可以耗尽存储器迫使设备丢弃到达的任何额外的数据报。
Source-quench messages are used by receiving devices to help prevent their buffers from overflowing. The receiving device sends source-quench messages to request that the source reduce its current rate of data transmission. First, the receiving device begins discarding received data due to overflowing buffers. Second, the receiving device begins sending source-quench messages to the transmitting device at the rate of one message for each packet dropped. The source device receives the source-quench messages and lowers the data rate until it stops receiving the messages. Finally, the source device then gradually increases the data rate as long as no further source-quench requests are received.
收紧发送源的消息是接收设备用来通知发送源,以帮助阻止缓存器溢出。接收设备向发送源发出发出发送源收紧通知的消息,申请降低当前数据的发送速率。开始,接收设备由于缓存器溢出而丢弃数据,接着,接收设备对于每个被丢失的数据包以单消息的速率向发送设备发回收紧消息。发送源收到后就不断降低数据速率直至停止接收到消息s。之后,发生源逐步增加数据速率,只要收不到申请消息。
Windowing is a flow-control scheme in which the source device requires an acknowledgment from the destination after a certain number of packets have been transmitted. With a window size of 3, the source requires an acknowledgment after sending three packets, as follows. First, the source device sends three packets to the destination device. Then, after receiving the three packets, the destination device sends an acknowledgment to the source. The source receives the acknowledgment and sends three more packets. If the destination does not receive one or more of the packets for some reason, such as overflowing buffers, it does not receive enough packets to send an acknowledgment. The source then retransmits the packets at a reduced transmission rate.
收发窗是一种流量控制策略,其中源设备在发送了指定数(例如,三个)的数据包后就等待接收数据的目的设备返回认可消息,然后再发(三个)。如果发送源(在一定的时间间隔内)收不到认可,发送源就会降低发送速率重发。
8. Error-Checking Basics错误检验基本原理
Error-checking schemes determine whether transmitted data has become corrupt or otherwise damaged while traveling from the source to the destination. Error checking is implemented at several of the OSI layers.
错误检验策略确定被传送的数据,在传送过程中是否损毁或被损坏。错误检验可以在几个OSI层中实现。
One common error-checking scheme is the cyclic redundancy check (CRC), which detects and discards corrupted data. Error-correction functions (such as data retransmission) are left to higher-layer protocols. A CRC value is generated by a calculation that is performed at the source device. The destination device compares this value to its own calculation to determine whether errors occurred during transmission. First, the source device performs a predetermined set of calculations over the contents of the packet to be sent. Then, the source places the calculated value in the packet and sends the packet to the destination. The destination performs the same predetermined set of calculations over the contents of the packet and then compares its computed value with that contained in the packet. If the values are equal, the packet is considered valid. If the values are unequal, the packet contains errors and is discarded.
一个公共的错误检验策略是CRC(循环冗余校验)，它检测和丢弃被损坏的数据。错误纠正功能(如数据重发)则留给更高层的协议。CRC值是在源设备处通过计算产生。目的设备将此值与自己的计算值相比较,以确定在传输的过程中是否发生错误。······
9. Multiplexing Basics复用基本原理
Multiplexing is a process in which multiple data channels are combined into a single data or physical channel at the source. Multiplexing can be implemented at any of the OSI layers. Conversely, demultiplexing is the process of separating multiplexed data channels at the destination. One example of multiplexing is when data from multiple applications is multiplexed into a single lower-layer data packet.
复用是一种过程,将多个数据通道,在源端,组合到单一数据通道或物理通道中。复用可以在OSI任一层实现。解复用则是一个相反的过程,在目的端将复用的数据通道分解开来。复用的一个例子是来自多个应用的数据复用到低层单一数据分组包中。
Figure 9.1: Multiple Applications Can Be Multiplexed into a Single Lower-Layer Data Packet illustrates this example.
图9.1说明了这个例子。
9.1 Figure: Multiple Applications Can Be Multiplexed into a Single Lower-Layer Data Packet
多个应用可以复用到低层单一数据分组包
Another example of multiplexing is when data from multiple devices is combined into a single physical channel (using a device called a multiplexer).
复用的另一个例子是,来自多个设备的数据,组合到单一物理通道中(使用一个称为复用器的设备)。
Figure 9.2: Multiple Devices Can Be Multiplexed into a Single Physical Channel illustrates this example.
图9.2说明了这个例子。
9.2 Figure: Multiple Devices Can Be Multiplexed into a Single Physical Channel 多个设备可以复用到单一物理通道中
A multiplexer is a physical layer device that combines multiple data streams into one or more output channels at the source. Multiplexers demultiplex the channels into multiple data streams at the remote end and thus maximize the use of the bandwidth of the physical medium by enabling it to be shared by multiple traffic sources.
复用器是一种物理层设备,它在源端组合多个数据流到一个或更多的输出通道。在远程端的复用器解复用这些通道到多个数据流，于是,多个信流源共享同一介质,使之最大限度地使用物理介质的带宽。
Some methods used for multiplexing data are time-division multiplexing (TDM), asynchronous time-division multiplexing (ATDM), frequency-division multiplexing (FDM), and statistical multiplexing.
用于复用数据的一些方法是TDM(时分复用),ATDM(异步时分复用),FTDM(频分复用),和随机复用(码分复用)。
In TDM, information from each data channel is allocated bandwidth based on preassigned time slots, regardless of whether there is data to transmit. In ATDM, information from data channels is allocated bandwidth as needed by using dynamically assigned time slots. In FDM, information from each data channel is allocated bandwidth based on the signal frequency of the traffic. In statistical multiplexing, bandwidth is dynamically allocated to any data channels that have information to transmit.
在TDM中,来自每个数据通道的信息是基于预先指定时间槽来分配带宽的,不管有否数据发送。在ATDM中, 来自每个数据通道的信息是按需动态指定时间槽来分配带宽的。在FDM中, 来自每个数据通道的信息是基于信流的(载波)信号频率来分配带宽的。在随机复用中,带宽动态分配给有数据要发送的任何通道。
10. Standards Organizations标准s化的组织机构s
A wide variety of organizations contribute to internetworking standards by providing forums for discussion, turning informal discussion into formal specifications, and proliferating specifications after they are standardized.
各式各样的机构组织致力于互联网络组网的标准化。它们通过提供论坛来展开讨论,把非正式的讨论转化为正式的规范，并在标准化后推广和扩展规范。
Most standards organizations create formal standards by using specific processes: organizing ideas, discussing the approach, developing draft standards, voting on all or certain aspects of the standards, and then formally releasing the completed standard to the public.
大多数标准化组织是通过特别的程序产生标准的：规划概念,讨论方法,开发标准草案,就标准的全部或某方面进行投票表决,然后向公众发布完整的标准。
Some of the best-known standards organizations that contribute to internetworking standards include these:
一些著名的致力于互联网络联网标准的标准化组织有：
International Organization for Standardization (ISO)-ISO is an international standards organization responsible for a wide range of standards, including many that are relevant to networking. Its best-known contribution is the development of the OSI reference model and the OSI protocol suite.
ISO(国际标准化组织)---ISO是一个国际标准化组织，其职责在于各种范围很广的标准化,其中很多是与组网有关。她最著名的贡献是开发OSI参考模型,以及OSI协议栈。
American National Standards Institute (ANSI)-ANSI, which is also a member of the ISO, is the coordinating body for voluntary standards groups within the United States. ANSI developed the Fiber Distributed Data Interface (FDDI) and other communications standards.
ANSI(美国国家标准化协会)---ANSI也是ISO的一个成员,是一个美国国内义务标准化各团体的协调机构。ANSI开发了FDDI(光纤分布式数据接口)和其他通信标准。
Electronic Industries Association (EIA)-EIA specifies electrical transmission standards, including those used in networking. The EIA developed the widely used EIA/TIA-232 standard (formerly known as RS-232).
EIA(电子工业协会)---
Institute of Electrical and Electronic Engineers (IEEE)-IEEE is a professional organization that defines networking and other standards. The IEEE developed the widely used LAN standards IEEE 802.3 and IEEE 802.5.
International Telecommunication Union Telecommunication Standardization Sector (ITU-T)-Formerly called the Committee for International Telegraph and Telephone (CCITT), ITU-T is now an international organization that develops communication standards. The ITU-T developed X.25 and other communications standards.
ITU-T(国际电信联盟电信标准化部)---
Internet Activities Board (IAB)-IAB is a group of internetwork researchers who discuss issues pertinent to the Internet and set Internet policies through decisions and task forces. The IAB designates some Request For Comments (RFC) documents as Internet standards, including Transmission Control Protocol/Internet Protocol (TCP/IP) and the Simple Network Management Protocol (SNMP).
IAB(互联网活动委员会)---
11. Summary总结
This article introduced the building blocks on which internetworks are built. Under-standing where complex pieces of internetworks fit into the OSI model will help you understand the concepts better. Internetworks are complex systems that, when viewed as a whole, are too much to understand. Only by breaking the network down into the conceptual pieces can it be easily understood. As you read and experience internetworks, try to think of them in terms of OSI layers and conceptual pieces.
本文介绍了建立互联网联网的积木块。能够了解复杂的互联网络条块相当于OSI模型的何处,将有助于更好地理解概念。互联网络是复杂系统,如果看作为一个整体,要了解的实在是太多了。只有将网络分成概念的条块,可能就易于理解。当你在了解和体验互联网络时,试着从OSI层次和概念条块的角度来思考它们。
Understanding the interaction between various layers and protocols makes designing, configuring, and diagnosing internetworks possible. Without understanding of the building blocks, you cannot understand the interaction between them.
理解了各层和各协议间的相互作用就有可能实现设计,配置,和诊断互联网络。不理解积木块,就无法理解它们间的相互作用。
12. Review Questions复习问答
Q - What are the layers of the OSI model?
A - Application, presentation, session, transport, network, data link, physical. Remember the sentence "All people seem to need data processing."
Q - Which layer determines path selection in an internetwork?
A - Layer 3, the network layer.
Q - What types of things are defined at the physical layer?
A - Voltage levels, time of voltage changes, physical data rates, maximum transmission distances, physical connectors, and type of media.
Q - What is one method of mapping network addresses to MAC addresses?
A - ARP, Hello, predictable.
Q - Which includes more overhead, connection-oriented or connectionless services?
A - Connection-oriented.
13. For More Information 了解更多信息
Cisco's web site (www.cisco.com) is a wonderful source for more information about these topics. The Documentation section includes in-depth discussions on many of the topics covered in this article.
Teare, Diane. Designing Cisco Networks. Indianapolis: Cisco Press, July 1999.
Retrieved from "http://docwiki.cisco.com/wiki/Internetworking_Basics"
Category:IOS Technology Handbook
Retrieved from "http://docwiki.cisco.com/wiki/Internetworking_Basics "
Category :IOS Technology Handbook分类 ：IOS技术手册
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====================================================================
二． Introduction to LAN Protocols 介绍LAN协议
联网技术手册 指导目录
Contents
Part1 Internetworking Basics 互联网络联网基本原理
Part2 LAN Technologies 局域网技术
part3 WAN Technologies 广域网技术
part4 Internet Protocols 互联网协议
part5 Bridging and Switching 桥接和交换
part6 Routing 路由
part7 Network Management 网络管理
part8 Voice/Data Integration Technologies 声音/数据集成技术
part9 Wireless Technologies 无线技术
part10 Cable Access Technologies 有线接入技术
part11 Dial-up Technology 拨号技术
part12 Security Technologies 安全技术
part13 Quality of Service Networking 联网服务质量
part14 Network Caching Technologies 网络缓存技术
part15 IBM Network Management IBM网络管理
part16 Multiservice Access Technologies 多业务接入技术
http://docwiki.cisco.com/wiki/Introduction_to_LAN_Protocols
This page was last modified on 17 December 2009, at 21:54
二 ． Introduction to LAN Protocols 介绍LAN(局域网)协议
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This article introduces the various media-access methods, transmission methods, topologies, and devices used in a local-area network (LAN). Topics addressed focus on the methods and devices used in Ethernet/IEEE 802.3, Token Ring/IEEE 802.5, and Fiber Distributed Data Interface (FDDI). Subsequent articles in LAN Protocols (Part 2) address specific protocols in more detail.
本文介绍了用于局域网(LAN)的各种媒体访问方法,传送方法,拓扑,和设备。所讲的题目集中在用于以太网/IEEE 802.3，令牌环/IEEE 802.5，和FDDI(光纤分布式数据接口)。在手册第二部分LAN协议各篇文章中将更详细地讲述具体的协议。
Figure 1: Three LAN Implementations Are Used Most Commonly illustrates the basic layout of these three implementations.
图1 三种最普遍使用的LAN实现 举例说明了这三种实现的基本布置。
Contents
[hide]
1 Figure: Three LAN Implementations Are Used Most Commonly
图1.：三种最常见的局域网实现
2 What Is a LAN? 什么是局域网？
3 LAN Protocols and the OSI Reference Model 局域网协议和OSI参考模型
3.1 Figure: Popular LAN Protocols Mapped to the OSI Reference Model
图3.1：流行的局域网协议与OSI参考模型的映射关系
4 LAN Media-Access Methods 局域网介质访问方法
5 LAN Transmission Methods 局域网传送方法
6 LAN Topologies 局域网拓扑
6.1 Figure: Some Networks Implement a Local Bus Topology
图6.1：某些网络实现本地总线拓扑
6.2 Figure: Some Networks Implement a Logical Ring Topology
图6.2：某些网络实现本地逻辑环网拓扑
6.3 Figure: A Logical Tree Topology Can Contain Multiple Nodes
图6.3：一个逻辑树形拓扑可以包含多个节点
7 LAN Devices 局域网设备
7.1 Figure: A Repeater Connects Two Network Segments
图7.1：一个中继器连接两段网络段
7.2 Figure: Multiple LAN Extenders Can Connect to the Host Router Through a WAN 图7.2：多个局域网扩展器可以通过广域网连接到主机路由器
8 Review Questions 复习问答
9 For More Information 参阅更多信息
1. Figure: Three LAN Implementations Are Used Most Commonly
图1 三种最普遍使用的LAN实现
2. What Is a LAN? LAN是什么？
A LAN is a high-speed data network that covers a relatively small geographic area. It typically connects workstations, personal computers, printers, servers, and other devices. LANs offer computer users many advantages, including shared access to devices and applications, file exchange between connected users, and communication between users via electronic mail and other applications.
LAN是一个高速数据网络,它覆盖了相对较小的地理区域。它通常连接工作站s,PC,打印机,服务器,和其它设备。
对计算机用户而言,LANs有很多优点,包括共享所访问的设备s和应用s,已连接用户间的文件交换,以及用户间经由电子邮件和其它(包括多媒体)应用s的通信。
3. LAN Protocols and the OSI Reference Model
LAN协议s及其OSI参考模型
LAN protocols function at the lowest two layers of the OSI reference model, as discussed in article Internetworking Basics between the physical layer and the data link layer.
LAN协议是在OSI参考模型的最低两层上起作用,如在第一章'互联网络联网的基本原理'中所讨论的,在物理层和数据链路层之间。
Figure: Popular LAN Protocols Mapped to the OSI Reference Model illustrates how several popular LAN protocols map to the OSI reference model.
Figure: Popular LAN Protocols Mapped to the OSI Reference Model
4. LAN Media-Access Methods LAN介质访问方法
Media contention occurs when two or more network devices have data to send at the same time. Because multiple devices cannot talk on the network simultaneously, some type of method must be used to allow one device access to the network media at a time. This is done in two main ways: carrier sense multiple access collision detect (CSMA/CD) and token passing.
介质抢占发生在两个或更多的设备,要在同一时间发送数据情况下。由于多个设备不能在网络上同时交谈,必须使用某种方法以允许在同一时间内只有一个设备访问网络介质。这可以用两种主要的方法来完成: CSMA/CD(载波感知多重访问/冲突检出)和令牌传送。
In networks using CSMA/CD technology such as Ethernet, network devices contend for the network media. When a device has data to send, it first listens to see if any other device is currently using the network. If not, it starts sending its data. After finishing its transmission, it listens again to see if a collision occurred. A collision occurs when two devices send data simultaneously. When a collision happens, each device waits a random length of time before resending its data. In most cases, a collision will not occur again between the two devices. Because of this type of network contention, the busier a network becomes, the more collisions occur. This is why performance of Ethernet degrades rapidly as the number of devices on a single network increases.
在使用CSMA/CD技术,如以太网,的网络中,网络设备为网络介质而争夺。当一个设备有数据要发送,它就要侦听是否有其它设备当前在使用网络。如果没有,它就开始发送数据。在完成发送后,它再次侦听是否有竞争发生。当两个设备同时发送数据时,竞争就发生了。此时,每个设备在发送它的数据前先等待一段随机时间间隔。在大多数的情况下,两个设备间的竞争不会再发生。然而这类网络越忙碌,竞争就越多,这就是为什么以太网在单一网络中随着设备数量的增加而性能迅速退化。
In token-passing networks such as Token Ring and FDDI, a special network frame called a token is passed around the network from device to device. When a device has data to send, it must wait until it has the token and then sends its data. When the data transmission is complete, the token is released so that other devices may use the network media. The main advantage of token-passing networks is that they are deterministic. In other words, it is easy to calculate the maximum time that will pass before a device has the opportunity to send data. This explains the popularity of token-passing networks in some real-time environments such as factories, where machinery must be capable of communicating at a determinable interval.
在令牌传送网络,如令牌环和FDDI,一种特殊的帧叫令牌帧,围绕网络,从一个设备到另一个设备,被传递着。当一个设备有数据要传送,它必须等待令牌到来,然后发送数据,完成后,释放令牌,以便其它设备使用。令牌传递网络是确定性的。换句话说,在设备有机会发送数据前,它易于计算传递的最长时间。这就解释了令牌传递网络s在某些实时环境中为何流行,如在工厂里,装置必须能够在可确定的时间区间内进行通信。
For CSMA/CD networks, switches segment the network into multiple collision domains. This reduces the number of devices per network segment that must contend for the media. By creating smaller collision domains, the performance of a network can be increased significantly without requiring addressing changes.
网络交换机将网络分段成几个区段。
对于CSMA/CD,这就减少了一个区段中必须为使用媒体而竞争的设备数目。通过创建较小的竞争区段,一个网络的性能可以显著地增加而不需要改变寻址；(并可引进全双工)。
Normally CSMA/CD networks are half-duplex, meaning that while a device sends information, it cannot receive at the time. While that device is talking, it is incapable of also listening for other traffic. This is much like a walkie-talkie. When one person wants to talk, he presses the transmit button and begins speaking. While he is talking, no one else on the same frequency can talk. When the sending person is finished, he releases the transmit button and the frequency is available to others.
通常CSMA/CD是半双工的,即当一个设备发送信息时它无法同时接收信息。它在说话时也无法侦听其它信流。这很像步话机。
When switches are introduced, full-duplex operation is possible. Full-duplex works much like a telephone-you can listen as well as talk at the same time. When a network device is attached directly to the port of a network switch, the two devices may be capable of operating in full-duplex mode. In full-duplex mode, performance can be increased, but not quite as much as some like to claim. A 100-Mbps Ethernet segment is capable of transmitting 200 Mbps of data, but only 100 Mbps can travel in one direction at a time. Because most data connections are asymmetric (with more data traveling in one direction than the other), the gain is not as great as many claim. However, full-duplex operation does increase the throughput of most applications because the network media is no longer shared. Two devices on a full-duplex connection can send data as soon as it is ready.
当引进交换机以后,全双工就可能了。全双工十分像电话,可以同时听和说。当一个网络设备直接加到一个网络交换机接口时,两个设备就能运行在全双工模式下。尽管全双工能提高性能,但没有声称的那么多。···两个全双工连接的设备一旦准备好就能立刻发送。
Token-passing networks such as Token Ring can also benefit from network switches. In large networks, the delay between turns to transmit may be significant because the token is passed around the network.
令牌传递网络也能从网络交换机中得益。在大型网络中，轮转发送由于令牌围绕着网络传递而可能延迟显著。(交换机可显著降低令牌传递延迟。)
5. LAN Transmission Methods LAN发送方法
LAN data transmissions fall into three classifications: unicast, multicast, and broadcast. In each type of transmission, a single packet is sent to one or more nodes.
LAN数据发送分成三类: 单播,组播,和广播。在每类传送中,单个数据包被送到一个或更多节点。
In a unicast transmission, a single packet is sent from the source to a destination on a network. First, the source node addresses the packet by using the address of the destination node. The package is then sent onto the network, and finally, the network passes the packet to its destination.
单播传送是在一个网络中将单个数据包从信源送到信宿。首先,源节点使用目的节点地址作为数据包的地址；然后,数据包被送到网络上；最后,网络传递数据包到目的地。
A multicast transmission consists of a single data packet that is copied and sent to a specific subset of nodes on the network. First, the source node addresses the packet by using a multicast address. The packet is then sent into the network, which makes copies of the packet and sends a copy to each node that is part of the multicast address. 一个组播传送是将单一数据包复制并送往网络的一组指定节点组成的子集。首先,源节点使用组播地址作为数据包地址；然后,数据包被送往网络,实现数据包复制,并将拷贝送到属于组播地址范围内的每个节点上。
A broadcast transmission consists of a single data packet that is copied and sent to all nodes on the network. In these types of transmissions, the source node addresses the packet by using the broadcast address. The packet is then sent on to the network, which makes copies of the packet and sends a copy to every node on the network. 一个广播传送是将单一数据包复制并送到网络的所有节点上。源节点使用广播地址作为数据包地址,然后将包送向网络,实现数据包复制,并将拷贝送到网络的每个节点上。
6. LAN Topologies LAN拓扑
LAN topologies define the manner in which network devices are organized. Four common LAN topologies exist: bus, ring, star, and tree. These topologies are logical architectures, but the actual devices need not be physically organized in these configurations. Logical bus and ring topologies, for example, are commonly organized physically as a star. A bus topology is a linear LAN architecture in which transmissions from network stations propagate the length of the medium and are received by all other stations. Of the three most widely used LAN implementations, Ethernet/IEEE 802.3 networks-including 100BaseT-implement a bus topology, which is illustrated inFigure 6.1: Some Networks Implement a Local Bus Topology.
LAN拓扑定义了网络设备逻辑上如何组织起来的方法。这些拓扑是逻辑结构,实际的设备在物理上并不需要按照这些配置来组织。有四种LAN拓扑：总线,环形,星形,和树形。一个总线拓扑是一种线性LAN结构,从网络站发送,沿着介质的长度方向传播,并为所有其它站点所接收。三种最广泛使用的LAN实现之一,以太网/IEEE 802.3网络---包括100BaseT---实现了一种总线拓扑,它举例说明在下图6.1中。
Figure 6.1: Some Networks Implement a Local Bus Topology
A ring topology is a LAN architecture that consists of a series of devices connected to one another by unidirectional transmission links to form a single closed loop. Both Token Ring/IEEE 802.5 and FDDI networks implement a ring topology.
环形拓扑是一种LAN结构,它是由一串设备按单向传送链路彼此连接构成单个封闭回路。令牌环/IEEE 802.5和FDDI网络实现了一种环形拓扑。图2-4说明了一种逻辑环形拓扑。,,
Figure: Some Networks Implement a Logical Ring Topology depicts a logical ring topology.
Figure: Some Networks Implement a Logical Ring Topology
A star topology is a LAN architecture in which the endpoints on a network are connected to a common central hub, or switch, by dedicated links. Logical bus and ring topologies are often implemented physically in a star topology, which is illustrated in the following figure.
星形拓扑是一种LAN结构,其中网络的各端点都通过专有链路s连向一个公共的中心集线器,或交换机。逻辑总线拓扑和环形拓扑被用于星形拓扑的物理实现，图2-5作了举例说明。
A tree topology is a LAN architecture that is identical to the bus topology, except that branches with multiple nodes are possible in this case.
树形拓扑是一种与总线拓扑一样的拓扑,唯其可以带有多节点的分叉。图2-5举例说明了一种逻辑树形拓扑。
Figure: A Logical Tree Topology Can Contain Multiple Nodes illustrates a logical tree topology.
Figure: A Logical Tree Topology Can Contain Multiple Nodes
7. LAN Devices LAN设备
Devices commonly used in LANs include repeaters, hubs, LAN extenders, bridges, LAN switches, and routers.
通常用于LAN的设备包括中继器,集线器,LAN扩展器,网桥,交换机,和路由器。
Note:
Repeaters, hubs, and LAN extenders are discussed briefly in this section. The function and operation of bridges, switches, and routers are discussed generally in "Bridging and Switching Basics" and "Routing Basics."
中继器,集线器,和LAN扩展器在本节中作简单的讨论。网桥,交换机,和路由器在第四章,'桥接和交换的基本原理',作一般性的讨论。
A repeater is a physical layer device used to interconnect the media segments of an extended network. A repeater essentially enables a series of cable segments to be treated as a single cable. Repeaters receive signals from one network segment and amplify, retime, and retransmit those signals to another network segment. These actions prevent signal deterioration caused by long cable lengths and large numbers of connected devices. Repeaters are incapable of performing complex filtering and other traffic processing. In addition, all electrical signals, including electrical disturbances and other errors, are repeated and amplified. The total number of repeaters and network segments that can be connected is limited due to timing and other issues.
中继器是物理层的设备,用于一个扩展网络介质段的互连.中继器本质上使一连串缆线段处理成像一条缆线。中继器从一个网络段接收信号,放大,再定时,再发送这些信号到另一个网络段。通过这些作用,阻止了信号由于缆线太长和连接设备太多而引起的退化。中继器不可能执行复杂的过滤和其他的信流处理。另外,所有电气信号,包括电气干扰和其他错误,都被中继和放大。用中继器连接起来的网络段总数和中继器总数,由于时间同步引进其他方面的问题,是有限制的。
Figure: A Repeater Connects Two Network Segments illustrates a repeater connecting two network segments.
下图 举例说明了一个中继器连接了两段网络段。
Figure: A Repeater Connects Two Network Segments
A hub is a physical layer device that connects multiple user stations, each via a dedicated cable. Electrical interconnections are established inside the hub. Hubs are used to create a physical star network while maintaining the logical bus or ring configuration of the LAN. In some respects, a hub functions as a multiport repeater.
集线器是物理层设备,它连接着多个用户站,每一个都连上一条专用线缆。电气交换建立在集线器内部。集线器用于建立一个物理的星形网络,维持着LAN的逻辑总线或环网的配置。在某些方面,一个集线器的功能,就像是一个多口中继器。
A LAN extender is a remote-access multilayer switch that connects to a host router. LAN extenders forward traffic from all the standard network layer protocols (such as IP, IPX, and AppleTalk) and filter traffic based on the MAC address or network layer protocol type. LAN extenders scale well because the host router filters out unwanted broadcasts and multicasts. However, LAN extenders are not capable of segmenting traffic or creating security firewalls.
LAN扩展器是一个远端访问的多层交换机,它在逻辑上连接着一个主机路由器。LAN
扩展器根据所有标准网络层协议(如IP,IPX,和AppleTalk)转发信流,过滤出基于MAC地址或网络层协议类型的信流。LAN扩展器能很好地缩放,因为主机路由器能过滤出不需要的广播和组播。然而,不能分段信流或建立安全的防火墙。
Figure: Multiple LAN Extenders Can Connect to the Host Router Through a WAN illustrates multiple LAN extenders connected to the host router through a WAN.
下图 举例说明了通过一个WAN连接了主机路由器的多个LAN扩展器。
Figure: Multiple LAN Extenders Can Connect to the Host Router Through a WAN 多个LAN扩展器可以通过一个广域网连接到主机路由器
8. Review Questions 复习问答s
Q - Describe the type of media access used by Ethernet.
叙述以太网所使用的介质访问的类型。
A - Ethernet uses carrier sense multiple access collision detect (CSMA/CD). Each network station listens before and after transmitting data. If a collision is detected, both stations wait a random time before trying to resend.
以太网使用CSMA/CD。每个网站在发送数据前,后都在侦听。如果碰撞被检出,各发送网站就各自等待一段随机时间后再发送。
Q - Describe the type of media access used by Token Ring.
叙述令牌环网所使用的介质访问的类型。
A - Token Ring passes a special type of packet called a token around the network. If a network device has data to send, it must wait until it has the token to send it. After the data has been sent, the token is released back on the network.
令牌环网传递一个特殊类型的数据包,称为令牌,顺次在网络设备上环绕。如果一个网络设备有数据要发送,它必须等待,直到它有了送至该设备的令牌。数据发送后,令牌就被释放回网络。
Q - Describe unicast, multicast, and broadcast transmissions.
说明单播,组播,和广播传送。
A - A unicast is a transmission from one source to one destination. A multicast is a transmission from one source to many stations that register to receive the traffic. A broadcast is a transmission from one source to every station on the local network segment.
单播是一种从一个信源到信宿的传送。组播是一种从一个信源到许多注册要接收信流的站点的传送。广播是一种从一个信源到本地网段每个站点的传送。
9. For More Information了解更多的信息
Cisco's web site (www.cisco.com) is a wonderful source for more information about these topics. The Documentation section includes in-depth discussions on many of the topics covered in this article.
Cisco网站(www.cisco.com)是一个极好的,作为有关这些论题方面更多信息的来源的网站。其文档部分包含了在本章中所覆盖的许多论题的深入讨论。
Teare, Diane. Designing Cisco Networks. Indianapolis: Cisco Press, July 1999.
Retrieved from "http://docwiki.cisco.com/wiki/Introduction_to_LAN_Protocols"
Category:IOS Technology Handbook
====================================================================
三．Introduction to WAN Technologies介绍广域网技术
联网技术手册 指导目录
Contents
Part1 Internetworking Basics 互联网络联网基本原理
Part2 LAN Technologies局域网技术
part3 WAN Technologies 广域网技术
part4 Internet Protocols 互联网协议
part5 Bridging and Switching 桥接和交换
part6 Routing 路由
part7 Network Management 网络管理
part8 Voice/Data Integration Technologies 声音/数据集成技术
part9 Wireless Technologies 无线技术
part10 Cable Access Technologies 有线接入技术
part11 Dial-up Technology 拨号技术
part12 Security Technologies 安全技术
part13 Quality of Service Networking 联网服务质量
part14 Network Caching Technologies 网络缓存技术
part15 IBM Network Management IBM网络管理
part16 Multiservice Access Technologies 多业务接入技术
http://docwiki.cisco.com/wiki/Introduction_to_WAN_Technologies
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三．Introduction to WAN Technologies介绍WAN技术
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This article introduces the various protocols and technologies used in wide-area network (WAN) environments. Topics summarized here include point-to-point links, circuit switching, packet switching, virtual circuits, dialup services, and WAN devices. Chapters in Part 3, “WAN Protocols,” address specific technologies in more detail.
本章介绍了用于WAN(广域网)环境的各种协议和技术。在这里概述的题目包括点对点链路，电路交换，分组包交换，虚电路，拨号服务，和WAN设备。在手册第3部分,‘WAN协议’,的各章里将更详细地讲述专门技术。
Contents
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1 What Is a WAN? 什么是广域网？
1.1 Figure: WAN Technologies Operate at the Lowest Levels of the OSI Model 图1.1：广域网技术运行在OSI模型的最低三层
2 Point-to-Point Links 点对点链路
2.1 Figure: A Typical Point-to-Point Link Operates Through a WAN to a Remote Network
图2.1：一个通过广域网到远程网的,典型的点对点链路的运行
3 Circuit Switching 电路交换
3.1 Figure: A Circuit-Switched WAN Undergoes a Process Similar to That Used for a Telephone Call
图3.1：一个电路交换的广域网经历了类似于用于电话呼叫的过程
4 Packet Switching 分组交换
4.1 Figure: Packet Switching Transfers Packets Across a Carrier Network
图4.1：分组交换将数据包传输通越一个承载网络
5 WAN Virtual Circuits 广域网虚电路
6 WAN Dialup Services 广域网拨号电路
7 WAN Devices 广域网设备
7.1 WAN Switch 广域网交换机
7.1.1 Figure: Two Routers at Remote Ends of a WAN Can Be Connected by WAN Switches
图7.1.1广域网远程两端的两个路由器可以通过广域网交换机连接起来
7.2 Access Server 接入服务器
7.2.1 Figure: An Access Server Concentrates Dial-Out Connections into a WAN 图7.2.1：一个接入服务器将各拨出连接连到一个广域网内
7.3 Modem 调制解调器
7.3.1 Figure: A Modem Connection Through a WAN Handles Analog and Digital Signals
图7.3.1：一个通过广域网的调制解调器连接,处理模拟和数字信号
7.4 CSU/DSU通道服务单元/数字服务单元
7.4.1 Figure: The CSU/DSU Stands Between the Switch and the Terminal
图7.4.1：CSU/DSU置于交换机和终端之间
7.5 ISDN Terminal Adapter ISDN终端适配器
7.5.1 Figure: The Terminal Adapter Connects the ISDN Terminal Adapter to Other Interfaces
图7.5.1：终端适配器将ISDN终端适配器连接到其他接口
8 Review Questions复习问答
9 For More Information提供更多的信息
1. What Is a WAN? 什么是WAN(广域网)?
A WAN is a data communications network that covers a relatively broad geographic area and that often uses transmission facilities provided by common carriers, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer.Figure: WAN Technologies Operate at the Lowest Levels of the OSI Model illustrates the relationship between the common WAN technologies and the OSI model.
WAN(广域网)是一种数据通信网络,覆盖了一个相对广阔的地理区域,并通常使用由公共承运商,如电话公司,提供的传送设施。WAN技术通常在OSI参考模型的低三层起作用：物理层,数据链路层,和网络层。图1.1举例说明了常用WAN技术和OAI模型之间的关系。
（本章讲述了一大堆的术语和具体设备的名字，应该是互联网络联网长期进展的历史过程中形成的产物。似乎予人以堆积、少章、杂乱的感觉。既然基本原理是一种概念性的思路，可否这样来归纳一下：······）
1.1 Figure: WAN Technologies Operate at the Lowest Levels of the OSI Model
图1.1：广域网技术在OSI模型的最低三层上起作用
2. Point-to-Point Links 点对点链路
A point-to-point link provides a single, pre-established WAN communications path from the customer premises through a carrier network, such as a telephone company, to a remote network. Point-to-point lines are usually leased from a carrier and thus are often called leased lines. For a point-to-point line, the carrier allocates pairs of wire and facility hardware to your line only. These circuits are generally priced based on bandwidth required and distance between the two connected points. Point-to-point links are generally more expensive than shared services such as Frame Relay.Figure: A Typical Point-to-Point Link Operates Through a WAN to a Remote Network illustrates a typical point-to-point link through a WAN.
点对点链路提供了单一的预建的WAN通信通路,从用户附加的设备,通过一个承运商网络,如电话公司,到一个远端网络。点对点线路通常从一个承运商处租借而来,因而通常称为租借线路。对于一个租借线路,承运商只给你分配线对和设施硬件。这些电路,通常是基于所需要的带宽和两个连接点间的距离来计价。点对点链路s通常要比共享服务,如帧中继,更贵。
图2.1举例说明了一个典型的通过一个WAN的点对点链路。
2.1 Figure: A Typical Point-to-Point Link Operates Through a WAN to a Remote Network
3. Circuit Switching 电路交换
Switched circuits allow data connections that can be initiated when needed and terminated when communication is complete. This works much like a normal telephone line works for voice communication. Integrated Services Digital Network (ISDN) is a good example of circuit switching. When a router has data for a remote site, the switched circuit is initiated with the circuit number of the remote network. In the case of ISDN circuits, the device actually places a call to the telephone number of the remote ISDN circuit. When the two networks are connected and authenticated, they can transfer data. When the data transmission is complete, the call can be terminated.Figure: A Circuit-Switched WAN Undergoes a Process Similar to That Used for a Telephone Call illustrates an example of this type of circuit.
交换式电路允许数据连接在需要时启动(接通连接),通信完成后终止(断开连接)。这十分像用于声音通信的正常电话。ISDN(综合业务数字网)是电路交换的一个很好的例子。当一个路由器有用于远处的数据时,交换电路就被启动,并使用远方网络的电路号码。在ISDN电路的情况下,设备实际上对远方ISDN电路的电话号码安排一次呼叫。当两个网络连接并生效时,它们就可以传输数据。当数据传送完成了,呼叫就可以结束。
图3.1举例说明了这类电路的一个例子。
3.1 Figure: A Circuit-Switched WAN Undergoes a Process Similar to That Used for a Telephone Call
4. Packet Switching 分组包交换
Packet switching is a WAN technology in which users share common carrier resources. Because this allows the carrier to make more efficient use of its infrastructure, the cost to the customer is generally much better than with point-to-point lines. In a packet switching setup, networks have connections into the carrier's network, and many customers share the carrier's network. The carrier can then create virtual circuits between customers' sites by which packets of data are delivered from one to the other through the network. The section of the carrier's network that is shared is often referred to as a cloud.
分组包交换是一种WAN技术,在那里用户共享公共的承运商资源。由于这能使承运商更有效地使用其基础结构,用户所付出的代价会优于点对点线路。在分组交换建立中,网络已经连入承运商网络,并且许多用户共享了承运商的网络。承运商于是可以在用户所在地之间创建虚电路,从而分组包数据就可以通过网络从一处被交付到到另一处。承运商的被共享的那部分网络通常被称之为一片云。
Some examples of packet-switching networks include Asynchronous Transfer Mode (ATM), Frame Relay, Switched Multimegabit Data Services (SMDS), and X.25.Figure: Packet Switching Transfers Packets Across a Carrier Network shows an example packet-switched circuit. 分组交换网络的一些例子包括ATM(异步传输模式),FR(帧中继),SMDS(交换式多兆位数据服务),和X.25。图4.1示明了分组交换电路的一个例子。
4.1 Figure: Packet Switching Transfers Packets Across a Carrier Network图4.1 分组交换传输数据包通过承运商网络
5. WAN Virtual Circuits WAN虚电路
The virtual connections between customer sites are often referred to as a virtual circuit.
用户所在地之间的虚连接通常称之为一个虚电路。
A virtual circuit is a logical circuit created within a shared network between two network devices. Two types of virtual circuits exist: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).
一个虚电路是一个在共享的网络内两个网络设备之间产生的逻辑电路。有两类虚电路：SVC(交换式虚电路)和PVC(永久性虚电路)。
SVCs are virtual circuits that are dynamically established on demand and terminated when transmission is complete. Communication over an SVC consists of three phases: circuit establishment, data transfer, and circuit termination. The establishment phase involves creating the virtual circuit between the source and destination devices. Data transfer involves transmitting data between the devices over the virtual circuit, and the circuit termination phase involves tearing down the virtual circuit between the source and destination devices. SVCs are used in situations in which data transmission between devices is sporadic, largely because SVCs increase bandwidth used due to the circuit establishment and termination phases, but they decrease the cost associated with constant virtual circuit availability.
SVC是在需要时动态建立的,而在传送完成后被终止。在一个SVC上承载的通信由三个阶段组成：电路建立,数据传输,和电路终止。电路建立阶段涉及在源设备和目的设备之间创建虚电路。数据传输涉及在虚电路上的两个设备间传送数据,而电路终止阶段则涉及在源和目的设备间关闭虚电路。 SVCs用于设备间偶有数据传输的情况。
PVC is a permanently established virtual circuit that consists of one mode: data transfer. PVCs are used in situations in which data transfer between devices is constant. PVCs decrease the bandwidth use associated with the establishment and termination of virtual circuits, but they increase costs due to constant virtual circuit availability. PVCs are generally configured by the service provider when an order is placed for service.
PVC是一种永久建立的电路,它只有一种模式：数据传输模式,而且保持设备间进行传输的情况不变。PVCs减少了SVCs的使用带宽,却由于PVCs保持不变的可用性而增加了用户的代价。PVCs通常是由服务提供商根据用户要求服务的订单配置的。
6. WAN Dialup Services WAN拨号服务
Dialup services offer cost-effective methods for connectivity across WANs. Two popular dialup implementations are dial-on-demand routing (DDR) and dial backup. 拨号服务提供了一类用于WANs范围内有效连接计价的方法。拨号有两种流行的实现：DDR(需要时拨号路由)和拨号后备。
DDR is a technique whereby a router can dynamically initiate a call on a switched circuit when it needs to send data. In a DDR setup, the router is configured to initiate the call when certain criteria are met, such as a particular type of network traffic needing to be transmitted. When the connection is made, traffic passes over the line. The router configuration specifies an idle timer that tells the router to drop the connection when the circuit has remained idle for a certain period.
DDR是一种技术,当需要发送数据时,路由器可以动态地在一个交换电路上发起一个呼叫。在DDR建立期间,当某种条件满足时,例如需要发送一种具体类型的网络信流,就配置路由器,以发起呼叫。连接完成后,信流就在线路上传递。如果呼叫时线路空置,在超过指定时间后路由器就放弃连接。,
Dial backup is another way of configuring DDR. However, in dial backup, the switched circuit is used to provide backup service for another type of circuit, such as point-to-point or packet switching. The router is configured so that when a failure is detected on the primary circuit, the dial backup line is initiated. The dial backup line then supports the WAN connection until the primary circuit is restored. When this occurs, the dial backup connection is terminated.
拨号后备是另外一种配置DDR的方法,通常作为交换电路的后备,如点对点或分组交换。当发现交换电路失效时,路由器就启动拨号备用线路,以支持WAN连接,直至交换电路恢复。于是拨号后备连接终止。
7. WAN Devices WAN设备
WANs use numerous types of devices that are specific to WAN environments. WAN switches, access servers, modems, CSU/DSUs, and ISDN terminal adapters are discussed in the following sections. Other devices found in WAN environments that are used in WAN implementations include routers, ATM switches, and multiplexers.
有很多种用于WAN环境的设备。WAN交换机,接入服务器,调制解调器,CSU/DSU,和ISDN终端适配器，将在以下各节中讨论。其它WAN中用于WAN实现的设备,还包括路由器,ATM交换机,和复用器。
7.1 WAN Switch WAN交换机
A WAN switch is a multiport internetworking device used in carrier networks. These devices typically switch such traffic as Frame Relay, X.25, and SMDS, and operate at the data link layer of the OSI reference model.Figure: Two Routers at Remote Ends of a WAN Can Be Connected by WAN Switches illustrates two routers at remote ends of a WAN that are connected by WAN switches.
WAN交换机是一个用于承运商网络的多口网络互联设备。这些设备,通常用来交换如帧中继,X.25,和SMDS等的信流,它们运行在OSI参考模型的数据链路层上。图7.1.1举例说明了两个相距很远的WAN路由器用WAN交换机连接起来。
7.1.1 Figure: Two Routers at Remote Ends of a WAN Can Be Connected by WAN Switches
7.2 Access Server 接入服务器
An access server acts as a concentration point for dial-in and dial-out connections.Figure: An Access Server Concentrates Dial-Out Connections into a WAN illustrates an access server concentrating dial-out connections into a WAN.
一个接入服务器作为用于拨入拨出连接的一个集中点。图7.2.1举例说明了一个接入服务器是如何将拨出连接集中进入WAN的。
7.2.1 Figure: An Access Server Concentrates Dial-Out Connections into a WAN 图7.2.1 一个接入服务器将拨出连接集中进入WAN
7.3 Modem 调制解调器
A modem is a device that interprets digital and analog signals, enabling data to be transmitted over voice-grade telephone lines. At the source, digital signals are converted to a form suitable for transmission over analog communication facilities. At the destination, these analog signals are returned to their digital form.Figure: A Modem Connection Through a WAN Handles Analog and Digital Signals illustrates a simple modem-to-modem connection through a WAN.
调制解调器是一种设备,用来转换数字和模拟信号,使能在声音级电话线上传送数据。在源端,数字信号转换成适合于模拟通信设施上传送的形式;而在目的端,则将模拟信号转回数字信号。图7.3.1举例说明了一个调制解调器,通过一个WAN,连接到另一个调制解调器上。
7.3.1 Figure: A Modem Connection Through a WAN Handles Analog and Digital Signals
7.4 CSU/DSU
A channel service unit/digital service unit (CSU/DSU) is a digital-interface device used to connect a router to a digital circuit like a T1. The CSU/DSU also provides signal timing for communication between these devices.Figure: The CSU/DSU Stands Between the Switch and the Terminal illustrates the placement of the CSU/DSU in a WAN implementation.
CSU/DSU(通道服务单元/数字服务单元)是一种数字接口单元,用于路由器和数字电路,如T1,相连。CSU/DSU也提供信号同步用于设备间的通信。图7.4.1举例说明了在一个WAN实现中CSU/DSU的位置。
7.4.1 Figure: The CSU/DSU Stands Between the Switch and the Terminal
图7.4.1 在交换机和终端间CSU/DSU的位置
7.5 ISDN Terminal Adapter ISDN终端适配器
An ISDN terminal adapter is a device used to connect ISDN Basic Rate Interface (BRI) connections to other interfaces, such as EIA/TIA-232 on a router. A terminal adapter is essentially an ISDN modem, although it is called a terminal adapter because it does not actually convert analog to digital signals.Figure: The Terminal Adapter Connects the ISDN Terminal Adapter to Other Interfaces illustrates the placement of the terminal adapter in an ISDN environment.
ISDN终端适配器是一种用于将ISDN的BRI(基本速率接口)连接到其它的接口上,如路由器上的EIA/TIA-232。一个终端适配器本质上是一个ISDN调制解调器,但它不属于A/D-D/A的那一种。图7.5.1举例说明了ISDN环境中的终端适配器的位置。
7.5.1 Figure: The Terminal Adapter Connects the ISDN Terminal Adapter to Other Interfaces 图7.5.1 终端适配器用于将ISDN终端适配器的BRI(基本速率接口)连接到其他接口上
8. Review Questions 复习问答
Q - What are some types of WAN circuits?
WAN电路有哪些类型？
A - Point-to-point, packet-switched, and circuit-switched.
点对点,分组交换和电路交换。
Q - What is DDR, and how is it different from dial backup?
什么是DDR?它与拨号备用有何不同？
A - DDR is dial-on-demand routing. DDR dials up the remote site when traffic needs to be transmitted. Dial backup uses the same type of services, but for backup to a primary circuit. When the primary circuit fails, the dial backup line is initiated until the primary circuit is restored. 当信流需要传送时,DDR是按拨号需要路由。拨号是主电路的一种后备,它使用被备用的同一类服务。当主电路失效时,拨号备用线路启动,直到主电路恢复。
Q - What is a CSU/DSU used for?
CSU/DSU的作用是什么？
A - A CSU/DSU interfaces a router with a digital line such as a T1.
CSU/DSU接口s是带一条数字线路的路由器,如T1.
Q - What is the difference between a modem and an ISDN terminal adapter?
调制解调器与ISDN终端适配器有何不同?
A - A modem converts digital signals into analog for transmission over a telephone line. Because ISDN circuits are digital, the conversion from digital to analog is not required.
调制解调器将数字信号转换为模拟信号,以便在电话线路上进行传送。而ISDN电路是数字的,因而没有必要进行数模转换。
9. For More Information 了解更多信息
Mahler, Kevin. CCNA Training Guide. Indianapolis: New Riders, 1999.
Cisco IOS Dial Solutions. Indianapolis: Cisco Press, 1998.
Cisco IOS Wide Area Networking Solutions. Indianapolis: Cisco Press, 1999.
Retrieved from "http://docwiki.cisco.com/wiki/Introduction_to_WAN_Technologies"
Category:IOS Technology Handbook
=================================================================
四．Bridging and Switching Basics 桥接和交换的基本原理
Contents联网技术手册 指导目录
Part1 Internetworking Basics 互联网络联网基本原理
Part2 LAN Technologies 局域网技术
part3 WAN Technologies广域网技术
part4 Internet Protocols 互联网协议
part5 Bridging and Switching 桥接和交换
part6 Routing 路由
part7 Network Management 网络管理
part8 Voice/Data Integration Technologies 声音/数据集成技术
part9 Wireless Technologies 无线技术
part10 Cable Access Technologies 有线接入技术
part11 Dial-up Technology 拨号技术
part12 Security Technologies 安全技术
part13 Quality of Service Networking 联网服务质量
part14 Network Caching Technologies 网络缓存技术
part15 IBM Network Management IBM网络管理
part16 Multiservice Access Technologies 多业务接入技术
http://docwiki.cisco.com/wiki/Bridging_and_Switching_Basics
This page was last modified on 17 December 2009, at 21:50
四．Bridging and Switching Basics 桥接和交换的基本原理
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This article introduces the technologies employed in devices loosely referred to as bridges and switches. Topics summarized here include general link layer device operations, local and remote bridging, ATM switching, and LAN switching. Chapters in Part 5, “Bridging and Switching,” address specific technologies in more detail.
本章介绍用于,不严格地称为桥和交换机的设备,的技术。综述在这里的论题包括一般链路层设备的操作，本地和远程的桥接，ATM交换，和LAN交换。在手册第5部分"桥接和交换"的各章中将更详细地讲述具体技术。
Contents
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1 What Are Bridges and Switches? 什么是网桥和交换机？
2 Link Layer Device Overview 链路层设备概述
3 Types of Bridges 网桥的类型
3.1 Figure: Local and Remote Bridges Connect LAN Segments in Specific Areas 图3.1 本地和远程网桥在指定区连接局域网段
3.2 Figure: A MAC-Layer Bridge Connects the IEEE 802.3 and IEEE 802.5 Networks 图3.2 MAV层网桥连接IEEE 802.3和IEEE 802.5网络
4 Types of Switches 交换机的类型
4.1 ATM Switch ATM交换机
4.1.1 Figure: Multi-LAN Networks Can Use an ATM-Based Backbone When Switching Cells
图4.1.1 多局域网交换信元时可以使用基于ATM的骨干网
4.2 LAN Switch 局域网交换机
4.2.1 Figure: A LAN Switch Can Link 10-Mbps and 100-Mbps Ethernet Segments
图4.2.1 局域网交换机可以连接10-Mbps和100-Mbps以太网段
5 Review Questions 复习问答
6 For More Information 参阅更多信息
1. What Are Bridges and Switches? 什么是桥和交换机?
Bridges and switches are data communications devices that operate principally at Layer 2 of the OSI reference model. As such, they are widely referred to as data link layer devices.
桥和交换机都是通信设备,它们主要是在OSI参考模型的第二层上操作,所以它们被广泛地称之为数据链路层设备。
Bridges became commercially available in the early 1980s. At the time of their introduction, bridges connected and enabled packet forwarding between homogeneous networks. More recently, bridging between different networks has also been defined and standardized.
商业上使用桥始于1980年初。当时,桥用来在同质的网络间连接并实现数据包转发。最近,在不同网络间的桥接,已经被定义和标准化了。
Several kinds of bridging have proven important as internetworking devices. Transparent bridging is found primarily in Ethernet environments, while source-route bridging occurs primarily in Token Ring environments. Translational bridging provides translation between the formats and transit principles of different media types (usually Ethernet and Token Ring). Finally, source-route transparent bridging combines the algorithms of transparent bridging and source-route bridging to enable communication in mixed Ethernet/Token Ring environments.
有几种桥接已经被证实很重要,就像互联网络联网设备那样。透明的桥接主要在以太网环境中能找到,而源路由桥接则主要发生在令牌环网的环境中。透明的桥接提供格式间的转换和不同媒体类型(通常是以太网和令牌环网)的转送规则。源路由透明桥接则是组合了上述两种桥接的算法,以实现在以太网/令牌环网的混合环境中进行通信。
Today, switching technology has emerged as the evolutionary heir to bridging-based internetworking solutions. Switching implementations now dominate applications in which bridging technologies were implemented in prior network designs. Superior throughput performance, higher port density, lower per-port cost, and greater flexibility have contributed to the emergence of switches as replacement technology for bridges and as complements to routing technology.
今天,交换技术已经显露成为基于桥接的互联网络联网解案的革命性的继承技术。交换实现现在主要的应用是在那些以前的网络设计中曾用桥接技术实现的地方。超级的吞吐性能,较高的接口密度,更低的单口代价,和更大的灵活性等,由于交换机s作为桥接的替代技术和路由技术的补充而出现了。
2. Link Layer Device Overview 链路层设备概述
Bridging and switching occur at the link layer, which controls data flow, handles transmission errors, provides physical (as opposed to logical) addressing, and manages access to the physical medium. Bridges provide these functions by using various link layer protocols that dictate specific flow control, error handling, addressing, and media-access algorithms. Examples of popular link layer protocols include Ethernet, Token Ring, and FDDI.
桥接和交换发生在链路层。链路层控制数据流,处理传送错误,提供物理寻址,以及管理对物理介质的访问。桥提供这些功能是通过使用各种链路层协议来实现的。协议安排特殊的流量控制,错误处理,寻址,和媒体访问算法。流行的链路层协议的例子包括以太网,令牌环网,和FDDI。,
Bridges and switches are not complicated devices. They analyze incoming frames, make forwarding decisions based on information contained in the frames, and forward the frames toward the destination. In some cases, such as source-route bridging, the entire path to the destination is contained in each frame. In other cases, such as transparent bridging, frames are forwarded one hop at a time toward the destination.
桥和交换机并非复杂设备。它们解析收入的帧,根据包含在帧中的信息,作出转发决策,并将帧向目的地转发。在某些情况下,如源路由桥接,到目的地的整个通路都包含在每个帧中。在其他情况下,如透明桥接,帧是向着目的地,一次转发一跳。
Upper-layer protocol transparency is a primary advantage of both bridging and switching. Because both device types operate at the link layer, they are not required to examine upper-layer information. This means that they can rapidly forward traffic representing any network layer protocol. It is not uncommon for a bridge to move AppleTalk, DECnet, TCP/IP, XNS, and other traffic between two or more networks. 对上层协议的透明性是桥接和交换的主要优点。由于两种类型的设备都操作在链路层,它们并不需要检查上层信息。这意味着它们可以迅速转发表达任何网络层协议的信流。对于一个桥而言,在两个或更多的网络间移动AppleTalk, DECnet, TCP/IP, XNS, 和其它信流并不罕见。
Bridges are capable of filtering frames based on any Layer 2 fields. For example, a bridge can be programmed to reject (not forward) all frames sourced from a particular network. Because link layer information often includes a reference to an upper-layer protocol, bridges usually can filter on this parameter. Furthermore, filters can be helpful in dealing with unnecessary broadcast and multicast packets.
桥有能力过滤基于任何层2字段的帧。例如,一个桥可以编程以拒绝(不转发)源自一个具体网络的所有帧。由于链路层信息通常包括一个对一个上层协议的引用,桥常可过滤该参数。此外,过滤器可以帮助处理不必要广播和组播数据包。
By dividing large networks into self-contained units, bridges and switches provide several advantages. Because only a certain percentage of traffic is forwarded, a bridge or switch diminishes the traffic experienced by devices on all connected segments. The bridge or switch will act as a firewall for some potentially damaging network errors and will accommodate communication between a larger number of devices than would be supported on any single LAN connected to the bridge. Bridges and switches extend the effective length of a LAN, permitting the attachment of distant stations that was not previously permitted.
通过把大网络划分成自含单元,桥和交换机就能提供若干优点。因为只有部分信流被转发,一个桥或交换机就减少了在所有连接的网段上的设备不必要再历经信流。桥和交换机可以作为防火墙用于防止某些潜在地损坏网络的错误,容许大量设备间的通信,而不必对连接到桥的单个LAN,一个一个地支持。桥和交换机扩展了LAN的有效长度,允许附加以前不允许的远处站点。
Although bridges and switches share most relevant attributes, several distinctions differentiate these technologies. Bridges are generally used to segment a LAN into a couple of smaller segments. Switches are generally used to segment a large LAN into many smaller segments. Bridges generally have only a few ports for LAN connectivity, whereas switches generally have many. Small switches such as the Cisco Catalyst 2924XL have 24 ports capable of creating 24 different network segments for a LAN. Larger switches such as the Cisco Catalyst 6500 can have hundreds of ports. Switches can also be used to connect LANs with different media-for example, a 10-Mbps Ethernet LAN and a 100-Mbps Ethernet LAN can be connected using a switch. Some switches support cut-through switching, which reduces latency and delays in the network, while bridges support only store-and-forward traffic switching. Finally, switches reduce collisions on network segments because they provide dedicated bandwidth to each network segment.
虽然桥和交换机大多数有关的属性,若干特帧区分了这些技术。桥通常用于将LAN分段成几个较小段。桥一般只有几个端口用于LAN连接,而交换机通常有很多。小交换机如2924XL有24端口,对一个LAN有能力产生24段不同的网络段。大交换机如6500可有百数端口。交换机也可以用于将LAN与不同的媒体相连接---例如,一个10Mbps和一个100Mbps以太网LANs可以使用一个交换机连接起来。某些交换机支持交换切断,减少了网络中的潜伏和延迟,而桥只支持存储-和-转发信流交换。最后,交换机减少了网络段上的碰撞,因为它们提供了专用的带宽给每个网络段。
3. Types of Bridges 桥的种类
Bridges can be grouped into categories based on various product characteristics. Using one popular classification scheme, bridges are either local or remote. Local bridges provide a direct connection between multiple LAN segments in the same area. Remote bridges connect multiple LAN segments in different areas, usually over telecommunications lines.
桥可以根据各种产品的特性组成类别。使用一个流行的分类方案,桥可以分成本地的或远处的。本地桥s在同一域的多个LAN段之间提供一个直接连接。远处桥在不同域中,通常在电信线路上,连接多个LAN段。图4-1举例说明了这两种配置。
Figure: Local and Remote Bridges Connect LAN Segments in Specific Areas illustrates these two configurations.
Figure: Local and Remote Bridges Connect LAN Segments in Specific Areas
Remote bridging presents several unique internetworking challenges, one of which is the difference between LAN and WAN speeds. Although several fast WAN technologies now are establishing a presence in geographically dispersed internetworks, LAN speeds are often much faster than WAN speeds. Vast differences in LAN and WAN speeds can prevent users from running delay-sensitive LAN applications over the WAN.
远程桥接存在若干少见的互联网络联网的挑战,其中之一是LAN和WAN间速度不同。虽然在地理上分布的互联网络s中,若干快速WAN技术现在正在出现,然而LAN的速度通常总是十分快于WAN速度。LAN与WAN在速度上的巨大差别可能阻止用户在WAN上运行LAN的应用。
Remote bridges cannot improve WAN speeds, but they can compensate for speed discrepancies through a sufficient buffering capability. If a LAN device capable of a 3-Mbps transmission rate wants to communicate with a device on a remote LAN, the local bridge must regulate the 3-Mbps data stream so that it does not overwhelm the 64-kbps serial link. This is done by storing the incoming data in onboard buffers and sending it over the serial link at a rate that the serial link can accommodate. This buffering can be achieved only for short bursts of data that do not overwhelm the bridge's buffering capability.
远程桥不能改善WAN的速度,然而它们通过其足够的缓冲能力来补偿速度上的(巨大)差别。例如,,,
The Institute of Electrical and Electronic Engineers (IEEE) differentiates the OSI link layer into two separate sublayers: the Media Access Control (MAC) sublayer and the Logical Link Control (LLC) sublayer. The MAC sublayer permits and orchestrates media access, such as contention and token passing, while the LLC sublayer deals with framing, flow control, error control, and MAC sublayer addressing.
IEEE将OSI链路层分成两个子层：MAC(媒体访问控制)子层和LLC子层(逻辑链路控制)。MAC子层允许和安排媒体访问,如竞争和令牌传递,而LLC子层处理成帧,流量控制,错误控制,和MAC子层寻址。
Some bridges are MAC-layer bridges, which bridge between homogeneous networks (for example, IEEE 802.3 and IEEE 802.3), while other bridges can translate between different link layer protocols (for example, IEEE 802.3 and IEEE 802.5).
某些桥是MAC子层桥,处于同质网络之间(例如IEEE 802.3和IEEE 802.3),而其它桥则处在不同链路层协议之间,(例如,IEEE 802.3和IEEE 802.5),桥中可以实现转换。这样的一种转换的基本机制图示在图4.2中。
The basic mechanics of such a translation are shown inFigure: A MAC-Layer Bridge Connects the IEEE 802.3 and IEEE 802.5 Networks.
Figure: A MAC-Layer Bridge Connects the IEEE 802.3 and IEEE 802.5 Networks
The above figure illustrates an IEEE 802.3 host (Host A) formulating a packet that contains application information and encapsulating the packet in an IEEE 802.3-compatible frame for transit over the IEEE 802.3 medium to the bridge. At the bridge, the frame is stripped of its IEEE 802.3 header at the MAC sublayer of the link layer and is subsequently passed up to the LLC sublayer for further processing. After this processing, the packet is passed back down to an IEEE 802.5 implementation, which encapsulates the packet in an IEEE 802.5 header for transmission on the IEEE 802.5 network to the IEEE 802.5 host (Host B).
图4-2举例说明了一个连接主机sIEEE 802.3和IEEE 802.5的MAC层桥的工作过程。主机(主机A)将包含应用信息的数据包进行安排,用IEEE 802.3兼容的帧格式包装数据包,然后经由IEEE 802.3介质送至桥处。在桥的MAC子层,帧
IEEE 802.3头部被剥离,并上传到LLC子层作进一步处理。处理后,数据包回传到IEEE 802.5实现,封装IEEE 802.5头部,出桥,再经802.5介质,传送到802.5主机B。
A bridge's translation between networks of different types is never perfect because one network likely will support certain frame fields and protocol functions not supported by the other network.
桥在不同类型网络间的转换是不会完美的,因为一个网络支持某种帧字段s和协议功能s,但不一定为其它网络所支持。
4. Types of Switches 交换机s的种类
Switches are data link layer devices that, like bridges, enable multiple physical LAN segments to be interconnected into a single larger network. Similar to bridges, switches forward and flood traffic based on MAC addresses. Any network device will create some latency. Switches can use different forwarding techniques-two of these are store-and-forward switching and cut-through switching.
交换机s是数据链路层设备s,像桥s那样,能使多个物理LAN段连成单一大网络。也类似于桥s,能按照MAC地址s转发信流,也会淹没信流。任何网络设备都会产生某些潜伏。交换机s可以使用不同的转发技术---其中的两个是转发交换和穿越交换。
In store-and-forward switching, an entire frame must be received before it is forwarded. This means that the latency through the switch is relative to the frame size-the larger the frame size, the longer the delay through the switch. Cut-through switching allows the switch to begin forwarding the frame when enough of the frame is received to make a forwarding decision. This reduces the latency through the switch. Store-and-forward switching gives the switch the opportunity to evaluate the frame for errors before forwarding it. This capability to not forward frames containing errors is one of the advantages of switches over hubs. Cut-through switching does not offer this advantage, so the switch might forward frames containing errors. Many types of switches exist, including ATM switches, LAN switches, and various types of WAN switches.
在存储--转发交换中,必须接收到完整的一帧才能转发。这意味着通过交换机的潜伏时间,帧越大,延迟就越长。穿越交换则允许交换机在接收到部分的,但已达到指定大小的帧时,就按转发决策转发。这样就减少了通过交换机的潜伏时间。
存储---转发交换使交换机有机会在转发前检查帧的错误。比较集线器s,这是交换机s的优点之一。,,, 穿越交换则不提供这个优点,所以交换机可能转发包含错误的帧s。有很多种类的交换机s,包括ATM交换机s,LAN交换机s,以及各种WAN交换机s。
ATM Switch ATM交换机
Asynchronous Transfer Mode (ATM) switches provide high-speed switching and scalable bandwidths in the workgroup, the enterprise network backbone, and the wide area. ATM switches support voice, video, and data applications, and are designed to switch fixed-size information units called cells, which are used in ATM communications. ATM(异步传输模式)交换机s提供高速交换和可缩放带宽予工作组,企业网络骨干,和广域网。ATM交换机s支持声音,视频,和数据应用s。ATM设计了固定尺寸的信息单元,叫做信元,用于ATM通信s中作为交换的基本单元。图4-3举例说明了一个企业网络,由多个LAN互联,通越一个ATM骨干网组成。
Figure: Multi-LAN Networks Can Use an ATM-Based Backbone When Switching Cells illustrates an enterprise network comprised of multiple LANs interconnected across an ATM backbone.
Figure: Multi-LAN Networks Can Use an ATM-Based Backbone When Switching Cells
LAN Switch LAN交换机
LAN switches are used to interconnect multiple LAN segments. LAN switching provides dedicated, collision-free communication between network devices, with support for multiple simultaneous conversations. LAN switches are designed to switch data frames at high speeds.
LAN交换机s用于互联多个LAN段。LAN交换提供网络设备间中专用的无碰撞通信,支持多个同时对话。LAN交换机s设计用来以高速度交换数据帧s。图4-4举例说明了一个简单的网络,其中,一个LAN交换机将10Mbps的LAN和
100Mbps的LAN互联起来。
Figure: A LAN Switch Can Link 10-Mbps and 100-Mbps Ethernet Segments illustrates a simple network in which a LAN switch interconnects a 10-Mbps and a 100-Mbps Ethernet LAN.
Figure: A LAN Switch Can Link 10-Mbps and 100-Mbps Ethernet Segments
5. Review Questions 复习问答
Q - What layer of the OSI reference model to bridges and switches operate.
OSI参考模型那一层操作桥s和交换机s?
A - Bridges and switches are data communications devices that operate principally at Layer 2 of the OSI reference model. As such, they are widely referred to as data link-layer devices.
桥s和交换机s是数据通信设备。主要操作在OSI参考模型的第二层,因此,它们被称之为链路层设备s。,
Q - What is controlled at the link layer?
在链路层上受控制的是什么？
A - Bridging and switching occur at the link layer, which controls data flow, handles transmission errors, provides physical (as opposed to logical) addressing, and manages access to the physical medium.
桥接和交换发生在链路层,该层控制数据流,处理传送错误,提供物理(而不是逻辑)寻址,以及管理物理介质。
Q - Under one popular classification scheme what are bridges classified as?
在一个流行分类策略下对桥s如何分类?
A - Local or Remote: Local bridges provide a direct connection between multiple LAN segments in the same area. Remote bridges connect multiple LAN segments in different areas, usually over telecommunications lines.
本地和远程：本地桥s提供在同一域的多个LAN段间的一个直接连接。远程桥s在不同域,通常在电信线路s上,连接多个LAN段。
Q - What is a switch?
交换机是什么?
A - Switches are data link-layer devices that, like bridges, enable multiple physical LAN segments to be interconnected into a single larger network.
交换机s是数据链路层设备s,像桥s那样,能将多个网络LAN段连成单一较大的网络。
6. For More Information 了解更多信息
Cisco's web site (www.cisco.com) is a wonderful source for more information about these topics. The Documentation section includes in-depth discussions on many of the topics covered in this article.
Cisco的网站是了解更多信息的极好来源，其文档部分包含了许多深度的讨论，其中不少文章覆盖了本文中的论题。
Clark, Kennedy, and Kevin Hamilton. Cisco LAN Switching. Indianapolis: Cisco Press, August 1999.
Cisco Systems, Inc. Cisco IOS Bridging and IBM Network Solutions. Indianapolis: Cisco Press, June 1998.
Retrieved from "http://docwiki.cisco.com/wiki/Bridging_and_Switching_Basics"
Category:IOS Technology Handbook
===================================================================
五．Routing Basics 路由基本原理
指导目录
Contents联网技术手册 指导目录
Part1 Internetworking Basics 互联网络联网基本原理
Part2 LAN Technologies 局域网技术
part3 WAN Technologies 广域网技术
part4 Internet Protocols 互联网协议
part5 Bridging and Switching 桥接和交换
part6 Routing 路由
part7 Network Management 网络管理
part8 Voice/Data Integration Technologies 声音/数据集成技术
part9 Wireless Technologies 无线技术
part10 Cable Access Technologies 有线接入技术
part11 Dial-up Technology 拨号技术
part12 Security Technologies 安全技术
part13 Quality of Service Networking 联网服务质量
part14 Network Caching Technologies 网络缓存技术
part15 IBM Network Management IBM网络管理
part16 Multiservice Access Technologies 多业务接入技术
http://docwiki.cisco.com/wiki/Routing_Basics
This page was last modified on 17 December 2009, at 21:56
五．Routing Basics 路由基本原理
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This article introduces the underlying concepts widely used in routing protocols. Topics summarized here include routing protocol components and algorithms. In addition, the role of routing protocols is briefly contrasted with the role of routed or network protocols.
本章介绍广泛用于路由协议的基本概念。在这里概述的论题包括路由协议的组成部分和算法。另外,将路由协议的作用与被路由或网络协议的作用作一简短的对比。在第VII部分的章节"路由协议s,"里,会更详细地讲述特殊的路由协议s,而使用路由协议s的网络协议,是在第VI部分,"网络协议s",中讨论。
Contents
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1 What Is Routing?
2 Routing Components
2.1 Path Determination
2.1.1 Figure: Destination/Next Hop Associations Determine the Data’s Optimal Path
2.2 Switching
2.2.1 Figure: Numerous Routers May Come into Play During the Switching Process
3 Routing Algorithms
3.1 Design Goals
3.1.1 Figure: Slow Convergence and Routing Loops Can Hinder Progress
3.2 Algorithm Types
3.2.1 Static Versus Dynamic
3.2.2 Single-Path Versus Multipath
3.2.3 Flat Versus Hierarchical
3.2.4 Host-Intelligent Versus Router-Intelligent
3.2.5 Intradomain Versus Interdomain
3.2.6 Link-State Versus Distance Vector
3.2.7 Routing Metrics
4 Network Protocols
5 Review Questions
1. What Is Routing? 什么是路由？
Routing is the act of moving information across an internetwork from a source to a destination. Along the way, at least one intermediate node typically is encountered. Routing is often contrasted with bridging, which might seem to accomplish precisely the same thing to the casual observer. The primary difference between the two is that bridging occurs at Layer 2 (the link layer) of the OSI reference model, whereas routing occurs at Layer 3 (the network layer). This distinction provides routing and bridging with different information to use in the process of moving information from source to destination, so the two functions accomplish their tasks in different ways. 路由就是一种作用,将信息从源经过互联网络移动到目的地。沿着这条通路,通常会遇到至少一个中间节点。路由通常与桥接相对照,在陌生的旁观者看来,它们精确地完成同一件事情。两者主要的不同在于,桥接发生在OSI参考模型第二层(链路层),而路由则发生在第三层(网络层)。这种不同提供给路由和桥接用不同的信息,来处理从源到目的信息移动的过程，所以这两种功能(路由和桥接)是用不同的方法来完成它们的任务的。
The topic of routing has been covered in computer science literature for more than two decades, but routing achieved commercial popularity as late as the mid-1980s. The primary reason for this time lag is that networks in the 1970s were simple, homogeneous environments. Only relatively recently has large-scale internetworking become popular.
路由这个论题,出现在计算机科学文献里,已经有多于20年的时间,然而路由实现在商业上流行,则是迟至80年代中期。其原因在于70年代的网络s还是简单,同质的环境s。大规模互联网络联网的流行出现较晚。
2. Routing Components 路由的组成部分
Routing involves two basic activities: determining optimal routing paths and transporting information groups (typically called packets) through an internetwork. In the context of the routing process, the latter of these is referred to as packet switching. Although packet switching is relatively straightforward, path determination can be very complex.
路由涉及两种活动:确定优化路由通路s,和传输通过互联网络的信息组s(通常叫数据包s)。,在路由过程的上下文中,后一活动称为包交换。虽然包交换相对直观,通路确定却可能十分复杂。
Path Determination 通路确定
Routing protocols use metrics to evaluate what path will be the best for a packet to travel. A metric is a standard of measurement, such as path bandwidth, that is used by routing algorithms to determine the optimal path to a destination. To aid the process of path determination, routing algorithms initialize and maintain routing tables, which contain route information. Route information varies depending on the routing algorithm used.
路由协议使用度量来评价,这样使通路能最好地传输数据包。一次度量就是一次标准测量,如通路带宽,路由算法s使用它来确定到达一个目标的最佳路径。为了辅助路径确定的过程,路由算法s启动和维持包含路由信息的路由表s。路由信息的变化取决于所用的路由算法。
Routing algorithms fill routing tables with a variety of information. Destination/next hop associations tell a router that a particular destination can be reached optimally by sending the packet to a particular router representing the "next hop" on the way to the final destination. When a router receives an incoming packet, it checks the destination address and attempts to associate this address with a next hop.
路由算法s将各种信息填入路由表s。目标/下一跳组合告诉路由器,一个具体的目标可以优化地到达,只要将数据包,沿着通向最终目标的通路,送到表示'下一跳'的具体路由器。当路由器接收到一个数据包时,它就检查目标地址,并在目标地址上结合下一跳相关地址。图5-1说明了一个例样目标/效益跳路由表。
Figure: Destination/Next Hop Associations Determine the Data’s Optimal Path depicts a sample destination/next hop routing table.
Figure: Destination/Next Hop Associations Determine the Data’s Optimal Path
Routing tables also can contain other information, such as data about the desirability of a path. Routers compare metrics to determine optimal routes, and these metrics differ depending on the design of the routing algorithm used. A variety of common metrics will be introduced and described later in this article.
路由表s也可以包含其他信息,如有关所希望通路的数据。路由器s比较度量值来确定优化由路,而这些度量值则取决于所使用的路由算法的设计,而有所不同。各种通用的度量s将被介绍,并在本章后面予以说明。
Routers communicate with one another and maintain their routing tables through the transmission of a variety of messages. The routing update message is one such message that generally consists of all or a portion of a routing table. By analyzing routing updates from all other routers, a router can build a detailed picture of network topology. A link-state advertisement, another example of a message sent between routers, informs other routers of the state of the sender's links. Link information also can be used to build a complete picture of network topology to enable routers to determine optimal routes to network destinations.
路由器s彼此通信,并通过传输各种消息来维持它们的路由表s。路由更新消息是一种这样的消息,它们构成了路由表的全部或部分。通过分析来自所有其它路由器的路由更新消息,一个路由器就可以建立一张详细的网络投票图。一个链路状态通告,路由器间发送消息的另一种例子,通知其它路由器,告知发送者链路s的状态。链路信息也可以用来建立一张完整的网络拓扑图,以使路由器s能确定由路到网络目标s的优化通路。
Switching 交换
Switching algorithms is relatively simple; it is the same for most routing protocols. In most cases, a host determines that it must send a packet to another host. Having acquired a router's address by some means, the source host sends a packet addressed specifically to a router's physical (Media Access Control [MAC]-layer) address, this time with the protocol (network layer) address of the destination host.
交换算法相对简单;对于大多数路由协议s都是一样的。在大多数的情况下,一个主机确定它必须将一个数据包传送给另外一个主机。通过某些方法,得到所需要的路由器地址,源主机就将一个带有目的主机协议(网络层)地址的数据包,针对寻址路由器的物理(MAC(介质访问控制)层)地址发送出去。
As it examines the packet's destination protocol address, the router determines that it either knows or does not know how to forward the packet to the next hop. If the router does not know how to forward the packet, it typically drops the packet. If the router knows how to forward the packet, however, it changes the destination physical address to that of the next hop and transmits the packet. 当路由器检查数据包的协议目的地址时,如果路由器不知道如何转发数据包,通常就丢弃它;如果知道,就改变目的物理地址到下一跳,并传送该数据包。
The next hop may be the ultimate destination host. If not, the next hop is usually another router, which executes the same switching decision process. As the packet moves through the internetwork, its physical address changes, but its protocol address remains constant, as illustrated inFigure: Numerous Routers May Come into Play During the Switching Process.
这下一跳可能是最后目的主机。如果不是,下一跳通常是另一个路由器,它执行同样的交换决策过程。当数据包移动通过互联网络时,它的物理地址就改变了,但它的协议地址保持不变,如在图5-2中举例说明的那样。
The preceding discussion describes switching between a source and a destination end system. The International Organization for Standardization (ISO) has developed a hierarchical terminology that is useful in describing this process. Using this terminology, network devices without the capability to forward packets between subnetworks are called end systems (ESs), whereas network devices with these capabilities are called intermediate systems (ISs). ISs are further divided into those that can communicate within routing domains (intradomain ISs) and those that communicate both within and between routing domains (interdomain ISs). A routing domain generally is considered a portion of an internetwork under common administrative authority that is regulated by a particular set of administrative guidelines. Routing domains are also called autonomous systems. With certain protocols, routing domains can be divided into routing areas, but intradomain routing protocols are still used for switching both within and between areas.
前面的讨论叙述了一个源和一个目的端系统间的交换。ISO已经开发了一类层次的术语,用来描述这个过程。按此类术语,一种网络设备,如果在子网间没有转发数据包的能力,该种设备就叫做ES(端系统);有此能力的,叫IS(中间系统)。ISs进一步分为域内ISs(在路由域内可以通信),和域间ISs(在路由域内和域间)可以通信。一个路由域通常认为是一个互联网络的一部分,该互联网络在公共的管理授权下,受一组特殊的管理原则指导。路由域也叫ASs(自治系统s)。在某些协议中,路由域s可以细分成路由区s,域内路由协议s仍可用于区内交换和区间交换。
Figure: Numerous Routers May Come into Play During the Switching Process
3. Routing Algorithms 路由算法
Routing algorithms can be differentiated based on several key characteristics. First, the particular goals of the algorithm designer affect the operation of the resulting routing protocol. Second, various types of routing algorithms exist, and each algorithm has a different impact on network and router resources. Finally, routing algorithms use a variety of metrics that affect calculation of optimal routes. The following sections analyze these routing algorithm attributes.
路由算法s可以根据若干关键特征来区分。其一,算法设计者的具体目标s影响到最终路由协议的操作;其二,有各种类型的路由算法,每一种算法对于网络和路由资源有不同的影响;其三,路由算法使用各种影响优化由路计算的度量。下面各节分析这些路由算法的属性。
Design Goals 设计目标
Routing algorithms often have one or more of the following design goals:
路由算法通常使用下列一个或多个设计目标：
Optimality 优化性
Simplicity and low overhead 简单性和低开销
Robustness and stability 稳固性和稳定性
Rapid convergence 快速收敛
Flexibility 灵活性
Optimality refers to the capability of the routing algorithm to select the best route, which depends on the metrics and metric weightings used to make the calculation. For example, one routing algorithm may use a number of hops and delays, but it may weigh delay more heavily in the calculation. Naturally, routing protocols must define their metric calculation algorithms strictly.
优化性是路由算法选择最佳由路的能力。这取决于用于计算的度量和度量加权。例如,一种路由算法可以使用若干跳和若干延迟,可以在计算中对延迟更重地加权。当然,路由协议s必须严格地定义它们的度量计算算法。
Routing algorithms also are designed to be as simple as possible. In other words, the routing algorithm must offer its functionality efficiently, with a minimum of software and utilization overhead. Efficiency is particularly important when the software implementing the routing algorithm must run on a computer with limited physical resources.
路由算法也应该设计成尽可能地简单。换句话说,路由算法必须以最少的软件开销和使用开销,提供其有效的功能性。效率是特别重要的,如果路由算法必须采用采用软件实现,而计算机上的物理资源又很有限的话。
Routing algorithms must be robust, which means that they should perform correctly in the face of unusual or unforeseen circumstances, such as hardware failures, high load conditions, and incorrect implementations. Because routers are located at network junction points, they can cause considerable problems when they fail. The best routing algorithms are often those that have withstood the test of time and that have proven stable under a variety of network conditions.
路由算法必须稳固,这意味着在面向不长用或想不到的环境中仍能准确地执行,如硬件失效,重载状况,和不正确地实现。由于路由器s位于网络连接点s,它们的失效可能引发可观的问题s。最好的路由算法通常容许有测试的时间,以及在各种网络状况下能保证其稳定性。
In addition, routing algorithms must converge rapidly. Convergence is the process of agreement, by all routers, on optimal routes. When a network event causes routes to either go down or become available, routers distribute routing update messages that permeate networks, stimulating recalculation of optimal routes and eventually causing all routers to agree on these routes. Routing algorithms that converge slowly can cause routing loops or network outages.
另外,路由算法必须快速收敛。收敛是所有路由器,关于由路优化,都一致认可的过程。当一个网络事件引发由路关闭或成为可用时,路由器s发布路由更改消息遍布网络s,激发重新计算优化由路,并实际上引起所有路由器都要认可这些由路。收敛很慢的路由算法可能引起路由成环或网络断开。
In the routing loop displayed inFigure: Slow Convergence and Routing Loops Can Hinder Progress, a packet arrives at Router 1 at time t1. Router 1 already has been updated and thus knows that the optimal route to the destination calls for Router 2 to be the next stop. Router 1 therefore forwards the packet to Router 2, but because this router has not yet been updated, it believes that the optimal next hop is Router 1. Router 2 therefore forwards the packet back to Router 1, and the packet continues to bounce back and forth between the two routers until Router 2 receives its routing update or until the packet has been switched the maximum number of times allowed.
Figure: Slow Convergence and Routing Loops Can Hinder Progress
Routing algorithms should also be flexible, which means that they should quickly and accurately adapt to a variety of network circumstances. Assume, for example, that a network segment has gone down. As many routing algorithms become aware of the problem, they will quickly select the next-best path for all routes normally using that segment. Routing algorithms can be programmed to adapt to changes in network bandwidth, router queue size, and network delay, among other variables.
路由算法也应该是灵活的,意思是它们应当快而准确地适应各种网络环境s。例如,假定一个网络段已经断开了。当许多路由算法知道了问题,它们就迅速地从所有的由路中选择下一个最好的通路,以能正常地使用该网段。可以编程路由算法,以适应在网络带宽,路由器队列大小,和网络延迟,还有其它可变的因素等,而引起的变化。
Algorithm Types 路由算法种类
Routing algorithms can be classified by type. Key differentiators include these:
Static versus dynamic
Single-path versus multipath
Flat versus hierarchical
Host-intelligent versus router-intelligent
Intradomain versus interdomain
Link-state versus distance vector
Static Versus Dynamic 对照静态与动态
Static routing algorithms are hardly algorithms at all, but are table mappings established by the network administrator before the beginning of routing. These mappings do not change unless the network administrator alters them. Algorithms that use static routes are simple to design and work well in environments where network traffic is relatively predictable and where network design is relatively simple. 静态算法s毕竟是很难的算法s,是由网络管理员在路由开始前建立的表映射s。这些映射s不会变化,除非管理员改变它们。使用静态路由的算法,设计简单,在网络信流相对可预测,网络设计相对简单的的环境s里,工作得很好。
Because static routing systems cannot react to network changes, they generally are considered unsuitable for today's large, constantly changing networks. Most of the dominant routing algorithms today are dynamic routing algorithms, which adjust to changing network circumstances by analyzing incoming routing update messages. If the message indicates that a network change has occurred, the routing software recalculates routes and sends out new routing update messages. These messages permeate the network, stimulating routers to rerun their algorithms and change their routing tables accordingly.
由于静态路由系统不能反映网络的变化,它们通常被认为不适合当今大型常变的网络s。如今大多数主流路由算法是动态路由算法s,它们通过分析收入的路由
变更消息,调整变化的网络环境s。如果消息指示,一个网络的变化已经发生,路由软件就重新计算,并送出新的路由更改消息。这些消息遍布网络,激发路由器重新运行它们的算法s,并改变它们相应的路由表。,,
Dynamic routing algorithms can be supplemented with static routes where appropriate. A router of last resort (a router to which all unroutable packets are sent), for example, can be designated to act as a repository for all unroutable packets, ensuring that all messages are at least handled in some way.
动态路由算法可以被静态由路适当地补充。例如,一个最后求助的路由器(一个路由器,其所有不可路由的数据包都已被送出的路由器),可以被指定当作储藏所有不可路由的储藏室,以保证所有的消息,至少用某种方法,都能得到处理。
Single-Path Versus Multipath 对照单通路与多通路
Some sophisticated routing protocols support multiple paths to the same destination. Unlike single-path algorithms, these multipath algorithms permit traffic multiplexing over multiple lines. The advantages of multipath algorithms are obvious: They can provide substantially better throughput and reliability. This is generally called load sharing. 某些高级的路由协议支持指向同一信宿的多通路。不像单通路的算法,多通路算法在多条线路上执行信流复用。多通路算法的优点很明显:它们可以提供大得多的吞吐能力和可靠性。这通常称为负载共享。
Flat Versus Hierarchical 对照扁平与分层
Some routing algorithms operate in a flat space, while others use routing hierarchies. In a flat routing system, the routers are peers of all others. In a hierarchical routing system, some routers form what amounts to a routing backbone. Packets from nonbackbone routers travel to the backbone routers, where they are sent through the backbone until they reach the general area of the destination. At this point, they travel from the last backbone router through one or more nonbackbone routers to the final destination.
某些路由算法操作在扁平空间,另一些则使用分层路由。在扁平路由系统中,所有路由器都是对等的。在分层路由系统中,一些路由器构成一个路由骨干网。数据包从非骨干路由器s传输到骨干路由器s,经由路由骨干网,直到它们到达目的地的一般区域。在那里,它们从离得最近的骨干路由器,经过一个或多个非骨干路由器,到达最终目的地。
Routing systems often designate logical groups of nodes, called domains, autonomous systems, or areas. In hierarchical systems, some routers in a domain can communicate with routers in other domains, while others can communicate only with routers within their domain. In very large networks, additional hierarchical levels may exist, with routers at the highest hierarchical level forming the routing backbone. 路由系统s通常将一组节点命名为域s,ASs(自治系统s),或区。在分层系统s中,某些在域中的路由器,可以与其它域的路由器通信;而其它的,就只能与它们域内的路由器通信。在非常大的网络s中,可以有附加的分层级s,其最高层的路由器s构成路由骨干网。
The primary advantage of hierarchical routing is that it mimics the organization of most companies and therefore supports their traffic patterns well. Most network communication occurs within small company groups (domains). Because intradomain routers need to know only about other routers within their domain, their routing algorithms can be simplified, and, depending on the routing algorithm being used, routing update traffic can be reduced accordingly. 分层路由的主要优点是它模拟大多数公司的组织机构,因而能很好地支持它们的信流模式。大多数网络通信发生在小公司组s(域s)内。由于域内路由器s只需要知道有关它们自己域内的路由器s,它们的路由算法就可以简化,这取决于所使用的具体的路由算法;更新信流的路由可以得到相应的简化。
Host-Intelligent Versus Router-Intelligent 对照主机智能与路由器智能
Some routing algorithms assume that the source end node will determine the entire route. This is usually referred to as source routing. In source-routing systems, routers merely act as store-and-forward devices, mindlessly sending the packet to the next stop. 某些路由算法假定源端节点能确定整个路由。它们常被称之为源路由系统s。在源路由系统s中,路由器只作为存储-转发设备s,不关心数据包送向的下一站。
Other algorithms assume that hosts know nothing about routes. In these algorithms, routers determine the path through the internetwork based on their own calculations. In the first system, the hosts have the routing intelligence. In the latter system, routers have the routing intelligence.
其它算法s则假定主机s对路由器s一无所知。在这些算法中,路由器s根据它们自己的计算,确定通过互联网络的通路。一开始,主机s具有路由智能,之后,路由器s具有路由智能。
Intradomain Versus Interdomain 对照域内与域间
Some routing algorithms work only within domains; others work within and between domains. The nature of these two algorithm types is different. It stands to reason, therefore, that an optimal intradomain-routing algorithm would not necessarily be an optimal interdomain-routing algorithm.
某些路由算法只工作在域内;其它的,工作在域内和域外。这两种算法类型的本性是不同的。很明显,一种优化域内路由的算法,并不需要优化域间路由；(反之亦然。)
Link-State Versus Distance Vector 对照链路状态与距离向量
Link-state algorithms (also known as shortest path first algorithms) flood routing information to all nodes in the internetwork. Each router, however, sends only the portion of the routing table that describes the state of its own links. In link-state algorithms, each router builds a picture of the entire network in its routing tables. Distance vector algorithms (also known as Bellman-Ford algorithms) call for each router to send all or some portion of its routing table, but only to its neighbors. In essence, link-state algorithms send small updates everywhere, while distance vector algorithms send larger updates only to neighboring routers. Distance vector algorithms know only about their neighbors.
链路状态算法s(也称最短路径首选算法s),洪泛路由信息到互联网络所有的节点。然而,每个路由器只送路由表中只含它自己链路状态的那一部分。在链路状态算法s中,每个路由器在其路由表s中建立一张全网络图。距离向量算法s(也称Bellman-Ford算法s)调用每个路由器送出其路由表所有或部分表项,但只给它的邻居s。一般说来,链路状态算法s到处送小更新信息,而距离向量算法s则只向邻居s发送较大的更新信息。
Because they converge more quickly, link-state algorithms are somewhat less prone to routing loops than distance vector algorithms. On the other hand, link-state algorithms require more CPU power and memory than distance vector algorithms. Link-state algorithms, therefore, can be more expensive to implement and support. Link-state protocols are generally more scalable than distance vector protocols.
由于链路状态算法s比距离向量算法s收敛更快,距离向量算法s有点更倾向用于路由环路。另一方面,链路状态算法s需要更多的CPU能力和存储量,因此,其实现和支持的代价也更大。链路状态协议s通常比距离向量协议s更具有伸缩性。
Routing Metrics 路由度量
Routing tables contain information used by switching software to select the best route. But how, specifically, are routing tables built? What is the specific nature of the information that they contain? How do routing algorithms determine that one route is preferable to others? 路由表s包含了交换软件用来选择最佳路由的信息。然而,路由表s是如何建立的?什么是它们所包含信息的本性?如何确定路由算法s所得的一种由路比其它的更好？
Routing algorithms have used many different metrics to determine the best route. Sophisticated routing algorithms can base route selection on multiple metrics, combining them in a single (hybrid) metric. All the following metrics have been used: 路由算法s已经使用了许多不同种类的度量来确定最佳由路。高级路由算法s
可以根据多种度量,将它们组合成一种单一的度量,来进行由路选择。所有下面的度量都是已经使用了：
Path length 通路长度
Reliability可靠性
Delay路由延迟
Bandwidth 带宽
Load 载荷
Communication cost 通信代价
Path length is the most common routing metric. Some routing protocols allow network administrators to assign arbitrary costs to each network link. In this case, path length is the sum of the costs associated with each link traversed. Other routing protocols define hop count, a metric that specifies the number of passes through internetworking products, such as routers, that a packet must take en route from a source to a destination.
通路长度是最普通的路由度量。某些路由协议s允许网络管理员给每个网络链路指定任意的代价。在这种情况下,通路长度就是与每个链路相联系的代价的总和。其它路由协议s定义跳步数。跳步数是一种由路度量,用来指定在由路中,传递通过互联网络联网产品s,如路由器s,的数量,而这种由路是一个数据包从源到目的所必须取道的。
Reliability, in the context of routing algorithms, refers to the dependability (usually described in terms of the bit-error rate) of each network link. Some network links might go down more often than others. After a network fails, certain network links might be repaired more easily or more quickly than other links. Any reliability factors can be taken into account in the assignment of the reliability ratings, which are arbitrary numeric values usually assigned to network links by network administrators. 可靠性。在路由算法s的上下文中,指的是每个网络链路的可信度(通常指的是误位率)。一些网络链路s可能比另一些更容易断开。一旦网络失效,某种网络链路比其它的更易于修复或修复得更快。任何可靠性因素s都可以在指定的可靠性范围内计及。这是一个任意的数值,它通常由网络管理员指定给网络链路s。
Routing delay refers to the length of time required to move a packet from source to destination through the internetwork. Delay depends on many factors, including the bandwidth of intermediate network links, the port queues at each router along the way, network congestion on all intermediate network links, and the physical distance to be traveled. Because delay is a conglomeration of several important variables, it is a common and useful metric.
路由延迟指的是时间长短,用来度量一个数据包,从源通过互联网络,到目的所需要的时间。延迟取决于许多因素,包括中间网络链路的带宽,沿着通路的每个路由器上的端口排列,在所有中间网络链路的网络拥塞,以及传送的物理距离。因为延迟是若干重要变数的组合,因而它是一种普通而又有用的度量。
Bandwidth refers to the available traffic capacity of a link. All other things being equal, a 10-Mbps Ethernet link would be preferable to a 64-kbps leased line. Although bandwidth is a rating of the maximum attainable throughput on a link, routes through links with greater bandwidth do not necessarily provide better routes than routes through slower links. For example, if a faster link is busier, the actual time required to send a packet to the destination could be greater.
带宽指的是一条链路可用的信流能力。如果其它前提都一样,10Mbps以太网链路要比一条64kbps的租借线路更可取。虽然带宽是一种在一条链路上最值得
注意的吞吐能力的额定值,但通过有较大带宽链路s的由路s,不一定需要提供比通过较慢链路s的由路s更好的由路s。例如,如果一条较快的链路很忙,需要用来发送一个数据包到目的地的实际时间可能更大。
Load refers to the degree to which a network resource, such as a router, is busy. Load can be calculated in a variety of ways, including CPU utilization and packets processed per second. Monitoring these parameters on a continual basis can be resource-intensive itself. 载荷指的是一种网络资源,如路由器,忙碌的程度。载荷可以用各种方法计算,包括CPU使用率和每秒处理的数据包s数。不间断地监视这些参数可以改善资源本身。
Communication cost is another important metric, especially because some companies may not care about performance as much as they care about operating expenditures. Although line delay may be longer, they will send packets over their own lines rather than through the public lines that cost money for usage time.
通信代价是另外一种重要的度量,尤其是,有些公司可能在关注性能方面不如关注操作费用那样多。他们情愿将数据包在
他们自己的租借线路s上发送;虽然延迟可能更长,也不愿意通过公共线路s。在那里是按使用时间付钱的。
4. Network Protocols 网络协议s
Routed protocols are transported by routing protocols across an internetwork. In general, routed protocols in this context also are referred to as network protocols. These network protocols perform a variety of functions required for communication between user applications in source and destination devices, and these functions can differ widely among protocol suites. Network protocols occur at the upper five layers of the OSI reference model: the network layer, the transport layer, the session layer, the presentation layer, and the application layer.
网路协议s(也叫网络协议s)由路由协议s传送穿越一个互联网络。一般说来,网路协议s在这种上下文中也叫网络协议s;这些网络协议s执行各种功能;这些功能是为实现源和目的设备s中,用户应用程序s之间的通信所必需;它们普遍地包含在协议族s中,各不相同。网络协议发生在OSI参考模型的上五层。
Confusion about the terms routed protocol and routing protocol is common. Routed protocols are protocols that are routed over an internetwork. Examples of such protocols are the Internet Protocol (IP), DECnet, AppleTalk, Novell NetWare, OSI, Banyan VINES, and Xerox Network System (XNS). Routing protocols, on the other hand, are protocols that implement routing algorithms. Put simply, routing protocols are used by intermediate systems to build tables used in determining path selection of routed protocols. Examples of these protocols include Interior Gateway Routing Protocol (IGRP), Enhanced Interior Gateway Routing Protocol (Enhanced IGRP), Open Shortest Path First (OSPF), Exterior Gateway Protocol (EGP), Border Gateway Protocol (BGP), Intermediate System-to-Intermediate System (IS-IS), and Routing Information Protocol (RIP). Routed and routing protocols are discussed in detail later in this book.
关于术语网路协议和路由协议的混淆是很普遍的。网路协议s是在一个互联网络上被路由的协议。这样协议的例子是IP(互联网协议),DECnet, AppleTalk,
Novell NetWare, OSI, Banyan VINES,和XNS(Xerox网络系统)。而路由协议s是执行路由算法s的协议。简而言之,路由协议s用于ISs(中间系统s)建立用来确定被路由协议s的通路选择。这些协议s的例子包括IGRP(内部网关路由协议),增强的IGRP,OSPF(开放最短路径首选),EGP(外部网关协议),BGP(边界网关协议),IS-IS(中间系统至中间系统),和RIP(路由信息协议)。
5. Review Questions 复习问答
Q - Describe the process of routing packets.
说明数据包s的路由过程。
A - Routing is the act of moving information across an internetwork from a source to a destination.
路由是一种行为,它将信息从一个源端通过一个互联网络移动到一个目的端。
Q - What are some routing algorithm types?
路由算法有哪些类型？
A - Static, dynamic, flat, hierarchical, host-intelligent, router-intelligent, intradomain, interdomain, link-state, and distance vector.
静态,动态,扁平,层次,主机智能,路由器智能,域内,域间,链路状态,和距离向量。
Q - Describe the difference between static and dynamic routing.
说明静态路由和动态路由有何不同。
A - Static routing is configured by the network administrator and is not capable of adjusting to changes in the network without network administrator intervention. Dynamic routing adjusts to changing network circumstances by analyzing incoming routing update messages without administrator intervention.
静态路由由网络管理员配置,没有网络管理员的干预不能调整网络中的变化。动态路由调整网络环境中的变化是经由解析收入的路由变更消息而实现的,没有管理员的干预。
Q - What are some of the metrics used by routing protocols?
用于路由协议s的一些度量s是什么？
A - Path length, reliability, delay, bandwidth, load, and communication cost. 通路长度,可靠性,延迟,带宽,载荷,和通信代价。
Retrieved from "http://docwiki.cisco.com/wiki/Routing_Basics"
Category:IOS Technology Handbook
====================================================================
六．Network Management Basics 网络管理基本原理
指导目录
联网技术手册 指导目录
Part1 Internetworking Basics 互联网络联网基本原理
Part2 LAN Technologies局域网技术
part3 WAN Technologies 广域网技术
part4 Internet Protocols 互联网协议
part5 Bridging and Switching 桥接和交换
part6 Routing 路由
part7 Network Management 网络管理
part8 Voice/Data Integration Technologies 声音/数据集成技术
part9 Wireless Technologies 无线技术
part10 Cable Access Technologies 有线接入技术
part11 Dial-up Technology 拨号技术
part12 Security Technologies 安全技术
part13 Quality of Service Networking 联网服务质量
part14 Network Caching Technologies 网络缓存技术
part15 IBM Network Management IBM网络管理
part16 Multiservice Access Technologies 多业务接入技术
http://docwiki.cisco.com/wiki/Network_Management_Basics
This page was last modified on 17 December 2009, at 21:55
六．Network Management Basics 网络管理基本原理
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This article describes functions common to most network-management architectures and protocols. It also presents the five conceptual areas of management as defined by the International Organization for Standardization (ISO). Subsequent articles in Network Management address specific network management technologies, protocols, and platforms in more detail.
本章叙述了管理功能,它们对于大多数网络管理体系结构和协议s都是共同的。它也介绍了,如ISO所定义的,五种概念区域的管理。在第IX部分的各章,“网络管理”更详细地讲述具体的网络管理技术s,协议s,和平台s。
Contents
[hide]
1 What Is Network Management?
1.1 A Historical Perspective
2 Network Management Architecture
2.1 Figure: A Typical Network Management Architecture Maintains Many Relationships
3 ISO Network Management Model
3.1 Performance Management
3.2 Configuration Management
3.3 Accounting Management
3.4 Fault Management
3.5 Security Management
4 Review Questions
1. What Is Network Management? 什么是网络管理？
Network management means different things to different people. In some cases, it involves a solitary network consultant monitoring network activity with an outdated protocol analyzer. In other cases, network management involves a distributed database, autopolling of network devices, and high-end workstations generating real-time graphical views of network topology changes and traffic. In general, network management is a service that employs a variety of tools, applications, and devices to assist human network managers in monitoring and maintaining networks. 网络管理对于不同的人们意味着不同的事情。在某些情况下,它有一个独立的网络监护,用一个保持更新的协议分析器,监视着网络的活动。在其它的情况下,网络管理包含了一个分布式数据库,自动轮询网络设备s,并在高端工作站s上,产生网络拓扑变化和信流的实时图形视图。一般说来,网络管理是一种服务,它使用各种工具,应用程序s,和设备s,帮助网络管理者的人们来监督和维护网络s。
A Historical Perspective 历史透视
The early 1980s saw tremendous expansion in the area of network deployment. As companies realized the cost benefits and productivity gains created by network technology, they began to add networks and expand existing networks almost as rapidly as new network technologies and products were introduced. By the mid-1980s, certain companies were experiencing growing pains from deploying many different (and sometimes incompatible) network technologies.
八十年代早期,已经看到了网络部署区域极大的扩充。当公司s意识到了由于网络技术所产生的价值利益和生产收益,他们就开始增加网络s和扩充现有的网络s,几乎与新技术s出现的速度和产品的引进一样迅速。到了八十年代中期,某些公司从部署许多不同的(有时是不兼容的)网络技术中,已经体验到增长的苦涩。
The problems associated with network expansion affect both day-to-day network operation management and strategic network growth planning. Each new network technology requires its own set of experts. In the early 1980s, the staffing requirements alone for managing large, heterogeneous networks created a crisis for many organizations. An urgent need arose for automated network management (including what is typically called network capacity planning) integrated across diverse environments.
伴随网络扩充而带来的问题s,同时影响着日常网络操作管理和战略上的网络增长规划。每一种新的网络技术需要它自己的一组专家。在八十年代早期,对于许多机构而言,仅有管理大型,同质网络s的职能需求,就产生了一种危机。一种紧急的需要,要求自动管理综合了含盖各种环境的网络s(包括通常称为网络容量规划),出现了。
2. Network Management Architecture 网络管理的体系结构
Most network management architectures use the same basic structure and set of relationships. End stations (managed devices), such as computer systems and other network devices, run software that enables them to send alerts when they recognize problems (for example, when one or more user-determined thresholds are exceeded). Upon receiving these alerts, management entities are programmed to react by executing one, several, or a group of actions, including operator notification, event logging, system shutdown, and automatic attempts at system repair
大多数网络管理体系结构s使用同一种基本结构和同一组关系。端站(受管理的设备s),如计算机系统s和其它网络设备s,运行软件,当它们确认有问题s时(例如
,当超过一个或多个用户确定的门槛时),使它们发出警报s。在接收到这些警报后,管理实体s被编程,用执行一个,几个或一组动作,包括操作者提醒,事件登记,系统关闭,以及自动尝试系统修复,来作出反应。.
Management entities also can poll end stations to check the values of certain variables. Polling can be automatic or user-initiated, but agents in the managed devices respond to all polls. Agents are software modules that first compile information about the managed devices in which they reside, then store this information in a management database, and finally provide it (proactively or reactively) to management entities within network management systems (NMSs) via a network management protocol. Well-known network management protocols include the Simple Network Management Protocol (SNMP) and Common Management Information Protocol (CMIP). Management proxies are entities that provide management information on behalf of other entities.
管理实体也可以轮询端站s来检查某些变量的值。轮询可以自动启动或用户启动,而由在被管理设备s中的代理s来响应所有的轮询。代理s是软件模块s,居于管理设备s中。它们首先编译有关管理设备s的信息,然后,把这些信息存储在一个管理数据库中,最后经由一个网络管理协议,把信息提供给(或主动或被动)NMSs(网络管理系统s)内的实体s。已知的网络管理协议s包括SNMP(简单网络管理协议),和CMIP(公用管理信息协议)。管理代理s是实体s,代表其它实体s提供管理信息。图6-1说明了一个典型的网络管理体系结构.
Figure: A Typical Network Management Architecture Maintains Many Relationships depicts a typical network management architecture.
Figure: A Typical Network Management Architecture Maintains Many Relationships
3. ISO Network Management Model ISO网络管理模型
The ISO has contributed a great deal to network standardization. Its network management model is the primary means for understanding the major functions of network management systems. This model consists of five conceptual areas, as discussed in the next sections. ISO已经为网络标准化做了大量的贡献。它的网络管理模型是用于理解网络管理系统s主要功能s的主要工具。该模型
如在下一节讨论中指出的,由五个概念区域组成。
3.1 Performance Management 性能管理域
The goal of performance management is to measure and make available various aspects of network performance so that internetwork performance can be maintained at an acceptable level. Examples of performance variables that might be provided include network throughput, user response times, and line utilization.
性能管理的目标是测量和实现网络性能可用的各个方面,以使互联网络性能能持在可接受的水平。可以提供的性能变量s的例子s,包括网络吞吐量,用户响应时间,和线路利用。
Performance management involves three main steps. First, performance data is gathered on variables of interest to network administrators. Second, the data is analyzed to determine normal (baseline) levels. Finally, appropriate performance thresholds are determined for each important variable so that exceeding these thresholds indicates a network problem worthy of attention. 性能管理包括三个主要步骤。首先,
收集信息数据,并把它放到网络管理员
感兴趣的变量s上。其次,分析数据以确定正常水平。最后,对每个重要变量,确认适当的性能门槛,超过门槛,就指示一个值得注意的网络问题。
Management entities continually monitor performance variables. When a performance threshold is exceeded, an alert is generated and sent to the network management system.
管理实体不断监视性能变量s。当一个性能门槛被超过,就产生一个报警,并送到网络管理系统。
Each of the steps just described is part of the process to set up a reactive system. When performance becomes unacceptable because of an exceeded user-defined threshold, the system reacts by sending a message. Performance management also permits proactive methods: For example, network simulation can be used to project how network growth will affect performance metrics. Such simulation can alert administrators to impending problems so that counteractive measures can be taken. 上面所叙述的每一步,是建立一个反应系统的部分过程。当性能由于超过用户定义的门槛,而变得不可接受时,系统就通过发送一个消息来反应。性能管理也允许前摄的方法s:例如,网络仿真可以用于规划,如何将影响网络性能的度量s。这样的仿真可以向管理员报警即将发生的问题s,以及时采取预防措施。
3.2 Configuration Management 配置管理域
The goal of configuration management is to monitor network and system configuration information so that the effects on network operation of various versions of hardware and software elements can be tracked and managed.
配置管理的目标是监督网络和系统配置信息,以使能对影响网络操作的各种型式的软件和硬件组件,进行跟踪和管理。
Each network device has a variety of version information associated with it. An engineering workstation, for example, may be configured as follows:
每个网络设备都有各种与它有关的版本信息。例如,一个工程工作站可以配置如下：
Operating system, Version 3.2
Ethernet interface, Version 5.4
TCP/IP software, Version 2.0
NetWare software, Version 4.1
NFS software, Version 5.1
Serial communications controller, Version 1.1
X.25 software, Version 1.0
SNMP software, Version 3.1
Configuration management subsystems store this information in a database for easy access. When a problem occurs, this database can be searched for clues that may help solve the problem. 配置管理子系统s将该信息存储在一个数据库中,以便于访问。当一个问题发生时,就可以搜索该数据库,以求取线索,帮助解决问题。
3.3 Accounting Management 审计管理域
The goal of accounting management is to measure network utilization parameters so that individual or group uses on the network can be regulated appropriately. Such regulation minimizes network problems (because network resources can be apportioned based on resource capacities) and maximizes the fairness of network access across all users.
审计管理的目标是,测量网络使用参数,以使网络上的单体或成组可以适当地调整。这样的调整会使网络问题s最小化,(因为网络资源可以根据资源能力进行分摊),并使访问网络的所有用户能得到最大的公正性。
As with performance management, the first step toward appropriate accounting management is to measure utilization of all important network resources. Analysis of the results provides insight into current usage patterns, and usage quotas can be set at this point. Some correction, of course, will be required to reach optimal access practices. From this point, ongoing measurement of resource use can yield billing information as well as information used to assess continued fair and optimal resource utilization.
与性能管理结合在一起,进行适当的审计管理的第一步,是测量所有重要网络资源的利用度。进而分析其结果s,弄清当前用法的模式s,并设置用途的指标。当然,某些修正是必要的,以达到优化访问的实现。从这一点出发,继续测量资源使用就能得到信息清单和信息,用以访问利用度持续良好和优化的资源。
3.4 Fault Management 故障管理域
The goal of fault management is to detect, log, notify users of, and (to the extent possible) automatically fix network problems to keep the network running effectively. Because faults can cause downtime or unacceptable network degradation, fault management is perhaps the most widely implemented of the ISO network management elements. 故障管理的目标是检测,登记,通知用户s,和(尽可能)修复网络问题s以有效地保持网络运行。因为故障能够引起停机或不可接受的网络退化,故障管理或许是ISO网络管理部件s中实现最为广泛的部件s。
Fault management involves first determining symptoms and isolating the problem. Then the problem is fixed and the solution is tested on all-important subsystems. Finally, the detection and resolution of the problem is recorded.
故障管理包括确定症状和隔离问题;然后修复问题,并在所有重要的子系统上进行测试,取得结果;最后,记录被测出和被解决的问题。
3.5 Security Management 安全管理域
The goal of security management is to control access to network resources according to local guidelines so that the network cannot be sabotaged (intentionally or unintentionally) and sensitive information cannot be accessed by those without appropriate authorization. A security management subsystem, for example, can monitor users logging on to a network resource and can refuse access to those who enter inappropriate access codes.
安全管理的目标是按照局限的原则s,有控制地访问网络资源,以使网络不会被破坏(有意或无意),或敏感信息没有适当的授权就不能被访问。例如,一个安全管理子系统可以监督用户s对网络资源的使用,可以拒绝用不恰当代码访问的用户s进入。
Security management subsystems work by partitioning network resources into authorized and unauthorized areas. For some users, access to any network resource is inappropriate, mostly because such users are usually company outsiders. For other (internal) network users, access to information originating from a particular department is inappropriate. Access to Human Resource files, for example, is inappropriate for most users outside the Human Resources department.
安全管理子系统把网络资源按在授权区和不授权区分开。对于某些用户s,访问任何网络资源都是不适当的,这通常指的是公司外部人员。对于其他(内部)网络
用户s,访问源于一个具体部门的信息,是不适当的。例如,访问人事资源文件s,对于人事资源部门外部的大多数用户,都是不适当的。
Security management subsystems perform several functions. They identify sensitive network resources (including systems, files, and other entities) and determine mappings between sensitive network resources and user sets. They also monitor access points to sensitive network resources and log inappropriate access to sensitive network resources.
安全管理子系统执行若干功能。它们识别敏感网络资源s(包括系统s,文件s,和其他实体s),确定敏感网络资源和用户组的映射关系。它们也监视敏感网络资源的入口s,以及登录不适当地对敏感网络资源访问的事件s。
4. Review Questions 复习问答
Q - Name the different areas of network management.
网络管理不同区域的名字。
A - Configuration, accounting, fault, security, and performance.
配置,审计,故障,安全,和性能。
Q - What are the goals of performance management?
性能管理的目标s是什么？
A - Measure and make available various aspects of network performance so that internetwork performance can be maintained at an acceptable level.
测量并尽可能实现网络性能的各个方面,使网络性能维持在可接受的水平。
Q - What are the goals of configuration management?
配置管理的目标s是什么？
A - Monitor network and system configuration information so that the effects on network operation of various versions of hardware and software elements can be tracked and managed.
监督网络和系统配置信息,以便跟踪和管理各种硬件部分和软件部分在网络操作上的效能。
Q - What are the goals of accounting management?
审计管理的目标s是什么？
A - Measure network utilization parameters so that individual or group uses on the network can be regulated appropriately.
测量反映网络利用情况的参数,据此可以对个别或成联网络资源,进行适当调整。
Q - What are the goals of fault management?
故障管理的目标s是什么？
A - Detect, log, notify users of, and automatically fix network problems to keep the network running effectively.
检测,登记,通知用户,和自动修复网络问题,以有效保持网络运行。
Q - What are the goals of security management?
安全管理的目标s是什么？
A - Control access to network resources according to local guidelines so that the network cannot be sabotaged and so that sensitive information cannot be accessed by those without appropriate authorization.
按照就地的原则s对网络资源的访问进行控制,以免网络被破坏,并使敏感信息不会被没有适当授权的用户所访问。
Retrieved from "http://docwiki.cisco.com/wiki/Network_Management_Basics"
Category:IOS Technology Handbook
===================================================================
七．Open System Interconnection Protocols 开放系统互连协议
指导目录
联网技术手册 指导目录
Part1 Internetworking Basics 互联网络联网基本原理
Part2 LAN Technologies 局域网技术
part3 WAN Technologies 广域网技术
part4 Internet Protocols 互联网协议
part5 Bridging and Switching 桥接和交换
part6 Routing 路由
part7 Network Management 网络管理
part8 Voice/Data Integration Technologies 声音/数据集成技术
part9 Wireless Technologies 无线技术
part10 Cable Access Technologies 有线接入技术
part11 Dial-up Technology 拨号技术
part12 Security Technologies 安全技术
part13 Quality of Service Networking 联网服务质量
part14 Network Caching Technologies 网络缓存技术
part15 IBM Network Management IBM网络管理
part16 Multiservice Access Technologies 多业务接入技术
http://docwiki.cisco.com/wiki/Open_System_Interconnection_Protocols
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七．Open System Interconnection Protocols 开放系统互连协议
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The Open System Interconnection (OSI) protocol suite is comprised of numerous standard protocols that are based on the OSI reference model. These protocols are part of an international program to develop data-networking protocols and other standards that facilitate multivendor equipment interoperability. The OSI program grew out of a need for international networking standards and is designed to facilitate communication between hardware and software systems despite differences in underlying architectures.
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开放系统互连（OSI）协议栈由许多标准协议组成,它们都基于OSI参考模型。这些协议是，一项国际安排,用来开发数据联网协议s,以及促进多厂商设备互操作性的其它标准s,的一部分。OSI的安排是出于一种对国际联网标准化的需要，设计成便于硬件系统和软件系统之间通信,而不管基础结构的差异。
The OSI specifications were conceived and implemented by two international standards organizations: the International Organization for Standardization (ISO) and the International Telecommunication Union-Telecommunications Standards Sector (ITU-T). This article provides a summary of the OSI protocol suite and illustrates its mapping to the general OSI reference model.
OSI规范s是由两个国际标准化组织构思和组织实施的：国际标准化组织（ISO）和国际电信联盟电信标准化部（ITU - T）。本文提供了一个OSI协议栈的汇总,举例说明了它与一般OSI参考模型的映射关系。
Contents
[hide]
1 OSI Networking Protocols
1.1 Figure: The OSI Protocol Suite Maps to All Layers of the OSI Reference Model
1.2 OSI Physical and Data Link layers
1.3 OSI Network Layer
1.3.1 OSI Layer Standards
1.3.2 OSI Connectionless Network Service
1.3.3 OSI Connection-Oriented Network Service
1.3.4 Network Layer Addressing
1.3.5 OSI Protocols Transport Layer
1.3.6 OSI Protocols Session Layer
1.3.7 OSI Protocols Presentation Layer
1.3.8 OSI Protocols Application Layer
2 Review Questions
目录
[hide]
1 OSI网络协议
1.1图：OSI协议参考模型套件映射到所有图层的OSI
1.2 OSI的物理和数据链路层
1.3 OSI网络层
1.3.1 OSI的层标准
1.3.2 OSI无连接网络服务
1.3.3 OSI的面向连接的网络服务
1.3.4 网络层寻址
1.4 传输层OSI协议
1.5 会话层OSI协议
1.6 表示层OSI协议
1.7 应用层OSI协议
2 Review Questions2 复习题
1. OSI Networking Protocols
Figure: The OSI Protocol Suite Maps to All Layers of the OSI Reference Model illustrates the entire OSI protocol suite and its relation to the layers of the OSI reference model. Each component of this protocol suite is discussed briefly in this article.
图：OSI协议栈映射到OSI参考模型所有层 说明了整个OSI协议栈,以及与OSI参考模型层的关系。该协议族的每个部分在本文中都作了简短的讨论。
Figure: The OSI Protocol Suite Maps to All Layers of the OSI Reference Model
1.1 图：OSI协议族映射到OSI参考模型的所有层
1.2 OSI Physical and Data Link layers
The OSI protocol suite supports numerous standard media-access protocols at the physical and data link layers. The wide variety of media-access protocols supported in the OSI protocol suite allows other protocol suites to exist easily alongside OSI on the same network media. Supported media-access protocols include IEEE 802.2 LLC, IEEE 802.3, Token Ring/IEEE 802.5, Fiber Distributed Data Interface (FDDI), and X.25.
1.2 OSI物理和数据链路层
OSI协议族支持物理层和数据链路层上的多种标准媒体访问协议。在OSI协议栈中的大量媒体访问协议,允许其他多种协议与ISO一道,共存在同一个网络媒体上。所支持的媒体访问协议,包括IEEE 802.2 LLC,IEEE 802.3,令牌环/IEEE 802.5,光纤分布式数据接口（FDDI）和X.25。
1.3 OSI Network Layer
The OSI protocol suite specifies two routing protocols at the network layer: End System-to-Intermediate System (ES-IS) and Intermediate System-to-Intermediate System (IS-IS). In addition, the OSI suite implements two types of network services: connectionless service and connection-oriented service.
OSI网络层
OSI协议栈在网络层上指定了两个路由协议：端系统到中间系统（ES - IS）和中间系统到中间系统（IS - IS）。此外， OSI栈实现了两类网络服务：无连接服务和面向连接服务。
1.3.1 OSI Layer Standards
In addition to the standards specifying the OSI network layer protocols and services, the following documents describe other OSI network layer specifications:
1.3.1 OSI的层标准s
除了指定OSI网络层协议和服务的那些标准外，下列文档还描述了其他OSI网络层规范：
ISO 8648 - This standard defines the internal organization of the network layer (IONL), which divides the network layer into three distinct sublayers to support different subnetwork types.
ISO 8348 - This standard defines network layer addressing and describes the connection-oriented and connectionless services provided by the OSI network layer.
ISO TR 9575 - This standard describes the framework, concepts, and terminology used in relation to OSI routing protocols.
ISO 8648-此标准定义了网络层的内部机构（IONL），它将网络层分为三个明确的子层，以支持不同的子网类型。
ISO 8348 -本标准规定了网络层寻址,并描述了为OSI网络层所提供的,面向连接和无连接的服务。
ISO TR 9575 –本标准描述了用于与OSI路由协议s有关的框架，概念和术语
1.3.2 OSI Connectionless Network Service
OSI connectionless network service is implemented by using the Connectionless Network Protocol (CLNP) and Connectionless Network Service (CLNS). CLNP and CLNS are described in the ISO 8473 standard. CLNP is an OSI network layer protocol that carries upper-layer data and error indications over connectionless links. CLNP provides the interface between the Connectionless Network Service (CLNS) and upper layers. CLNS provides network layer services to the transport layer via CLNP. CLNS does not perform connection setup or termination because paths are determined independently for each packet that is transmitted through a network. This contrasts with Connection-Mode Network Service (CMNS). In addition, CLNS provides best-effort delivery, which means that no guarantee exists that data will not be lost, corrupted, misordered, or duplicated. CLNS relies on transport layer protocols to perform error detection and correction.
1.3.2 OSI无连接网络服务
OSI无连接网络服务是通过使用无连接网络协议（CLNP）和无连接网络服务（CLNS）来实现的。CLNP和CLNS是在ISO 8473标准中描述的。CLNP是一个OSI网络层协议,承载着上一层的数据,以及在无连接链路s以上的错误指示。CLNP提供了CLNS(无连接网络服务)和上层s之间的接口。CLNS通过CLNP,为传输层提供了网络层服务。CLNS并不执行连接的建立或终止，因为路径s是独立地用于每一个通过网络传送的数据包。这不同于连接模式的网络服务（CMNS）。此外，CLNS提供尽力传送，这意味着，不能保证数据不会丢失，损坏，乱序，或重复。 CLNS依靠传输层协议，来执行错误检测和纠错。
1.3.3 OSI Connection-Oriented Network Service
OSI connection-oriented network service is implemented by using the Connection-Oriented Network Protocol (CONP) and Connection-Mode Network Service (CMNS). CONP is an OSI network layer protocol that carries upper-layer data and error indications over connection-oriented links. CONP is based on the X.25 Packet-Layer Protocol (PLP) and is described in the ISO 8208 standard, "X.25 Packet-Layer Protocol for DTE." CONP provides the interface between CMNS and upper layers. It is a network layer service that acts as the interface between the transport layer and CONP, and it is described in the ISO 8878 standard. CMNS performs functions related to the explicit establishment of paths between communicating transport layer entities. These functions include connection setup, maintenance, and termination. CMNS also provides a mechanism for requesting a specific quality of service (QoS). This contrasts with CLNS.
1.3.3 OSI的面向连接的网络服务
OSI面向连接的网络服务是通过使用CONP（面向连接的网络协议）和CMNS（连接模式网络服务）来实现的。CONP是一个OSI网络层协议，它承载着上一层的数据,以及在面向连接链路s以上的错误指示。CONP基于X.25分组层协议（PLP），在ISO 8208标准“X.25分组层协议的DTE”中描述。CONP提供CMNS及其上层s间的接口。这是一个网络层服务，起着传输层和CONP之间接口的作用，它在ISO 8878标准中描述。CMNS执行的功能，跟明确建立,在传输层实体间通信的通路,有关。这些功能包括连接的建立，维持和终止。CMNS还提供了一个用于申请特殊的服务质量（QoS）的机制。这不同于CLNS。
1.3.4 Network Layer Addressing
OSI network layer addressing is implemented by using two types of hierarchical addresses: network service access point addresses and network entity titles. A network service access point (NSAP) is a conceptual point on the boundary between the network and the transport layers. The NSAP is the location at which OSI network services are provided to the transport layer. Each transport layer entity is assigned a single NSAP, which is individually addressed in an OSI internetwork using NSAP addresses.Figure: The OSI NSAP Address Is Assigned to Each Transport Layer Entity illustrates the format of the OSI NSAP address, which identifies individual NSAPs.
1.3.4 网络层寻址
OSI网络层寻址,是通过使用两种类型的分层地址来实现的：网络服务访问点(NSAP)地址和网络实体标题s一个网络服务访问点（NSAP）是一个概念点,位于网络层与传输层的边界处。NSAP处于将网络服务提供给传输层的位置上。每个传输层实体被指定给单一的NSAP，它在使用NSAP地址的OSI互联网络中是独立寻址的。图：OSI的NSAP地址分配给每个传输层实体说明了用来识别单个NSAP的 OSI NSAP地址格式。
图： OSI的NSAP地址分配给每个传输层实体
Figure: The OSI NSAP Address Is Assigned to Each Transport Layer Entity
NSAP Address Fields
Two NSAP Address fields exist: the initial domain part (IDP) and the domain-specific part (DSP). The IDP field is divided into two parts: the authority format identifier (AFI) and the initial domain identifier (IDI). The AFI provides information about the structure and content of the IDI and DSP fields, such as whether the IDI is of variable length and whether the DSP uses decimal or binary notation. The IDI specifies the entity that can assign values to the DSP portion of the NSAP address. The DSP is subdivided into four parts by the authority responsible for its administration. The Address Administration fields allow for the further administration of addressing by adding a second authority identifier and by delegating address administration to subauthorities. The Area field identifies the specific area within a domain and is used for routing purposes. The Station field identifies a specific station within an area and also is used for routing purposes. The Selector field provides the specific n-selector within a station and, much like the other fields, is used for routing purposes. The reserved n-selector 00 identifies the address as a network entity title (NET).
NSAP地址字段s
（NSAP地址字段的这种划分是基于ES-IS和IS-IS系统的结构。为便于理解，请先了解它们。）
有两个NSAP地址的字段：IDP（初始域部分）和DSP（指定域部分）的。IDP字段分为两部分：AFI（授权格式标识符）和IDI（初始域标识符）。 AFI提供了有关IDI和DSP字段的信息,如IDI是否是变长的,DSP使用的表示法是十进制还是二进制。IDI指定的实体用于将值分配到NSAP地址的DSP部分。DSP根据对它管理的授权责任,再细分为四个部分。地址管理address administration字段允许,通过加入第二个授权标识符,来解决进一步管理，并下放地址管理部分授权。区Area字段识别一个域内的指定区域,系用于路由目的。站Station字段识别一个域内的指定站,也是用于路由目的。选择器Selector字段提供了一个站内指定的n-选择器，很像其他字段，也是用于路由目的。保留的n-选择器 00 识别为网络实体标题(NET)的地址。
End-System NSAPs
An OSI end system (ES) often has multiple NSAP addresses, one for each transport entity that it contains. If this is the case, the NSAP address for each transport entity usually differs only in the last byte (called the n-selector).
端点系统的NSAPs
一个OSI的端点系统(ES)通常有多个NSAP地址，每一个NSAP地址对应一个传输层实体。如果是这种情况，每个传输实体NSAP地址通常只有在最后一个字节才是不同的(称为n-选择器)。
图：NSAP提供了一个在运输层实体,NSAP,和网络服务之间的链路说明了运输实体,NSAP,和网络服务间的关系。
图： NSAP提供了一个在传输实体和一个网络服务之间的关系
Figure: The NSAP Provides a Link Between a Transport Entity and a Network Service illustrates the relationship between a transport entity, the NSAP, and the network service.
Figure: The NSAP Provides a Link Between a Transport Entity and a Network Service
A network entity title (NET) is used to identify the network layer of a system without associating that system with a specific transport layer entity (as an NSAP address does). NETs are useful for addressing intermediate systems (ISs), such as routers, that do not interface with the transport layer. An IS can have a single NET or multiple NETs, if it participates in multiple areas or domains.
一个NAT(网络实体标题)是用来识别一种网络层,该网络层所在系统并没有指定的传输层实体(即没有相应的NSAP地址)。NAT对于寻址ISs(中间系统s),如路由器s,是有用的，因为它们没有与传输层的接口。一个IS可以有一个，或多个NAT,如果它参与在多个区或域内。
1.4 OSI Protocols Transport Layer
The OSI protocol suite implements two types of services at the transport layer: connection-oriented transport service and connectionless transport service. Five connection-oriented transport layer protocols exist in the OSI suite, ranging from Transport Protocol Class 0 through Transport Protocol Class 4. Connectionless transport service is supported only by Transport Protocol Class 4. Transport Protocol Class 0 (TP0), the simplest OSI transport protocol, performs segmentation and reassembly functions. TP0 requires connection-oriented network service. Transport Protocol Class 1 (TP1) performs segmentation and reassembly, and offers basic error recovery. TP1 sequences protocol data units (PDUs) and will retransmit PDUs or reinitiate the connection if an excessive number of PDUs are unacknowledged. TP1 requires connection-oriented network service. Transport Protocol Class 2 (TP2) performs segmentation and reassembly, as well as multiplexing and demultiplexing of data streams over a single virtual circuit. TP2 requires connection-oriented network service. Transport Protocol Class 3 (TP3) offers basic error recovery and performs segmentation and reassembly, in addition to multiplexing and demultiplexing of data streams over a single virtual circuit. TP3 also sequences PDUs and retransmits them or reinitiates the connection if an excessive number are unacknowledged. TP3 requires connection-oriented network service. Transport Protocol Class 4 (TP4) offers basic error recovery, performs segmentation and reassembly, and supplies multiplexing and demultiplexing of data streams over a single virtual circuit. TP4 sequences PDUs and retransmits them or reinitiates the connection if an excessive number are unacknowledged. TP4 provides reliable transport service and functions with either connection-oriented or connectionless network service. It is based on the Transmission Control Protocol (TCP) in the Internet Protocols suite and is the only OSI protocol class that supports connectionless network service.
1.4 OSI协议栈的传输层
在传输层,OSI协议栈实现两类服务：面向连接的传输服务和无连接的传输服务。在OSI协议栈中,有五种面向连接的传输层协议s，范围从传输协议类0到类4(TP0到TP4)。无连接传输服务只支持TP4(传输协议类4)。TP0 (第0类传输协议),一种最简单的OSI传输协议,执行分段和重组功能。TP0需要面向连接的网络服务。TP1（第1类传输协议）除了执行分段和重组，还提供基本的错误恢复。TP1还以协议数据单元（PDU）为单位进行排序, 如果过多的PDU不被认可,就重新发送PDUs或重新启动连接。TP1需要面向连接的网络服务。TP2（第2类传输协议）执行分段和重组，以及在一个单一的虚电路上复用和解复用。TP2上要求面向连接的网络服务。TP3（第3类传输协议）除了在一个单一的虚电路上数据流的复用和解复用外,还提供基本的错误恢复和执行分割及重组。TP3还以协议数据单元（PDU）为单位进行排序, 如果过多的PDU不被认可,就重新发送PDUs或重新启动连接。TP3需要面向连接的网络服务。TP4（第四类传输协议）提供了基本的错误恢复，执行分段和重组，并提供在一个单一的虚电路上数据流的复用和解复用。TP4还以协议数据单元（PDU）为单位进行排序, 如果过多的PDU不被认可,就重新发送PDUs或重新启动连接。TP4提供可靠的传输服务和功能,无论是面向连接还是无连接网络服务。这是基于互联网协议栈的TCP（传输控制协议）,是唯一支持无连接网络服务的OSI传输协议类。
1.5 OSI Protocols Session Layer
The session layer implementation of the OSI protocol suite consists of a session protocol and a session service. The session protocol allows session-service users (SS-users) to communicate with the session service. An SS-user is an entity that requests the services of the session layer. Such requests are made at session-service access points (SSAPs), and SS-users are uniquely identified by using an SSAP address. Session service provides four basic services to SS-users. First, it establishes and terminates connections between SS-users and synchronizes the data exchange between them. Second, it performs various negotiations for the use of session layer tokens, which the SS-user must possess to begin communicating. Third, it inserts synchronization points in transmitted data that allow the session to be recovered in the event of errors or interruptions. Finally, it enables SS-users to interrupt a session and resume it later at a specific point.
1.5 OSI Protocols Session Layer OSI协议栈的会话层
OSI协议栈的会话层实现,包含了一个会话协议和一个会话服务。会话协议允许会话服务用户（SS-用户s）与会话服务相通信。一个SS-用户是一个实体,他向会话层申请服务s。这样的申请是在SSAPs(会话服务访问点s)提出的，通过使用SSAP地址,唯一地识别SS-用户。会话服务为SS-用户s提供了四种基本服务。第一种基本服务，它建立和终止SS -用户间的连接，同步它们之间的数据交换。第二种基本服务，因为SS-用户必须占有令牌才能进行通信，为了使用会话层令牌s而必须进行各种协商。第三种基本服务，在发生错误或中断事件时,于被发送的数据中插入同步点,从而允许恢复会话。最后一种基本服务，它使SS-用户s中断一次会话,并在以后在指定处重新开始。
图：会话层功能,经由SSAP,提供服务给表示层功能显示了SS-用户s,SSAP,会话协议和会话服务之间的关系。
图：会话层功能,经由SSAP,提供服务给表示层功能
Figure: Session Layer Functions Provide Service to Presentation Layer Functions via an SSAP shows the relationship between the SS-user, the SSAP, the session protocol, and the session service.
Figure: Session Layer Functions Provide Service to Presentation Layer Functions via an SSAP
Session service is defined in the ISO 8306 standard and in the ITU-T X.215 recommendation. The session protocol is defined in the ISO 8307 standard and in the ITU-T X.225 recommendation. A connectionless version of the session protocol is specified in the ISO 9548 standard.
会话服务定义在ISO 8306标准中,以及在ITU - T的X.215建议中。会话协议定义在ISO 8307标准中,以及在ITU - T的X.225建议中。一个会话协议的无连接版本被指定在ISO 9548标准中。
1.6 OSI Protocols Presentation Layer
The presentation layer implementation of the OSI protocol suite consists of a presentation protocol and a presentation service. The presentation protocol enables presentation-service users (PS-users) to communicate with the presentation service. A PS-user is an entity that requests the services of the presentation layer. Such requests are made at presentation-service access points (PSAPs). PS-users are uniquely identified by using PSAP addresses. Presentation service negotiates transfer syntax and translates data to and from the transfer syntax for PS-users, which represent data using different syntaxes. The presentation service is used by two PS-users to agree upon the transfer syntax that will be used. When a transfer syntax is agreed upon, presentation-service entities must translate the data from the PS-user to the correct transfer syntax. The OSI presentation layer service is defined in the ISO 8822 standard and in the ITU-T X.216 recommendation. The OSI presentation protocol is defined in the ISO 8823 standard and in the ITU-T X.226 recommendation. A connectionless version of the presentation protocol is specified in the ISO 9576 standard.
1.6 OSI协议栈的表示层
OSI协议栈的表示层实现,包含一个表示协议和一个表示服务。表示协议使表示服务用户s（PS-用户s）与表示服务通信。一个PS用户是一个实体,它申请表示层服务。(PSAPs).这样的申请是在PSAPs（表示服务访问点s）完成的。 PS-用户是通过使用PSAP地址来唯一标识的。表示服务协商传输语法,并根据用于PS-用户s的传输语法来转换数据，这说明,使用不同的语法来表示数据。表示服务实现两个PS-用户经协商确认,它们将要使用的传送语法。当一个传输语法商定了，表示服务实体s必须把数据从PS -用户转换到正确的传输语法。OSI表示层服务定义在ISO 8822标准和ITU - T的X.216建议中。OSI表示协议定义在ISO 8823标准和ITU - T的X.226建议中。一个表示协议的无连接版本被指定在ISO 9576标准中。
1.7 OSI Protocols Application Layer
The application layer implementation of the OSI protocol suite consists of various application entities. An application entity is the part of an application process that is relevant to the operation of the OSI protocol suite. An application entity is composed of the user element and the application service element (ASE). The user element is the part of an application entity that uses ASEs to satisfy the communication needs of the application process. The ASE is the part of an application entity that provides services to user elements and, therefore, to application processes. ASEs also provide interfaces to the lower OSI layers.
1.7 OSI协议应用层
在OSI协议栈的应用层实现包括各种应用实体。应用程序实体是,与OSI协议栈操作有关的,应用程序进程的一部分。一个应用程序的实体是由用户组件和ASE（应用服务组件）组成。用户组件是 应用程序实体的一部分，它使用的ASEs来满足应用程序进程的通信需要。ASE是一个应用程序实体的一部分,它向用户组件提供服务,因而也就是向应用程序进程提供了服务。ASEs也向OSI低层提供了接口。
图：一个应用程序进程依赖于PSAP和表示服务描述了单一应用进程的构成（由应用实体,用户组件,和ASEs组成）以及与PSAP和表示服务的关系。
图：一个应用程序进程依赖于PSAP和表示服务
Figure: An Application Process Relies on the PSAP and Presentation Service portrays the composition of a single application process (composed of the application entity, the user element, and the ASEs) and its relation to the PSAP and presentation service.
Figure: An Application Process Relies on the PSAP and Presentation Service
ASEs fall into one of the two following classifications: common-application service elements (CASEs) and specific-application service elements (SASEs). Both of these might be present in a single application entity.
ASEs分属下面两种类别之一：CASEs(公共应用服务组件)和SASEs(特殊应用服务组件)。两者都可以在单一应用实体中存在。
Common-Application Service Elements
Common-application service elements (CASEs) are ASEs that provide services used by a wide variety of application processes. In many cases, multiple CASEs are used by a single application entity. The following four CASEs are defined in the OSI specification:
公共应用服务组件
CASEs(公共应用服务组件)是一种ASEs,各种各样的应用程序进程使用它们提供的服务。在许多情况下，单一应用实体使用多个CASE。在OSI规范中定义了下面四种CASE：
Association control service element (ACSE) - Creates associations between two application entities in preparation for application-to-application communication
Remote operations service element (ROSE) - Implements a request-reply mechanism that permits various remote operations across an application association established by the ACSE
Reliable transfer service element (RTSE) - Allows ASEs to reliably transfer messages while preserving the transparency of complex lower-layer facilities
Commitment, concurrence, and recovery service elements (CCRSE) - Coordinates dialogues among multiple application entities.
关联控制服务组件（ACSE） --建立两个应用实体间的关联,为应用与应用间的通信作好准备。
远程操作服务组件（ROSE） --实现一个请求-应答机制,允许各种远程操作通过一个由ACSE建立的应用关联。
可靠传输服务组件（RTSE） --允许ASEs可靠地传递信息,同时保持复杂的低层设施的透明度。
承诺，同意和恢复服务组件（CCRSE） --多个应用实体的协商对话。
Specific-Application Service Elements
Specific-application service elements (SASEs) are ASEs that provide services used only by a specific application process, such as file transfer, database access, and order entry, among others.
特殊应用服务组件
特殊应用服务组件(SASEs)是一种ASEs,只提供服务给一个特殊应用进程,如文件传输,数据库访问,和订单输入,以及其它等。
OSI Protocols Application Processes
An application process is the element of an application that provides the interface between the application itself and the OSI application layer. Some of the standard OSI application processes include the following:
OSI协议应用进程s
一个应用进程是一个应用的组成部分,它提供应用本身和OSI应用层之间的接口。某些标准应用进程s包括以下几个部分：
Common management-information protocol (CMIP) - Performs network-management functions, allowing the exchange of management information between ESs and management stations. CMIP is specified in the ITU-T X.700 recommendation and is functionally similar to the Simple Network Management Protocol (SNMP) and NetView.
Directory services (DS) - Serves as a distributed directory that is used for node identification and addressing in OSI internetworks. DS is specified in the ITU-T X.500 recommendation.
File transfer, access, and management (FTAM) - Provides file-transfer service and distributed file-access facilities.
Message handling system (MHS) - Provides a transport mechanism for electronic messaging applications and other applications by using store-and-forward services.
Virtual terminal protocol (VTP) - Provides terminal emulation that allows a computer system to appear to a remote ES as if it were a directly attached terminal.
公共管理-信息协议（CMIP） --执行网络管理功能，允许在ESs和管理站s之间交换管理信息。CMIP是在ITU - T X.700建议中规定的,功能上类似于SNMP(简单网络管理协议)和 NetView。
目录服务s（DS） --用作分布式目录,用于OSI互联网络中节点的识别和寻址。DS是在ITU - T X.500建议中规定的。
文件传输，访问和管理（FTAM） --提供文件传输服务和分布式文件访问设施s。
消息处理系统（MHS） -提供一种传输机制,用于电子消息应用s以及使用存储-转发服务s的其他应用s。
虚拟终端协议（VTP） -提供终端仿真,允许一个计算机系统表现为一个远程ES,就好像它是一个直接附加的终端。
2. Review Questions
Q - What are the two routing protocols specified in the OSI suite?
A - End System-to-Intermediate System (ES-IS) and Intermediate System-to-Intermediate System (IS-IS).
Q - Describe the OSI connectionless network protocol.
A - OSI connectionless network service is implemented by using the Connectionless Network Protocol (CLNP) and Connectionless Network Service (CLNS). CLNP and CLNS are described in the ISO 8473 standard.
Q - Describe the OSI connection-oriented network protocol.
A - OSI connection-oriented network service is implemented by using the Connection-Oriented Network Protocol (CONP) and Connection-Mode Network Service (CMNS).
Q - How are requests to services at the session layer made within OSI protocols?
A - Requests are made at session-service access points (SSAPs), and SS-users are uniquely identified by using an SSAP address.
Q - Describe common-application service elements (CASEs).
A - Common-application service elements (CASEs) are ASEs that provide services used by a wide variety of application processes. In many cases, multiple CASEs are used by a single application entity.
Q - Name some of the media types that the OSI protocol suite supports.
A - IEEE 802.2 LLC, IEEE 802.3, Token Ring/IEEE 802.5, Fiber Distributed Data Interface (FDDI), and X.25.
Q - Why was the OSI protocol suite created?
A - The OSI specifications were conceived and implemented by two international standards organizations: the International Organization for Standardization (ISO) and the International Telecommunication Union-Telecommunications Standards Sector (ITU-T).
Q - Describe the session layer protocols within the OSI protocol suite.
A - The session layer implementation of the OSI protocol suite consists of a session protocol and a session service. The session protocol enables session-service users (SS-users) to communicate with the session service. An SS-user is an entity that requests the services of the session layer. Such requests are made at session-service access points (SSAPs), and SS-users are uniquely identified by using an SSAP address.
Q - Describe the presentation layer protocols of the OSI protocol suite.
A - The presentation layer implementation of the OSI protocol suite consists of a presentation protocol and a presentation service. The presentation protocol enables presentation-service users (PS-users) to communicate with the presentation service.
Q - What are the two types of ASEs?
A - ASEs fall into one of the two following classifications: common-application service elements (CASEs) and specific-application service elements (SASEs). Both of these might be present in a single application entity.
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二、Introduction to LAN ProtocolsLAN协议简介 P.19
三、Introduction to WAN Technologies广域网技术简介 P.22
四、Bridging and Switching Basics桥接和交换基础 P.26
五、Routing Basics路由基础知识 P.
六、Network Management Basics网络管理基础知识 P.
七、Open System Interconnection Protocols开放系统互连各协议 P.35
http://docwiki.cisco.com/wiki/Internetworking_Technology_Handbook
Internetworking Technology Handbook 互联网络联网技术手册概述
http://docwiki.cisco.com/wiki/Internetworking_Technology_Handbook
Internetworking Technology Handbook
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This article provides guidance for understanding internetworking technology. Different components of internetwork and the protocols used are described.
本文指导你理解互联网络联网技术。叙述了互联网络的各个组成部分和所使用的协议。
联网技术手册 指导目录
PART 1. Internetworking Basics
An internetwork is a collection of individual networks, connected by intermediate networking devices, that functions as a single large network. Internetworking refers to the industry, products, and procedures that meet the challenge of creating and administering internetworks.
PART 1.互联网络联网基础知识
互联网络是众独立网络的一个集合，通过中间联网设备s连接起来，其功能就像单一的大型网络。互联网络联网指的是,满足创建和管理互联网络所挑战的,工业,产品,和例程。
The following articles provide information about internetworking basics:
一、 Internetworking Basics
二、 Introduction to LAN Protocols
三、 Introduction to WAN Technologies
四、 Bridging and Switching Basics
五、 Routing Basics
六、 Network Management Basics
七、 Open System Interconnection Protocols
下面各篇文章提供了互联网络联网基础知识的有关资料：
一、 互联网络联网基础知识
二、 LAN协议简介
三、 广域网技术简介
四、 桥接和交换基础
五、 路由基础知识
六、 网络管理基础知识
七、 开放系统互连各协议
PART 2. LAN Technologies 局域网技术
A LAN is a high-speed data network that covers a relatively small geographic area. It typically connects workstations, personal computers, printers, servers, and other devices. LANs offer computer users many advantages, including shared access to devices and applications, file exchange between connected users, and communication between users via electronic mail and other applications.
局域网是一种高速数据网络，覆盖了相对较小的地理区域。它通常连接工作站,个人电脑,打印机,服务器和其他设备。对于计算机用户s而言,LANs提供了很多优点：包括对设备s和应用s的共享访问，已连接用户间交换文件，用户间通过电子邮件进行通信，以及其他应用。
The following articles provide information different LAN technologies:
下列文章提供了不同局域网技术的信息：
PART 3. WAN Technologies
A WAN is a data communications network that covers a relatively broad geographic area and that often uses transmission facilities provided by common carriers, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer.
PART 3.广域网技术
广域网是一个数据通信网络，覆盖了相对广阔的地理区域，通常使用由公共承运商,如电话公司,提供的传输设施。广域网技术一般作用在OSI参考模型低三层：物理层，数据链路层和网络层。
The following articles provide information about the various protocols and technologies used in WAN environments:
下列文章提供了用于广域网环境下的各种协议的有关资料：
PART 4. Internet Protocols Internet协议组
The Internet protocols are the world's most popular open-system (nonproprietary) protocol suite because they can be used to communicate across any set of interconnected networks and are equally well suited for LAN and WAN communications. The Internet protocols consist of a suite of communication protocols, of which the two best known are the Transmission Control Protocol (TCP) and the Internet Protocol (IP). The Internet protocol suite not only includes lower-layer protocols (such as TCP and IP), but it also specifies common applications such as electronic mail, terminal emulation, and file transfer. This article provides a broad introduction to specifications that comprise the Internet protocols. Discussions include IP addressing and key upper-layer protocols used in the Internet. Specific routing protocols are addressed individually later in this document.
互联网协议是世界上最流行的开放式系统（非专利）协议栈，因为它们可在遍及任意组已互联的网络上用于通信,而且也同样能很好地适合于LAN和WAN的通信。互联网协议s由一套通信协议组成，其中两个最有名协议是TCP（传输控制协议）和IP（互联网协议）。互联网协议栈不仅包括较低层协议s(如TCP和IP),而且也指定了,如电子邮件,终端仿真,以及文件传输等常见的应用协议。本文对含盖互联网协议栈的各种规范,提供一个广泛的介绍。讨论包括IP寻址,以及用于互联网的关键上层协议s。特定的路由协议是在本文档稍后单独讲解。
The following articles provide information about different IOS IP technologies:
下面的文章提供了有关各种OSI IP技术的有关资料：
PART 5. Bridging and Switching
Bridges and switches are data communication devices that operate principally at Layer 2 of the OSI reference model. As such, they are widely referred to as data link layer devices. Several kinds of bridging have proven important as internetworking devices. Transparent bridging is found primarily in Ethernet environments, while source-route bridging occurs primarily in Token Ring environments. Translational bridging provides translation between the formats and transit principles of different media types (usually Ethernet and Token Ring). Finally, source-route transparent bridging combines the algorithms of transparent bridging and source-route bridging to enable communication in mixed Ethernet/Token Ring environments. Today, switching technology has emerged as the evolutionary heir to bridging-based internetworking solutions. Switching implementations now dominate applications in which bridging technologies were implemented in prior network designs. Superior throughput performance, higher port density, lower per-port cost, and greater flexibility have contributed to the emergence of switches as replacement technology for bridges and as complements to routing technology.
PART 5. Bridging and Switching 桥接和交换
网桥和交换机是数据通信设备，主要操作在OSI参考模型的第二层。因此，它们被广泛称为数据链路层设备。若干种桥接已经证实了它们作为互联网络联网设备的重要性。透明桥接主要出现在以太网环境中，而源路由桥接主要发生在令牌环环境。桥接转换提供了不同媒体类型（通常是以太网和令牌环）的格式和传输原理之间的转换。最后，源路由透明桥接结合了透明桥接和源路由桥接两者的算法,以使混合的以太网/令牌环环境能实现通信。今天，交换技术已经出现,革命性地继承了基于桥接的互联网络联网的解决方案。以前曾经是在网络设计中采用桥接技术的地方, 现在,已是交换实现占统治地位的主要应用领域。卓越的吞吐性能，更高的端口密度，每端口成本的降低，和更大的灵活性,交换机的出现为替代网桥技术以及作为路由技术的补充手段作出了贡献。
The following articles provide information about the technologies employed in devices loosely referred to as bridges and switches:
下面的文章所提供的,是关于通常被泛称为网桥和交换机的设备,所采用的技术,的有关资料：
PART 6. Routing
Routing is the act of moving information across an internetwork from a source to a destination. Along the way, at least one intermediate node typically is encountered. Routing is often contrasted with bridging, which might seem to accomplish precisely the same thing to the casual observer. The primary difference between the two is that bridging occurs at Layer 2 (the link layer) of the OSI reference model, whereas routing occurs at Layer 3 (the network layer). This distinction provides routing and bridging with different information to use in the process of moving information from source to destination, so the two functions accomplish their tasks in different ways.
PART 6. Routing 路由
路由是一种活动,将信息从信源,通过互联网络,传送到信宿。沿此通路,通常至少遇到一个中间节点。泛泛一看,对比路由与桥接,似乎这两者要完成的是同样的事情。两者的主要区别在于,桥接发生在OSI参考模型第二层（链路层），而路由发生在第3层（网络层）。这一区别,使得路由和桥接,从源到目的地的过程中所要移动的信息,是不同的；因此，完成他们任务的方法也是不同的。
The following articles provide information different routing technologies:
下面的文章提供了不同路由技术的有关资料：
PART 7. Network Management 网络管理
Network management means different things to different people. In some cases, it involves a solitary network consultant monitoring network activity with an outdated protocol analyzer. In other cases, network management involves a distributed database, auto polling of network devices, and high-end workstations generating real-time graphical views of network topology changes and traffic. In general, network management is a service that employs a variety of tools, applications, and devices to assist human network managers in monitoring and maintaining networks.
网络管理对于不同的人们意味着不同的事情。在某些情况下，它包含一个单独的网络，使用无用协议分析器,以协助监视网络活动。在其他情况下，网络管理包括一个分布式数据库,网络设备的自动轮询,以及能生成网络拓扑变化和信流的实时图形视图的高端工作站s。一般来说，网络管理是一种服务，通过使用各种工具,应用程序和设备，协助网络管理人员监视和维护网络。
The following articles provide information different network management technologies:
下面的文章提供不同网络管理技术的有关资料：
PART 8.Voice/Data Integration Technologies 语音/数据集成技术
Voice/data integration is important to network designers of both service providers and enterprise. Service providers are attracted by the lower-cost model-the cost of packet voice is currently estimated to be only 20 to 50 percent of the cost of a traditional circuit-based voice network. Likewise, enterprise network designers are interested in direct cost savings associated with toll-bypass and tandem switching. Both are also interested in so-called "soft savings" associated with reduced maintenance costs and more efficient network control and management. Finally, packet-based voice systems offer access to newly enhanced services such as Unified Messaging and application control. These, in turn, promise to increase the productivity of users and differentiate services.
语音/数据集成,对于服务提供商和企业这两种网络设计者,是很重要的。吸引服务提供者的是低成本模型---分组语音的成本,目前估计只有传统的基于电路的语音网络成本的百分之20至50。.同样，企业网络设计者的兴趣在于,与收费的旁路和汇接交换相比,节约了直接成本。两者也有兴趣在降低维护成本和更有效的网络控制和管理的同时,还有所谓的“软节约”。最后，基于分组的语音系统能提供访问新的增强服务，如统一发布消息和统一控制应用。从而能承诺提高用户的工作效率和增进差异化服务。
Integration of voice and data technologies has accelerated rapidly in recent years because of both supply- and demand-side interactions. On the demand side, customers are leveraging investment in network infrastructure to take advantage of integrated applications such as voice applications. On the supply side, vendors have been able to take advantage of breakthroughs in many areas, including standards, technology, and network performance.
语音和数据的集成技术,由于供应和需求双方的互动,近年来进展非常迅速。在需求方面，客户正促进着网络基础设施投资的转化，以取得集成应用的优点,如语音应用。在供给方面，厂商已经能够在许多领域，包括标准,技术,和网络性能等方面，取得突破性的优势。
The following article provides information about Voice/Data Integration Technologies:
Voice/Data Integration Technologies
下面的文章提供了有关语音/数据集成技术的有关资料：
语音/数据集成技术
PART 9. Wireless Technologies 无线技术
Wireless communication is the transfer of information over a distance without the use of electrical conductors or "wires".[1] The distances involved may be short (a few meters as in television remote control) or long (thousands or millions of kilometers for radio communications). When the context is clear, the term is often shortened to "wireless". Wireless communication is generally considered to be a branch of telecommunications.
无线通信是一种,在某种距离下,不使用电导体或“电线”,就能实现信息传送。 [1]，距离可短（如电视遥控,短至数米）可长（数千或数百万公里的无线电通讯）。在上下文是明确的情况下，这个术语经常简称为“无线”。无线通讯通常认为是电信的一个分支。
It encompasses various types of fixed, mobile, and portable two way radios, cellular telephones, personal digital assistants (PDAs), and wireless networking. Other examples of wireless technology include GPS units, garage door openers and or garage doors, wireless computer mice, keyboards and headsets, satellite television and cordless telephones.
它包括各种类型的固定,移动和便携式双向无线电通讯设备,蜂窝电话,个人数字助理PDA(掌上电脑),和无线联网。无线技术的其他例子包括GPS设备，车库门开门闭,无线鼠标，键盘和耳机，卫星电视和无绳电话等等。
The following article provides information about Wireless Technologies:
下面的文章提供了有关无线技术的资料：
PART 10. Cable Access Technologies 有线接入技术
Historically, CATV has been a unidirectional medium designed to carry broadcast analog video channels to the maximum number of customers at the lowest possible cost. Since the introduction of CATV more than 50 years ago, little has changed beyond increasing the number of channels supported. The technology to provide high-margin, two-way services remained elusive to the operator.
历史上，有线电视是一种单向媒介,设计用来以最低的成本,承载模拟视频广播频道,传播给最大数量的客户。由于有线电视的引入是在50多年前，除了增加可支持频道的数量外并没有多大改变。提供高利润,双向服务的技术，对于经营者而言,仍然难以捉摸。
Cable television (CATV) is a unidirectional medium carrying broadcast analog video channels to the most customers possible at the lowest possible cost to the CATV service provider. Since the introduction of CATV more than 50 years ago, little has changed beyond increasing the number of channels supported.
The following article provides information about Cable Access Technologies:
下面的文章提供了有关有线接入技术的资料：
有线接入技术
Cable Access Technologies
PART 11. Dial-up Technology 拨号技术
Dialup is simply the application of the Public Switched Telephone Network (PSTN) to carry data on behalf of the end user. It involves customer premises equipment (CPE) device sending the telephone switch a phone number to direct a connection to. The AS3600, AS5200, AS5300, and AS5800 are all examples of routers that have the capability to run a PRI along with banks of digital modems. The AS2511, on the other hand, is an example of a router that communicates with external modems.
拨号是公共交换式电话网（PSTN）的简单应用,承载着代表最终用户利益的数据。它需要客户附属装备（CPE）设备发送一个指向要连接处的电话号码给电话交换机。AS3600，AS5200，AS5300和AS5800是路由器的所有实例,它们数字调制解调器堆一起,有能力运行一个PRI。另一方面，AS2511是一个与外部调制解调器通信的,路由器实例。
Since the time of Internetworking Technologies Handbook, 2nd edition, the carrier market has continued to grow, and there have been demands for higher modem densities. The answer to this need was a higher degree of interoperation with the telco equipment and the refinement of the digital modem: a modem capable of direct digital access to the PSTN. This has allowed the development of faster CPE modems that take advantage of the clarity of signal that the digital modems enjoy. The fact that the digital modems connecting into the PSTN through a PRI or a BRI can transmit data at more than 53 K using the V.90 communication standard attests to the success of the idea.
自从互联网络技术手册第二版以来，承运器市场持续增长，出现了对更高密度调制解调器的需求。对于这种需求的回答是一种高度互操作的电信设备和改进的数字调制解调器：一种调制解调器,有能力对PSTN进行直接数字访问。这就允许开发更快的,能取得清晰信号的CPE调制解调器,而这正是数字调制解调器所需要的。事实上，数字调制解调器通过PRI或BRI连接到PSTN,使用V.90通信标准，可以传输数据的速率超过53k波特率。证实了想法的成功。
The following article provides information about Dial-up Technology:
Dial-up Technology
下面的文章提供了有关拨号技术的资料：
拨号技术
PART 12. Security Technologies 安全技术
With the rapid growth of interest in the Internet, network security has become a major concern to companies throughout the world. The fact that the information and tools needed to penetrate the security of corporate networks are widely available has increased that concern.
随着对互联网的兴趣迅速增长，网络安全已成为整个世界各公司的主要关注。事实是,渗透到企业网络,破坏其安全性所用的信息和工具,已被广泛使用；增加了人们的担心。
Because of this increased focus on network security, network administrators often spend more effort protecting their networks than on actual network setup and administration. Tools that probe for system vulnerabilities, such as the Security Administrator Tool for Analyzing Networks (SATAN), and some of the newly available scanning and intrusion detection packages and appliances, assist in these efforts, but these tools only point out areas of weakness and may not provide a means to protect networks from all possible attacks. Thus, as a network administrator, you must constantly try to keep abreast of the large number of security issues confronting you in today's world. This article describes many of the security issues that arise when connecting a private network to the Internet.
由于这种网络安全的日益重视，网络管理员除了日常网络建立和网络管理外,常常要花费更多的精力来保护他们的网络。能探测系统薄弱环节的工具，如SATAN(用于分析网络的安全管理员工具),和一些新的用于扫描和入侵检测的软件包和装置，有助于管理员的努力效果；但这些工具只能指出的薄弱范围，还没有可能提供一种方法来保护网络免于所有可能的攻击。因此，作为一个网络管理员，你必须不懈地保持努力,以面对各种当今世界出现的大量安全问题。本文(指安全技术)将介绍许多出现在,当专用网络与互联网相连时,的安全问题。
The following article provides information about Security Technologies:
Security Technologies
下面的文章提供了有关安全技术的资料：
安全技术
PART 13. Quality of Service Networking 网络服务质量
Quality of Service (QoS) refers to the capability of a network to provide better service to selected network traffic over various technologies, including Frame Relay, Asynchronous Transfer Mode (ATM), Ethernet and 802.1 networks, SONET, and IP-routed networks that may use any or all of these underlying technologies. The primary goal of QoS is to provide priority including dedicated bandwidth, controlled jitter and latency (required by some real-time and interactive traffic), and improved loss characteristics. Also important is making sure that providing priority for one or more flows does not make other flows fail. QoS technologies provide the elemental building blocks that will be used for future business applications in campus, WAN, and service provider networks. This article outlines the features and benefits of the QoS provided by the Cisco IOS QoS.
服务质量（QoS）是指一种网络服务能力，对于建立在不同技术上的网络信流,包括帧中继,异步传输模式（ATM）,以太网和802.1网络,SONET,以及IP路由网络---其能使用任何或所有这些基础技术---，来为信流选择提供更好的服务。 QoS的主要目标是提供优先权，包括专用带宽,控制抖动和延迟（为某些实时性和交互性信流所要求的）,并改善损耗特性。.同样重要的是,确保为一个或多个流量提供优先权,而不会使其它流量失效。QoS技术提供的基本积木块，它们将在校园网,广域网,和服务提供商网络s中用于将来的业务应用。本文概述了Cisco IOS QoS 所具有的QoS的特点和效益。
The following articles provide information about Quality of Service:
Quality of Service Networking
Resource Reservation Protocol
下面的文章提供了有关服务质量的资料：
网络服务质量
资源预留协议
PART 14. Network Caching Technologies 网络缓存技术
Although the volume of Web traffic on the Internet is staggering, a large percentage of that traffic is redundant-multiple users at any given site request much of the same content. This means that a significant percentage of the WAN infrastructure carries the identical content (and identical requests for it) day after day. Eliminating a significant amount of recurring telecommunications charges offers an enormous savings opportunity for enterprise and service provider customers.
虽然在互联网上网站信流量大得惊人，信流中占很大的百分比是冗余的--在任意地点的多个用户,其申请的大部分内容是相同的。这意味着,广域网基础设施,日复一日地承载的内容,其相同的部分(以及相同的申请)占很大的比例。取消大量重复电信的资费,为企业和服务提供商客户,提供了巨大的节约机会。
Web caching performs the local storage of Web content to serve these redundant user requests more quickly, without sending the requests and the resulting content over the WAN.
网站缓存执行网站内容的本地存储,使这些冗余的用户申请更迅速,而不必再在广域网上发送申请及其内容。
The following article provides information about Network Caching Technologies:
Network Caching Technologies
下面的文章提供了有关网络缓存技术的资料：
网络缓存技术
PART 15.IBM Network Management IBM网络管理
IBM network management refers to any architecture used to manage IBM Systems Network Architecture (SNA) networks or Advanced Peer-to-Peer Networking (APPN) networks. IBM network management is part of the IBM Open-Network Architecture (ONA) and is performed centrally by using management platforms such as NetView and others. It is divided into five functions that are similar to the network management functions specified under the Open System Interconnection (OSI) model. This article summarizes the IBM network management functional areas, ONA network management architecture, and management platforms.
IBM网络管理是指用于管理IBM系统网络体系结构（SNA）网络或高级对等联网（APPN）网络上的任何体系结构。IBM网络管理是IBM的开放式网络架构（ONA）的一部分,是通过使用管理平台,诸如NetView和其他的,执行集中化。它分为五个功能,类似于指定在开放系统互连（OSI）模型下的网络管理功能。本文总结了IBM网络管理功能区s,ONA网络管理体系结构,和管理平台s。
下面的文章提供了有关IBM网络管理资源：
IBM网络管理
The following article provides information about IBM Network Management:
IBM Network Management
PART 16. Multiservice Access Technologies 多业务接入技术
Multiservice networking is emerging as a strategically important issue for enterprise and public service provider infrastructures alike. The proposition of multiservice networking is the combination of all types of communications, all types of data, voice, and video over a single packet-cell-based infrastructure. The benefits of multiservice networking are reduced operational costs, higher performance, greater flexibility, integration and control, and faster new application and service deployment.
多业务联网,在为企业和公共服务提供商安排基础设施方面,正作为一种战略上的重要议题而出现。多业务联网的命题是结合所有类型的通信，所有类型的数据,语音,和视频，承载在单一的基于分组包—信元的基础设施上。多业务联网的好处是降低运营成本，增强更高的性能,提高更大的灵活性,集成和控制，以及更快部署新的应用和服务。
The following article provides information about Multiservice Access Technologies:
Multiservice Access Technologies
下面的文章提供了有关多业务接入技术资料：
多业务接入技术
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本文指导你理解互联网络联网技术。叙述了互联网络的各个组成部分和所使用的协议s。
Contents
[hide]
Part1 Internetworking Basics 互联网络联网基本原理
Part2 LAN Technologies局域网技术
part3 WAN Technologies 广域网技术
part4 Internet Protocols 互联网协议
part5 Bridging and Switching 桥接和交换
part6 Routing 路由
part7 Network Management 网络管理
part8 Voice/Data Integration Technologies 声音/数据集成技术
part9 Wireless Technologies 无线技术
part10 Cable Access Technologies 有线接入技术
part11 Dial-up Technology 拨号技术
part12 Security Technologies 安全技术
part13 Quality of Service Networking 联网服务质量
part14 Network Caching Technologies 网络缓存技术
part15 IBM Network Management IBM网络管理
part16 Multiservice Access Technologies 多业务接入技术
第一部分 互联网络联网基础知识
互联网络是众独立网络的一个集合，通过中间联网设备s连接起来，其功能就像单一的大型网络。互联网络联网指的是,满足创建和管理互联网络所挑战的,工业,产品s,和例程s。
下面各篇文章提供了互联网络联网基础知识的有关资料：
互联网络联网基础知识
LAN协议简介
广域网技术简介
桥接和交换基础
路由基础知识
网络管理基础知识
开放系统互连各协议
第二部分 局域网技术
局域网是一种高速数据网络，覆盖了相对较小的地理区域。它通常连接工作站,个人电脑,打印机,服务器和其他设备。对于计算机用户s而言,LANs提供了很多优点：包括对设备s和应用s的共享访问，已连接用户间交换文件，用户间通过电子邮件进行通信，以及其他应用。
下列文章提供了不同局域网技术的信息：
以太网技术
令牌环网/IEEE 802.5
Fiber Distributed Data Interface(FDDI)（见第六篇：‘路由’）
第三部分 广域网技术
广域网是一个数据通信网络，覆盖了相对广阔的地理区域，通常使用由公共承运商,如电话公司,提供的传输设施。广域网技术一般运行在OSI参考模型的低三层：物理层，数据链路层和网络层。
下列文章提供了用于广域网环境下的各种协议的有关资料：
帧中继
高速串行接口
综合业务数字网
点至点协议
交换式多兆位数据服务
同步数据链路控制 及其派生协议s
X.25
数字用户线路
第四部分 Internet各协议
互联网协议是世界上最流行的开放式系统（非专利）协议栈，因为它们可在遍及任意组已互联的网络上用于通信,而且也同样能很好地适合于LAN和WAN的通信。互联网协议s由一套通信协议组成，其中两个最有名协议是TCP（传输控制协议）和IP（互联网协议）。互联网协议栈不仅包括较低层协议s(如TCP和IP),而且也指定了,如电子邮件,终端仿真,以及文件传输等常见的应用协议。本文对含盖互联网协议栈的各种规范,提供一个广泛的介绍。讨论包括IP寻址,以及用于互联网的关键上层协议s。特定的路由协议是在本文档稍后单独讲解。
下面的文章提供了有关各种OSI IP技术的有关资料：
Internet协议
AppleTalk
Banyan VINES
IBM系统网络体系结构协议
DECnet
简单组播路由协议
互联网协议组播
IPv6
第五部分 Bridging and Switching 桥接和交换
网桥和交换机是数据通信设备，主要操作在OSI参考模型的第二层。因此，它们被广泛称为数据链路层设备。若干种桥接已经证明了它们作为互联网络联网设备的重要性。透明桥接主要出现在以太网环境中，而源路由桥接主要发生在令牌环环境。桥接转换提供了不同媒体类型（通常是以太网和令牌环）的格式和传输原理之间的转换。最后，源路由透明桥接结合了透明桥接和源路由桥接两者的算法,以使混合的以太网/令牌环环境能实现通信。今天，交换技术已经出现,革命性地继承了基于桥接的互联网络联网的解决方案。现在,交换实现占统治地位的主要应用领域，以前曾经是在网络设计中采用桥接技术的地方。卓越的吞吐性能，更高的端口密度，降低每端口的成本，和更大的灵活性,交换机的出现为替代网桥技术以及作为路由技术的补充手段作出了贡献。
下面的文章所提供的,是关于通常被泛称为网桥和交换机的设备,所采用的技术,的有关资料：
透明桥接
混合媒体桥接
源路由桥接
异步传输模式交换
局域网交换和虚拟局域网
MPLS /标记交换
数据链路交换
标签交换
第六部分 Routing 路由
路由是一种活动,将信息从信源,通过互联网络,传送到信宿。沿此通路,通常至少遇到一个中间节点。泛泛一看,对比路由与桥接,似乎这两者要完成的是同样的事情。两者的主要区别在于,桥接发生在OSI参考模型第二层（链路层），而路由发生在第3层（网络层）。这一区别,使得路由和桥接,从源到目的地的过程中所要移动的信息,是不同的；因此，完成他们任务的方法也是不同的。
下面的文章提供了不同路由技术的有关资料：
光纤分布式数据接口(FDDI)
IBM系统s的网络体系结 构的路由
NetWare的链路服务协议
开放系统互连路由协议
开放最短路径优先
路由信息协议
边界网关协议
内部网关路由协议
增强式内部网关路由协议
Xerox网络系统
第七部分 Network Management 网络管理
.网络管理对于不同的人们意味着不同的事情。在某些情况下，它包含一个单独的网络，使用无用协议分析器,以协助监视网络活动。在其他情况下，网络管理包括一个分布式数据库,网络设备的自动轮询,以及能生成网络拓扑变化和信流的实时图形视图的高端工作站s。一般来说，网络管理是一种服务，通过使用各种工具,应用程序和设备，协助网络管理人员监视和维护网络。
下面的文章提供不同网络管理技术的有关资料：
虚拟专用网(VPN)
启用目录联网
远程监控
简单网络管理协议
第八部分 Voice/Data Integration Technologies 语音/数据集成技术
语音/数据集成,对于服务提供商和企业这两种网络设计者,是很重要的。吸引服务提供者的是低成本模型---分组语音的成本,目前估计只有传统的基于电路的语音网络成本的百分之20至50。.同样，企业网络设计者的兴趣在于,与收费的旁路和汇接交换相比,节约了直接成本。两者也有兴趣在降低维护成本和更有效的网络控制和管理的同时,还有所谓的“软节约”。最后，基于分组的语音系统提供访问新的增强服务，如消息统一发布和统一控制应用。这些，从而能承诺提高用户的工作效率和差异化服务。
语音和数据的集成技术,由于供应和需求双方的互动,近年来进展非常迅速。在需求方面，客户正促进着网络基础设施投资的转化，以取得集成应用的优点,如语音应用。在供给方面，厂商已经能够在许多领域，包括标准，技术和网络性能等方面,取得突破的优势。
下面的文章提供了有关语音/数据集成技术的有关资料：
语音/数据集成技术
第九部分 无线技术
无线通信是一种,在某种距离下,不使用电导体或“电线”,实现信息传送。[1]使用[1]，距离可短（如电视遥控,短至数米）可长（数千或数百万公里的无线电通讯）。.在上下文是明确的情况下，这个术语经常简称为“无线”。无线通讯通常认为是电信的一个分支。
它包括各种类型的固定,移动和便携式双向无线电通讯设备,蜂窝电话,个人数字助理PDA(掌上电脑),和无线联网。无线技术的其他例子包括GPS设备，车库门开闭合,无线鼠标，键盘和耳机，卫星电视和无绳电话等等。
下面的文章提供了有关无线技术的资料：
无线技术
第十部分 有线接入技术
历史上，有线电视是一种单向媒介,设计用来以最低的成本,承载模拟视频广播频道,传播给最大数量的客户。由于有线电视的引入是在50多年前，除了增加可支持频道的数量外并没有多大改变。提供高利润,双向服务的技术，对于经营者而言,仍然难以捉摸。
Cable television (CATV) is a unidirectional medium carrying broadcast analog video channels to the most customers possible at the lowest possible cost to the CATV service provider.有线电视（CATV）的执行是一个单向的媒介，以最低的成本向有线电视服务提供商播出模拟视频通道，以最可能的客户。 Since the introduction of CATV more than 50 years ago, little has changed beyond increasing the number of channels supported.由于有线电视超过50年前推出，并没有多大改变之外增加支持的通道数量。
下面的文章提供了有关有线接入技术的资料：
有线接入技术
第十一部分 拨号技术
拨号是公共交换式电话网（PSTN）的简单应用,承载着代表最终用户利益的数据。它需要客户附属装备（CPE）设备发送一个指向要连接处的电话号码给电话交换机。AS3600，AS5200，AS5300和AS5800是路由器的所有实例,它们数字调制解调器堆一起,有能力运行一个PRI。另一方面，AS2511是一个与外部调制解调器通信的,路由器实例。
自从互联网络技术手册第二版以来，承运器市场持续增长，出现了对更高密度调制解调器的需求。对于这种需求的回答是一种高度互操作的电信设备和改进的数字调制解调器：一种调制解调器,有能力对PSTN进行直接数字访问。这就允许开发更快的,能取得清晰信号的CPE调制解调器,而这正是数字调制解调器所需要的。事实上，数字调制解调器通过PRI或BRI连接到PSTN,使用V.90通信标准，可以传输数据的速率超过53k波特率,成功地证实了想法。
下面的文章提供了有关拨号技术的资料：
拨号技术
第十二部分 安全技术
随着对互联网的兴趣迅速增长，网络安全已成为整个世界各公司的主要关注。事实是,渗透到企业网络,破坏其安全性所用的信息和工具,已被广泛使用；增加了人们的担心。
由于这种网络安全的日益重视，网络管理员除了日常网络建立和网络管理外,常常要花费更多的精力来保护他们的网络。探测系统薄弱能力的工具，如SATAN(用于分析网络的安全管理员工具),和一些新的可用的扫描和入侵检测的软件包和装置，有助于管理员的努力，但这些工具只能指出的薄弱范围，还没有可能提供一种方法来保护网络免于所有可能的攻击。因此，作为一个网络管理员，你必须不懈地努力保持,同时面临在当今的世界里出现的,大量安全问题。本文(指安全技术)介绍了许多出现在专用网络连接到互联网时的安全问题。
下面的文章提供了有关安全技术的资料：
安全技术
第十三部分 网络服务质量
服务质量（QoS）是指一种网络服务能力，对于建立在不同技术上的网络信流,包括帧中继,异步传输模式（ATM）,以太网和802.1网络,SONET,以及IP路由网络---其能使用任何或所有这些基础技术---，来为信流选择提供更好的服务。 QoS的主要目标是提供优先权，包括专用带宽,控制抖动和延迟（为某些实时性和交互性信流所要求的）,并改善损耗特性。.同样重要的是,确保为一个或多个流量提供优先权,而不会使其它流量失效。QoS技术提供的基本积木块，它们将在校园网,广域网,和服务提供商网络s中用于将来的业务应用。本文概述了Cisco IOS QoS 所具有的QoS的特点和效益。
下面的文章提供了有关服务质量的资料：
网络服务质量
资源预留协议
第十四部分 网络缓存技术
虽然在互联网上网站信流量大得惊人，信流中占很大的百分比是冗余的---在任何所给位置的多个用户申请的大多数内容是相同的。这意味着,广域网基础设施,日复一日地,承载的内容,其相同的部分(以及相同的申请)占很大的比例。取消大量的重复电信资费,为企业和服务提供商客户,提供了巨大的节约机会。
网站缓存执行网站内容的本地存储,使这些冗余的用户申请更迅速,而不必再在广域网上发送申请和最终的内容。
下面的文章提供了有关网络缓存技术的资料：
网络缓存技术
第十五部分 IBM网络管理
IBM网络管理是指用于管理IBM系统网络体系结构（SNA）网络或高级对等联网（APPN）网络上的任何体系结构。IBM网络管理是IBM的开放式网络架构（ONA）的一部分,是通过使用管理平台,诸如NetView和其他的,执行集中化。它分为五个功能,类似于指定在开放系统互连（OSI）模型下的网络管理功能。本文总结了IBM网络管理功能区s,ONA网络管理体系结构,和管理平台s。
下面的文章提供了有关IBM网络管理资源：
IBM网络管理
第十六部分 多业务接入技术
多业务联网,在为企业和公共服务提供商的战略基础设施方面,正作为一种战略上的重要议题出现。多服务网络的命题是结合所有类型的通信，所有类型的数据,语音,和视频，承载在单一的基于分组包—信元的基础设施上。多业务联网的好处是降低运营成本，增强更高的性能,提高更大的灵活性,集成和控制，以及更快部署新的应用和服务。
下面的文章提供了有关多业务接入技术资料：
多业务接入技术
取自
“http://docwiki.cisco.com/wiki/Internetworking_Technology_Handbook”
分类 ：IOS技术手册
=======================================================================================================================================
PART1 Internetworking Basics 互联网络联网的基本原理
联网技术手册 指导目录
PART1 Internetworking Basics 互联网络联网基本原理
PART 1. Internetworking Basics
An internetwork is a collection of individual networks, connected by intermediate networking devices, that functions as a single large network. Internetworking refers to the industry, products, and procedures that meet the challenge of creating and administering internetworks.
1.互联网络联网基础知识
互联网络是众独立网络的一个集合，通过中间联网设备s连接起来，其功能就像单一的大型网络。互联网络联网指的是,满足创建和管理互联网络所挑战需要的工业,产品s,和例程s。
The following articles provide information about internetworking basics:
一． Internetworking Basics互联网络联网基础知识
二． Introduction to LAN ProtocolsLAN协议简介
三． Introduction to WAN Technologies广域网技术简介
四． Bridging and Switching Basics桥接和交换基础
五． Routing Basics路由基础知识
六． Network Management Basics网络管理基础知识
七． Open System Interconnection Protocols开放系统互连各协议
下面各篇文章提供了互联网络联网基础知识的有关资料：
一、 互联网络联网基础知识
二、 LAN协议简介
三、 广域网技术简介
四、 桥接和交换基础
五、 路由基础知识
六、 网络管理基础知识
七、 开放系统互连各协议
一Internetworking Basics 互联网络联网基础知识
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This article along with the next six articles acts as a foundation for the technology discussions that follow. In this article, some fundamental concepts and terms used in the evolving language of internetworking are addressed. In the same way that this book provides a foundation for understanding modern networking, this article summarizes some common themes presented throughout the remainder of this book. Topics include flow control, error checking, and multiplexing, but this article focuses mainly on mapping the Open System Interconnection (OSI) model to networking/internetworking functions, and also summarizing the general nature of addressing schemes within the context of the OSI model. The OSI model represents the building blocks for internetworks. Understanding the conceptual model helps you understand the complex pieces that make up an internetwork.
本文与下面的六篇文章一起(组成第一部分)用于讨论联网手册后继部分介绍的技术s的一个基础。在本文中,可以找到用于涉及互联网络联网语言的某些基本概念和术语。本手册提供了一种用于理解现代联网的基本方法。用此方法,本文概述了某些出现在通篇本手册其余部分的一般原理。论题包括流量控制，错误检验，和复用，但本文主要关注于,将OSI模型映射到网络互联/互联网络联网的功能，同时,也概述了OSI模型上下文内寻址策略的通性。OSI模型展示了用于互联网络的积木块。理解概念模型有助于理解构成一个互联网络的复杂条块。
Contents
[hide]
1 What Is an Internetwork? 什么是互联网络？
1.1 Figure: Different Network Technologies Can Be Connected to Create an Internetwork 图：不同的网络技术可以连接起来构成一个互联网络
1.2 History of Internetworking 互联网络联网的历史
1.3 Internetworking Challenges 互联网络联网的挑战
2 Open System Interconnection Reference Model 开放系统互联参考模型
2.1 Figure: The OSI Reference Model Contains Seven Independent Layers 图：OSI参考模型包括七个独立层
2.2 Characteristics of the OSI Layers OSI各层的特性
2.2.1 Figure: Two Sets of Layers Make Up the OSI Layers 图：两组层次构成OSI层次
2.3 Protocols 协议s
2.4 OSI Model and Communication Between Systems 系统间的OSI模型和通信
2.4.1 Interaction Between OSI Model Layers OSI模型各层间的相互作用
2.4.2 OSI Model Layers and Information Exchange OSI模型层次和信息交换
2.5 OSI Model Physical Layer OSI模型物理层
2.5.1 Figure: Physical Layer Implementations Can Be LAN or WAN Specifications 图：可能是LAN或WAN规范的物理层实现
2.6 OSI Model Data Link Layer OSI模型数据链路层
2.6.1 Figure: The Data Link Layer Contains Two Sublayers 图：数据链路层包含两个子层
2.7 OSI Model Network Layer OSI模型网络层
2.8 OSI Model Transport Layer OSI模型传输层
2.9 OSI Model Session Layer OSI模型会晤层
2.10 OSI Model Presentation Layer OSI模型表示层
2.11 OSI Model Application Layer OSI模型应用层
3 Information Formats 信息格式s
3.1 Figure: Data from Upper-Layer Entities Makes Up the Data Link Layer Frame 图：来自上层实体的数据构成数据链路层的帧
3.2 Figure: Three Basic Components Make Up a Network Layer Packet 图：三个基本组件构成一个网络层数据包
3.3 Figure: Two Components Make Up a Typical Cell 图：两个组件构成一个典型的信元
4 ISO Hierarchy of Networks 网络s的ISO分层结构
4.1 Figure: A Hierarchical Network Contains Numerous Components 图：一个分层网络包含许多组件
5 Connection-Oriented and Connectionless Network Services 面向连接和无连接的网络服务
6 Internetwork Addressing 互联网络寻址
6.1 Data Link Layer Addresses 数据链路层地址
6.1.1 Figure: Each Interface on a Device Is Uniquely Identified by a Data-Link Address 图：一个设备上的每个接口是用数据链路地址作唯一识别
6.2 MAC Addresses MAC地址
6.2.1 Figure: MAC Addresses, Data-Link Addresses, and the IEEE Sublayers of the Data Link Layer Are All Related 图：数据链路层的MAC地址,数据链路地址,和IEEE子层都是有关联的,
6.2.2 Figure: The MAC Address Contains a Unique Format of Hexadecimal Digits 图：MAC地址包含一个唯一的十六进制数的格式
6.3 Mapping Addresses 映射地址
6.4 Network Layer Addresses 网络层地址
6.4.1 Figure: Each Network Interface Must Be Assigned a Network Address for Each Protocol Supported 图：每个网络接口必须指定一个网络地址以用于每个所支持的协议
6.5 Hierarchical Versus Flat Address Space 对比分层和扁平地址
6.5.1 Figure: Hierarchical and Flat Address Spaces Differ in Comparison Operations 图：分层和扁平地址空间s在比较操作中的不同
6.6 Address Assignments 地址指定
6.7 Addresses Versus Names 对比地址s和名字s
7 Flow Control Basics 流量控制基本原理
8 Error-Checking Basics 错误检验基本原理
9 Multiplexing Basics 复用基本原理
9.1 Figure: Multiple Applications Can Be Multiplexed into a Single Lower-Layer Data Packet 图：多个应用可以复用到单一低层数据包中
9.2 Figure: Multiple Devices Can Be Multiplexed into a Single Physical Channel 图：多个设备可以复用到单一物理通道中
10 Standards Organizations 标准化组织
11 Summary 总结
12 Review Questions 复习问答
13 For More Information 更多的参考信息
1. What Is an Internetwork? 什么是互联网络？
An internetwork is a collection of individual networks, connected by intermediate networking devices, that functions as a single large network. Internetworking refers to the industry, products, and procedures that meet the challenge of creating and administering internetworks.
一个互联网络是独立网络的一种集合，用中间联网设备连接起来，能起到单一大型网络的作用。互联网络联网指的是,能满足创建和实施互联网络挑战的工业，产品s和规程s。
Figure 1.1: Different Network Technologies Can Be Connected to Create an Internetwork illustrates some different kinds of network technologies that can be interconnected by routers and other networking devices to create an internetwork.
图1-1举例说明了,某些不同种类的网络技术,可以通过路由器和其他联网设备互联起来，以创建一个互联网络。
1.1 Figure 1.1: Different Network Technologies Can Be Connected to Create an Internetwork
各种网络技术可以连接起来创建一个互联网络
1.2 History of Internetworking 互联网络联网的历史
The first networks were time-sharing networks that used mainframes and attached terminals. Such environments were implemented by both IBM's Systems Network Architecture (SNA) and Digital's network architecture.
首批网络是时分网络，使用主机s和附加的终端s。这样的环境是由IBM的SNA(系统网络结构)和DEC网络结构两者实现的。
Local-area networks (LANs) evolved around the PC revolution. LANs enabled multiple users in a relatively small geographical area to exchange files and messages, as well as access shared resources such as file servers and printers. LANs(局域网)是围绕着PC的变革演变而来。LANs使多个用户能在一个相对小的地理区域范围内交换文件和消息,以及共享资源，如文件服务器和打印机。
Wide-area networks (WANs) interconnect LANs with geographically dispersed users to create connectivity. Some of the technologies used for connecting LANs include T1, T3, ATM, ISDN, ADSL, Frame Relay, radio links, and others. New methods of connecting dispersed LANs are appearing everyday.
WANs(广域网)将LANs互联起来，为地理上分散的用户创建了可连接性。用于连接LANs的一些技术，包括有T1, T3, ATM, ISDN, ADSL, 帧中继, 无线链路, 以及其它。把分散的LANs连接起来的新方法每天都在出现。
Today, high-speed LANs and switched internetworks are becoming widely used, largely because they operate at very high speeds and support such high-bandwidth applications as multimedia and videoconferencing.
今天，高速LANs和交换式的互联网络能被广泛地使用，很大程度上是由于它们能以极高速度进行操作,并能支持高带宽的应用,如多媒体和视频会议。
Internetworking evolved as a solution to three key problems: isolated LANs, duplication of resources, and a lack of network management. Isolated LANs made electronic communication between different offices or departments impossible. Duplication of resources meant that the same hardware and software had to be supplied to each office or department, as did separate support staff. This lack of network management meant that no centralized method of managing and troubleshooting networks existed.
演变中的互联网络联网,已经成为三大关键问题的解决方案：相互隔离的LANs孤岛，资源重复，和网络缺乏管理。隔离的LANs孤岛使不同办公室和部门间不可能通信；资源重复意味着同样的硬件和软件必须给每个办公室和部门重复提供，就像以前分别支持各成员那样；网络缺乏管理就是没有集中化的方法来管理和排障现有的网络。
1.3 Internetworking Challenges互联网络联网的挑战
Implementing a functional internetwork is no simple task. Many challenges must be faced, especially in the areas of connectivity, reliability, network management, and flexibility. Each area is key in establishing an efficient and effective internetwork. 实现一个功能上实用的互联网络并不是简单的任务。必须面对很多挑战，特别是在连接性，可靠性，网络管理，和灵活性上。要建立一个效能高及实用性好的互联网络，每个方面都是关键。
The challenge when connecting various systems is to support communication among disparate technologies. Different sites, for example, may use different types of media operating at varying speeds, or may even include different types of systems that need to communicate.
当连接各种系统时，其挑战在于,要求处于根本不同的技术环境下的各种媒体和系统,都能支持相互通信。例如,不同地点可能使用类型不同,以不同速度运行的介质，或者,甚至可能包括不同类型的系统，却需要互相通信。
Because companies rely heavily on data communication, internetworks must provide a certain level of reliability. This is an unpredictable world, so many large internetworks include redundancy to allow for communication even when problems occur.
由于公司s非常依赖于数据通信,互联网络必须提供某种程度的可靠性。然而，这是一个不可预测的世界，所以许多大型互联网络包含了冗余性，甚至当问题发生时仍容许保持通信。
Furthermore, network management must provide centralized support and troubleshooting capabilities in an internetwork. Configuration, security, performance, and other issues must be adequately addressed for the internetwork to function smoothly. Security within an internetwork is essential. Many people think of network security from the perspective of protecting the private network from outside attacks. However, it is just as important to protect the network from internal attacks, especially because most security breaches come from inside. Networks must also be secured so that the internal network cannot be used as a tool to attack other external sites.
此外，在一个互联网络中，网络管理必须提供集中化的支持和排障能力。配置,安全,性能,和其他问题都必须适当地解决以使互联网络平稳地发挥作用。在互联网络中,安全是必须的。许多人认为网络安全是从保护私有网络的观点免于受到外部的攻击，然而更为重要的是,保护网络免于受到内部攻击，尤其是,因为大多数安全漏洞来自内部。网络s也必须是安全的,以使内部网络不能被用作攻击其他外部网点的工具。
Early in the year 2000, many major web sites were the victims of distributed denial of service (DDOS) attacks. These attacks were possible because a great number of private networks currently connected with the Internet were not properly secured. These private networks were used as tools for the attackers.
在2000年早期,许多主要网站曾是DDOS (分布式拒接服务)的受害者。这些攻击之所以能发生,是因为当前与互联网连接的大量私有网络并不安全。这些私有网络常被用作攻击者的工具。
Because nothing in this world is stagnant, internetworks must be flexible enough to change with new demands.
由于在这个世界上一切都在发展，互联网络s必须具有足够的灵活性以适应新的需求。(尤其是当前就急需适应对上层网络结构动态变化快速响应的迫切需求)
2.Open System Interconnection Reference Model开放系统互连参考模型
The Open System Interconnection (OSI) reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer. The OSI reference model is a conceptual model composed of seven layers, each specifying particular network functions. The model was developed by the International Organization for Standardization (ISO) in 1984, and it is now considered the primary architectural model for intercomputer communications. The OSI model divides the tasks involved with moving information between networked computers into seven smaller, more manageable task groups. A task or group of tasks is then assigned to each of the seven OSI layers. Each layer is reasonably self-contained so that the tasks assigned to each layer can be implemented independently. This enables the solutions offered by one layer to be updated without adversely affecting the other layers. The following list details the seven layers of the Open System Interconnection (OSI) reference model:
OSI参考模型叙述了信息是如何,从一台计算机中的应用软件,通过一个网络介质,移动到另一台计算机的应用软件中。OSI参考模型是一个概念模型,由七层组成，每一层指定了具体的网络功能。该模型由ISO于1984年开发，现被认为是用于计算机间通信的主要结构模型。OSI模型将联网计算机间移动信息的任务s,分成七个较小的,更易管理的任务组。一个任务或任务组于是被指定给七层OSI层的每一层。每一层合理地自我包含以使指定给每一层的任务可以独立完成。这就解决了当某层升级时不会反过来影响其他层。下面的列表详细列出了OSI参考模型的七层：(表示层中有些部分似乎应在会晤层之下(外层),例如,尤其是加密部分;否则会给会晤造成困难。或者,会晤宜采用统一的对等一致的方式,以独立于表示层。)
Note:
A handy way to remember the seven layers is the sentence "All people seem to need data processing." The beginning letter of each word corresponds to a layer.
有一种方便的方法用来记忆七层。即：英文句“All People Seem To Need Data Processing.”(“所有的人看来都需要数据处理”)中,每字第一个字母就是七层层名的第一个字母。
Figure 2.1: The OSI Reference Model Contains Seven Independent Layers illustrates the seven-layer OSI reference model.
图2-1 举例说明了七层OSI参考模型
2.1 Figure: The OSI Reference Model Contains Seven Independent Layers
OSI参考模型包含了七个独立层
2.2 Characteristics of the OSI Layers OSI层次特性
The seven layers of the OSI reference model can be divided into two categories: upper layers and lower layers.
OSI参考模型的七层可以分成两大类：上三层和低四层。
The upper layers of the OSI model deal with application issues and generally are implemented only in software. The highest layer, the application layer, is closest to the end user. Both users and application layer processes interact with software applications that contain a communications component. The term upper layer is sometimes used to refer to any layer above another layer in the OSI model.
上三层处理应用问题(整体处理有关应用的通信问题)，通常只用软件执行。最高的一层,应用层,最接近最终用户。用户s和应用层进程s,同时与包含有一个通信组件的应用软件相互作用。术语‘较高层’有时指的是OSI模型中另一层之上的任何层。
The lower layers of the OSI model handle data transport issues. The physical layer and the data link layer are implemented in hardware and software. The lowest layer, the physical layer, is closest to the physical network medium (the network cabling, for example) and is responsible for actually placing information on the medium.
下四层处理数据传输问题。物理层和数据链路层是用硬件和软件实现的。最低层,物理层,最接近物理网络介质(例如,网络布缆),其职责是将信息放到实际介质上去。
Figure: Two Sets of Layers Make Up the OSI Layers illustrates the division between the upper and lower OSI layers.
图2.2.1说明了上三层和下四层的区分。
2.2.1 Figure: Two Sets of Layers Make Up the OSI Layers
两组层次构成了OSI层次
2.3 Protocols 协议
The OSI model provides a conceptual framework for communication between computers, but the model itself is not a method of communication. Actual communication is made possible by using communication protocols. In the context of data networking, a protocol is a formal set of rules and conventions that governs how computers exchange information over a network medium. A protocol implements the functions of one or more of the OSI layers.
OSI模型提供了一个概念性的框架用于计算机间的通信，模型本身并不是一种通信的方法。实际通信可以使用通信协议s来完成。在数据联网的上下文中,一个协议是一组规则和约定的一种形式,管理计算机如何在一个网络介质上交换信息。一个协议执行OSI的一层或更多层的功能。
A wide variety of communication protocols exist. Some of these protocols include LAN protocols, WAN protocols, network protocols, and routing protocols. LAN protocols operate at the physical and data link layers of the OSI model and define communication over the various LAN media. WAN protocols operate at the lowest three layers of the OSI model and define communication over the various wide-area media. Routing protocols are network layer protocols that are responsible for exchanging information between routers so that the routers can select the proper path for network traffic. Finally, network protocols are the various upper-layer protocols that exist in a given protocol suite. Many protocols rely on others for operation. For example, many routing protocols use network protocols to exchange information between routers. This concept of building upon the layers already in existence is the foundation of the OSI model.
OSI有一大类范围很广的协议s。其中包括有LAN协议s,WAN协议s,网络协议s,和路由协议s。LAN协议操作在OSI模型的物理层和数据链路层，并定义了在各种LAN介质上的通信。WAN协议操作在最低三层，它们定义了在各种广域网介质上的通信。路由协议是网络层协议s，其职责是用于在路由器之间交换信息，以使路由器可以选择用于网络信流的适当通路。最后,网络协议栈是各种上层协议s，它们存在于所给的一组协议族中。许多协议依赖其它协议才可操作。例如,许多路由协议使用网络协议s来交换路由器之间的信息。这种建立在已经存在的层上(的执行结果上,所谓’服务’)的概念已经成为OSI模型的基础。
2.4 OSI Model and Communication Between Systems
Information being transferred from a software application in one computer system to a software application in another must pass through the OSI layers. For example, if a software application in System A has information to transmit to a software application in System B, the application program in System A will pass its information to the application layer (Layer 7) of System A. The application layer then passes the information to the presentation layer (Layer 6), which relays the data to the session layer (Layer 5), and so on down to the physical layer (Layer 1). At the physical layer, the information is placed on the physical network medium and is sent across the medium to System B. The physical layer of System B removes the information from the physical medium, and then its physical layer passes the information up to the data link layer (Layer 2), which passes it to the network layer (Layer 3), and so on, until it reaches the application layer (Layer 7) of System B. Finally, the application layer of System B passes the information to the recipient application program to complete the communication process.
被传输的信息,从一个计算机系统的应用软件到另一个计算机系统的应用软件,必须通过OSI层次······
2.4.1 Interaction Between OSI Model Layers OSI模型各层间的互相作用
A given layer in the OSI model generally communicates with three other OSI layers: the layer directly above it, the layer directly below it, and its peer layer in other networked computer systems. The data link layer in System A, for example, communicates with the network layer of System A, the physical layer of System A, and the data link layer in System B.
OSI模型中所给的一层,通常与其它三层相通信：直接在它上面的一层，直接在它下面的一层，以及它的对等层---它处在已联网的其它计算机系统中。例如,系统A的数据链路层与系统A的网络层,系统A的物理层,以及系统B的数据链路层相通信。
Figure2.4.1.1: OSI Model Layers Communicate with Other Layers illustrates this example. 图2.4.1.1举例说明了这一例子。
2.4.1.1 Figure: OSI Model Layers Communicate with Other Layers
OSI模型层次与其它OSI层次的通信
2.4.1.2 OSI Layer Services OSI层服务s
One OSI layer communicates with another layer to make use of the services provided by the second layer. The services provided by adjacent layers help a given OSI layer communicate with its peer layer in other computer systems. Three basic elements are involved in layer services: the service user, the service provider, and the service access point (SAP).
一个给定的OSI层,通过与其它层通信,来使用由次层所提供的服务。邻层提供的服务,帮助所给OSI层与它在其他计算机系统中的对等层实现通信。在层服务中有三个基本组成部分：服务用户,服务提供者,和服务访问点(SAP)(见图2.4.1.2)。
In this context, the service user is the OSI layer that requests services from an adjacent OSI layer. The service provider is the OSI layer that provides services to service users. OSI layers can provide services to multiple service users. The SAP is a conceptual location at which one OSI layer can request the services of another OSI layer.
在这种上下文关系中,服务用户是申请服务的OSI层，它申请来自相邻(下层)OSI层的服务。服务提供者则是提供服务s给服务用户s的OSI层。OSI层s可以提供服务给多个服务用户。SAP是一个概念性的位置，在那里，一个OSI层可以申请其它OSI层的服务s。(所谓概念性是否指只是原理性、理解性,而不是实体，没有如协议一类的东西可以明确具体实现？我们认为，对于OSI和SAPs而言，其实不应该仅限于此！OSI既然是一个框架，就应该有格式来支撑；也就是说，OSI框架就是体现实现的软件框架和硬件框架，这种框架的具体体现就应该在每层SAP的头部(和/或尾部)的统一格式,以及对格式的统一处理(即:协议)上。它们应该成为实体性的夹层，即所谓：As:1:P:1: :1:S:1:T:1:N:1:D:1:P，甚至还应该在最前面加上一个SAP夹层；而SAP不但要’承上---对等层前作还原服务’，而且要‘’启下---选下层功能或下层路由’，以实现双向传递。承上的控制信息可以放在头部，应在对等层释放；启下的控制信息可以放在尾部(SDU?服务数据单元)，应在本SAP中完成启下任务进入下邻层时释放。尾部的控制信息结构，可以取下邻层SAP作承上头部的格式和结构)
（话到这里，就值得继续说下去。SAP承上起下的工作，应该智能实现；更进一步，应该‘语义+智能’实现。从而隔离了两个(乃至多个)知识领域的人工交互(例如，需要专家和编程者的交互)。）
Figure: Service Users, Providers, and SAPs Interact at the Network and Data Link Layers illustrates how these three elements interact at the network and data link layers.
图2.4.1.2举例说明在网络层和数据链路层上这三个组成部分是如何相互作用的。
Figure 2.4.1.2: Service Users, Providers, and SAPs Interact at the Network and Data Link Layers
服务用户s,服务提供者s,和服务访问点s在网络层和数据链路层上的相互作用
2.4.2 OSI Model Layers and Information Exchange OSI模型层次和信息交换
The seven OSI layers use various forms of control information to communicate with their peer layers in other computer systems. This control information consists of specific requests and instructions that are exchanged between peer OSI layers. 七层OSI层,使用各种形式的控制信息,与相应的,在其它计算机系统s中的对等层s,进行通信。控制信息是由特殊的申请s和指令s组成，它们在对等的OSI层间进行交换。
Control information typically takes one of two forms: headers and trailers. Headers are prepended to data that has been passed down from upper layers. Trailers are appended to data that has been passed down from upper layers. An OSI layer is not required to attach a header or a trailer to data from upper layers.
(本层的)控制信息通常取两种形式之一：(本层的)头部和尾部。头部是由前加到来自上层的数据构成。尾部则后拼来自上层的数据。一个OSI层并不需要为来自于上层的数据加一个(上层的)头部或尾部。
Headers, trailers, and data are relative concepts, depending on the layer that analyzes the information unit. At the network layer, for example, an information unit consists of a Layer 3 header and data. At the data link layer, however, all the information passed down by the network layer (the Layer 3 header and the data) is treated as data. 头部,尾部,和数据都是相对的概念，取决于解析信息单元的层。例如,在网络层,一个信息单元是由层3的头部和数据组成。然而,在数据链路层,所有传输通过网络层(层3头部和数据)的信息都被当作数据处理。
In other words, the data portion of an information unit at a given OSI layer potentially can contain headers, trailers, and data from all the higher layers. This is known as encapsulation.
换句话说,一个所给OSI层的信息单元,都有可能包含来自上层s的头部,尾部,和数据。这称之为封装。(能够逐层把信息单元封装起来的依据,正是建立在OSI层次中,低层是高层的外围,层层包含,且各层间的功能独立,的基础上)
Figure 2.4.2.1: Headers and Data Can Be Encapsulated During Information Exchange shows how the header and data from one layer are encapsulated into the header of the next lowest layer.
图2.4.2.1显示了来自某层的头部和数据是如何被封装到邻接低层的头部 (?数据)的。
2.4.2.1 Figure: Headers and Data Can Be Encapsulated During Information Exchange 在信息交换期间头部和数据可以被封装起来
2.4.2.2 Information Exchange Process信息交换过程
The information exchange process occurs between peer OSI layers. Each layer in the source system adds control information to data, and each layer in the destination system analyzes and removes the control information from that data.
在OSI对等层之间发生信息交换。在源系统的每一层把控制信息加到数据中，而在目的系统的每一层则解析,并从数据中移去控制信息。
If System A has data from a software application to send to System B, the data is passed to the application layer. The application layer in System A then communicates any control information required by the application layer in System B by prepending a header to the data. The resulting information unit (a header and the data) is passed to the presentation layer, which prepends its own header containing control information intended for the presentation layer in System B. The information unit grows in size as each layer prepends its own header (and, in some cases, a trailer) that contains control information to be used by its peer layer in System B. At the physical layer, the entire information unit is placed onto the network medium.
若系统A······
The physical layer in System B receives the information unit and passes it to the data link layer. The data link layer in System B then reads the control information contained in the header prepended by the data link layer in System A. The header is then removed, and the remainder of the information unit is passed to the network layer. Each layer performs the same actions: The layer reads the header from its peer layer, strips it off, and passes the remaining information unit to the next highest layer. After the application layer performs these actions, the data is passed to the recipient software application in System B, in exactly the form in which it was transmitted by the application in System A.
在系统A中的物理层······
2.5 OSI Model Physical Layer OSI模型的物理层
The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems. Physical layer specifications define characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors. Physical layer implementations can be categorized as either LAN or WAN specifications.
物理层定义了电气的,机械的,过程的,和功能的规范，用于激活,维持,和去激活通信网络系统之间的物理链路。物理层规范定义了,如电压电平,电压变化的时间关系,物理数据速率,最大传输距离,以及物理连接器,等等的物理特性。(不同的物理介质有不同的物理特性。)物理层的实现,可以按LAN或WAN规范来分类。
Figure 2.5.1: Physical Layer Implementations Can Be LAN or WAN Specifications illustrates some common LAN and WAN physical layer implementations.
图2.5.1举例说明了一些通常的LAN和WAN物理层的实现。
2.5.1 Figure: Physical Layer Implementations Can Be LAN or WAN Specifications 物理层的实现可能是LAN规范，或者是WAN规范
2.6 OSI Model Data Link Layer OSI模型的数据链路层
The data link layer provides reliable transit of data across a physical network link. Different data link layer specifications define different network and protocol characteristics, including physical addressing, network topology, error notification, sequencing of frames, and flow control. Physical addressing (as opposed to network addressing) defines how devices are addressed at the data link layer. Network topology consists of the data link layer specifications that often define how devices are to be physically connected, such as in a bus or a ring topology. Error notification alerts upper-layer protocols that a transmission error has occurred, and the sequencing of data frames reorders frames that are transmitted out of sequence. Finally, flow control moderates the transmission of data so that the receiving device is not overwhelmed with more traffic than it can handle at one time.
数据链路层为通过一个物理网络链路的数据,提供数据的可靠运送。不同的数据链路规范定义不同的网络和协议特性，包括物理寻址,网络拓扑,错误通告,帧排序,和流量控制。物理寻址(不是网络寻址。LLC地址涵盖MAC地址。LLC地址不是网络地址。MAC地址与物理层紧密结合。)定义了设备在数据链路层中是如何地址址。网络拓扑(应在MAC层上)由数据链路层的规范s构成，通常定义设备s在物理上是如何连接的，如,在一个总线中或是在路环拓扑中。错误通告通知上层协议：一个传送错误已经发生,数据帧排序要重新再排,因为发送出列了。最后,流量控制平缓了数据的发送,以使接收设备不会因为信流太多,超过它能一次处理的能力而被淹没。
The Institute of Electrical and Electronics Engineers (IEEE) has subdivided the data link layer into two sublayers: Logical Link Control (LLC) and Media Access Control (MAC).
IEEE已经细分了数据链路层为两个子层：LLC(逻辑链路控制)和MAC(介质访问控制)。
Figure: The Data Link Layer Contains Two Sublayers illustrates the IEEE sublayers of the data link layer.
图2.6.1 举例说明了IEEE的数据链路层的子层。
2.6.1 Figure: The Data Link Layer Contains Two Sublayers
The Logical Link Control (LLC) sublayer of the data link layer manages communications between devices over a single link of a network. LLC is defined in the IEEE 802.2 specification and supports both connectionless and connection-oriented services used by higher-layer protocols. IEEE 802.2 defines a number of fields in data link layer frames that enable multiple higher-layer protocols to share a single physical data link. The Media Access Control (MAC) sublayer of the data link layer manages protocol access to the physical network medium. The IEEE MAC specification defines MAC addresses, which enable multiple devices to uniquely identify one another at the data link layer.
数据链路层的LLC子层管理一个网络的单一链路上的设备间的(一次完整的)通信。LLC定义在IEEE 802.2的规范中,支持用于高层协议s的无连接和面向连接服务这两者。IEEE 802.2在数据链路帧中定义了若干字段,使多个高层协议能共享单一物理数据链路。MAC子层 管理协议对物理网络介质的访问。IEEE MAC规范定义MAC地址,使多个设备能在数据链路层上被唯一地识别,以区分彼此。
2.7 OSI Model Network Layer OSI模型的网络层
The network layer defines the network address, which differs from the MAC address. Some network layer implementations, such as the Internet Protocol (IP), define network addresses in a way that route selection can be determined systematically by comparing the source network address with the destination network address and applying the subnet mask. Because this layer defines the logical network layout, routers can use this layer to determine how to forward packets. Because of this, much of the design and configuration work for internetworks happens at Layer 3, the network layer.
网络层定义了网络地址,它不同于MAC地址。某些网络层地址的实现,如IP(互联网协议),可以使用一种定义网络地址的方法,即路由选择能从系统上通过比较源网络地址和目的网络地址,并施加子网屏蔽来实现。由于该层定义了逻辑网络布置,路由器就可以使用该层以确定如何转发数据包。因此,对于互联网络的许多设计和配置,都发生在第三层(网络层)。
2.8 OSI Model Transport Layer OSI模型的传输层
The transport layer accepts data from the session layer and segments the data for transport across the network. Generally, the transport layer is responsible for making sure that the data is delivered error-free and in the proper sequence. Flow control generally occurs at the transport layer. 传输层从会晤层接收数据,并将数据分段,以便于传输通过网络。通常,传输层的职责在于保证以适当的次序交付没有错误的数据。流量控制通常发生在传输层。
Flow control manages data transmission between devices so that the transmitting device does not send more data than the receiving device can process. Multiplexing enables data from several applications to be transmitted onto a single physical link. Virtual circuits are established, maintained, and terminated by the transport layer. Error checking involves creating various mechanisms for detecting transmission errors, while error recovery involves acting, such as requesting that data be retransmitted, to resolve any errors that occur.
流量控制对设备间的数据传送进行管理，以使发送设备不会送出大于接收设备处理能力的数据。复用,使来自几个应用程序要发送的数据,能发送到单一网络链路上。通过传输层,虚电路就被建立,维持和终止。错误检查涉及产生用于检测传送错误的各种机制,而纠错恢复需要各种活动,如申请数据重新发送,分辨发生的任何错误。
The transport protocols used on the Internet are TCP and UDP.
在互联网上的传输层协议是TCP和UDP。
2.9 OSI Model Session Layer OSI模型的会晤层
The session layer establishes, manages, and terminates communication sessions. Communication sessions consist of service requests and service responses that occur between applications located in different network devices. These requests and responses are coordinated by protocols implemented at the session layer. Some examples of session-layer implementations include Zone Information Protocol (ZIP), the AppleTalk protocol that coordinates the name binding process; and Session Control Protocol (SCP), the DECnet Phase IV session layer protocol.
会晤层实现 建立,管理,和终止通信会晤。通信会晤是由服务申请和服务响应组成，它们发生在位于不同网络地点设备中的应用程序之间(即对等层之间)。这些申请和响应,由在会晤层实现的协议s来协调。会晤层实现的某些例子包括ZIP(区域信息协议), ,协调名字捆绑过程的AppleTalk协议,和作为DECnet 阶段4会晤层协议的SCP (会晤控制协议)。(会晤层的一个重要职责,是分配资源(建立),控制资源(管理),释放资源(终止))
2.10 OSI Model Presentation Layer OSI模型的表示层
The presentation layer provides a variety of coding and conversion functions that are applied to application layer data. These functions ensure that information sent from the application layer of one system would be readable by the application layer of another system. Some examples of presentation layer coding and conversion schemes include common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes. 表示层提供了各种编码和变换功能，它们用在应用层数据上。这些功能保证由一个系统的应用层送出的信息能被另一个系统的应用层读出。表示层编码和变换策略的一些例子,包括常见的数据表示格式，字符表示格式的变换,通常的数据压缩策略,以及常见的数据加密策略。
Common data representation formats, or the use of standard image, sound, and video formats, enable the interchange of application data between different types of computer systems. Conversion schemes are used to exchange information with systems by using different text and data representations, such as EBCDIC and ASCII. Standard data compression schemes enable data that is compressed at the source device to be properly decompressed at the destination. Standard data encryption schemes enable data encrypted at the source device to be properly deciphered at the destination.
使用通用的数据表示格式,或使用标准的图像,声音,和视频格式,使你能在不同类型的计算机系统间互相交换应用数据。变换策略则用于所交换的信息,使用了文本和数据格式不相同的表示系统,如EBCDIC和ASICC。标准数据压缩策略,使得在源设备处压缩的数据,在目的处能适当地解压出来。标准数据加密策略使在源设备处加密的数据在目的处能适当地解密出来。
Presentation layer implementations are not typically associated with a particular protocol stack. Some well-known standards for video include QuickTime and Motion Picture Experts Group (MPEG). QuickTime is an Apple Computer specification for video and audio, and MPEG is a standard for video compression and coding.
表示层的实现通常并不与具体的协议栈有关。一些著名的视频标准包括QuickTime和MPEG(移动图像专家组)。QuickTime是苹果公司用于视频和音频的一个规范,MPEG是一个用于视频压缩和编码的标准。
Among the well-known graphic image formats are Graphics Interchange Format (GIF), Joint Photographic Experts Group (JPEG), and Tagged Image File Format (TIFF). GIF is a standard for compressing and coding graphic images. JPEG is another compression and coding standard for graphic images, and TIFF is a standard coding format for graphic images.
著名的图形图像格式是GIF(图形交换格式),JPEG(联合照片专家组),和TIFF(标志的影像文件格式)。GIF是一个图形影像的压缩和编码标准，JPEG是另一个这样的标准，TIFF是用于图形影像的编码格式。
2.11 OSI Model Application Layer OAI模型的应用层
The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application.
应用层是最接近用户的OSI层，也就是说,OSI应用层及用户,直接与应用软件相互作用。
This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication.
该层与一种应用软件相互作用，其中软件起到一个通信组件的作用。这样的应用程序已经超出OSI模型的范围。应用层的功能通常包括识别通信伙伴s,确定资源可用性,以及同步通信。
When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer.
在识别通信伙伴s时,对于一个欲传送数据的应用而言,应用层确认通信伙伴的身份和可用性。在确定资源的可用性时，应用层必须判断,对于被申请的,已存在的通信,是否有足够的网络资源。在同步通信中,应用程序间的所有通信,需要协调,合作,这是通过应用层来管理的。
Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).
应用层实现的某些例子包括Telnet,FTP(文件传输协议),和SMTP(简单邮件传输协议)。
3. Information Formats 信息格式
The data and control information that is transmitted through internetworks takes a variety of forms. The terms used to refer to these information formats are not used consistently in the internetworking industry but sometimes are used interchangeably. Common information formats include frames, packets, datagrams, segments, messages, cells, and data units.
通过互联网络发送的数据和控制信息(源层各层建立的,用于对等的目的层的控制信息—通常用于服务--存放在该层的头部或尾部)可取多种格式。用来引用这些信息格式的术语,在互联网络联网中,并不统一,有时可以互相交换。普通信息格式包括帧,数据包,数据报,分段,消息,信元和数据单元。
A frame is an information unit whose source and destination are data link layer entities. A frame is composed of the data link layer header (and possibly a trailer) and upper-layer data. The header and trailer contain control information intended for the data link layer entity in the destination system. Data from upper-layer entities is encapsulated in the data link layer header and trailer.
术语帧是一个信息单元,其源和目的是数据链路层的实体。一帧是由数据链路层的头部(也可能是尾部)和上层数据组成。头部和尾部包含了拟用于目的系统中链路层实体的控制信息。来自上层实体的数据与本层头部或尾部封装在数据链路层中。
Figure 3.1: Data from Upper-Layer Entities Makes Up the Data Link Layer Frame illustrates the basic components of a data link layer frame.
图3.1举例说明了一个数据链路层帧的基本组成部分
3.1 Figure: Data from Upper-Layer Entities Makes Up the Data Link Layer Frame
A packet is an information unit whose source and destination are network layer entities. A packet is composed of the network layer header (and possibly a trailer) and upper-layer data. The header and trailer contain control information intended for the network layer entity in the destination system. Data from upper-layer entities is encapsulated in the network layer header and trailer.
术语分组包是一个信息单元,它的源和目的是网络层实体。一个分组包是由网络层的头部(也可能是一个尾部)和上层数据组成。头部和尾部包含控制信息,拟用于目的系统的网络层实体。来自上层实体的数据与本层头部或尾部封装在网络层中。
Figure: Three Basic Components Make Up a Network Layer Packet illustrates the basic components of a network layer packet.
图1-10说明了一个网络层分组包的三个基本组成部分。
3.2 Figure: Three Basic Components Make Up a Network Layer Packet
三个基本组成部分组成了一个网络层分组包
The term datagram usually refers to an information unit whose source and destination are network layer entities that use connectionless network service.
术语数据报通常指的是一个信息单元,其源和目的是网络层实体,它们使用无连接网络服务。
The term segment usually refers to an information unit whose source and destination are transport layer entities.
术语分段通常指的是一个信息单元,其源和目的是传输层实体。
A message is an information unit whose source and destination entities exist above the network layer (often at the application layer).
术语消息是一个信息单元,其源和目的实体存在于网络层之上(通常在应用层)。
A cell is an information unit of a fixed size whose source and destination are data link layer entities. Cells are used in switched environments, such as Asynchronous Transfer Mode (ATM) and Switched Multimegabit Data Service (SMDS) networks. A cell is composed of the header and payload. The header contains control information intended for the destination data link layer entity and is typically 5 bytes long. The payload contains upper-layer data that is encapsulated in the cell header and is typically 48 bytes long.
一个信元是一个固定尺寸的信息单元,其源和目的是数据链路层实体。信元s用于交换环境中，如ATM(异步传输模式),和SMDS(交换的多兆位数据服务,)网络。一个信元由头部和有效载荷组成。头部包含有控制信息,拟用于目的数据链路层实体,通常是5字节长。有效载荷包含上层数据,它们封装在信元头部,通常是48字节长。
The length of the header and the payload fields always are the same for each cell.
对于每一个信元,头部和有效载荷字段s的长度都是一样的。
Figure: Two Components Make Up a Typical Cell depicts the components of a typical cell.
图1-11说明了一个典型的信元的组成。
3.3 Figure: Two Components Make Up a Typical Cell
Data unit is a generic term that refers to a variety of information units. Some common data units are service data units (SDUs), protocol data units, and bridge protocol data units (BPDUs). SDUs are information units from upper-layer protocols that define a service request to a lower-layer protocol. PDU is OSI terminology for a packet. BPDUs are used by the spanning-tree algorithm as hello messages.
数据单元是一个一般性的术语,指的是各种信息单元。一些通常的数据单元是服务数据单元(SDUs),协议数据单元(PDUs),和桥协议数据单元(BPDUs)。SDUs是来自上层协议的信息单元,它们定义了一种申请一个低层协议的服务。PDU是OSI术语,用于一个数据包。BPDUs被生成树算法用作hello消息。
4. ISO Hierarchy of Networks ISO网络的体系结构
Large networks typically are organized as hierarchies. A hierarchical organization provides such advantages as ease of management, flexibility, and a reduction in unnecessary traffic. Thus, the International Organization for Standardization (ISO) has adopted a number of terminology conventions for addressing network entities. Key terms defined in this section include end system (ES), intermediate system (IS), area, and autonomous system (AS). 大型网络通常组织成体系结构。一个体系结构组织提供了这样的的优点,如易于管理,灵活性,和减少不必要的信流。于是ISO采用了若干个术语惯例,用于选择网络实体s。在本节中定义的关键术语包括ES(端系统,指端设备),IS(中间系统,指转发设备,如路由器等),和AS(自治系统)。（参见图4.1）
An ES is a network device that does not perform routing or other traffic forwarding functions. Typical ESs include such devices as terminals, personal computers, and printers. An IS is a network device that performs routing or other traffic-forwarding functions. Typical ISs include such devices as routers, switches, and bridges. Two types of IS networks exist: intradomain IS and interdomain IS. An intradomain IS communicates within a single autonomous system, while an interdomain IS communicates within and between autonomous systems. An area is a logical group of network segments and their attached devices. Areas are subdivisions of autonomous systems (AS's). An AS is a collection of networks under a common administration that share a common routing strategy. Autonomous systems are subdivided into areas, and an AS is sometimes called a domain.
一个端系统ES是一种网络设备,但它并不执行路由或其它信流转发功能s。通常ESs包括如终端,PC,和打印机这样的设备。一个中间系统IS也是一种网络设备,它执行路由或其他信流转功能。通常ISs包括如路由器,交换机,和路桥。有两类IS网络：域内IS与域间IS。域内IS在单一自治系统内通信,而域间IS则是包含在自治系统内和自治系统间通信。一个自治系统AS是一个网络集合,处于共享同一路由策略的统一管理。有时称AS为域。自治系统细分为区Area，一个区Area是网络段s及其附加设备s的逻辑组合。区是自治系统s的细分部分s。。
（将整个互联网络---包括TCP/IP,OSI,AppleTalk，乃至不断衍生和创建崭新类型的互联网络,如已经出现的并迅速往深广发展的物联网,和必将出现的星际网---能涵盖在全网一致的,层次结构化的,可物理寻址的分层结构中，是一个十分重要而迫切的科研论题。）
Figure 4.1: A Hierarchical Network Contains Numerous Components illustrates a hierarchical network and its components.
图4.1举例说明了一个体系结构的网络和组成部分。
4.1 Figure: A Hierarchical Network Contains Numerous Components
一个体系结构的网络包含许多组成部分
5. Connection-Oriented and Connectionless Network Services
面向连接和无连接网络服务
（属于通信层次的传输层,网络层,和数据链路层都有一个面向连接或无连接的问题。面向连接的物理层通常是虚电路连接。会晤层是面向连接的。）
In general, transport protocols can be characterized as being either connection-oriented or connectionless. Connection-oriented services must first establish a connection with the desired service before passing any data. A connectionless service can send the data without any need to establish a connection first. In general, connection-oriented services provide some level of delivery guarantee, whereas connectionless services do not
通常,传输协议s可以按面向连接或无连接的特性分类。面向连接的服务必须在传送任何数据前以所希望的服务首先建立起一种连接。一种无连接服务则不必建立连接就可以发送数据。一般说来,面向连接的服务s通过某种水平的交付保证,而无连接服务则没有。.
Connection-oriented service involves three phases: connection establishment, data transfer, and connection termination.
面向连接的服务涉及三个阶段: 连接建立,数据传输,和连接终止。
During connection establishment, the end nodes may reserve resources for the connection. The end nodes also may negotiate and establish certain criteria for the transfer, such as a window size used in TCP connections. This resource reservation is one of the things exploited in some denial of service (DOS) attacks. An attacking system will send many requests for establishing a connection but then will never complete the connection. The attacked computer is then left with resources allocated for many never-completed connections. Then, when an end node tries to complete an actual connection, there are not enough resources for the valid connection.
在连接建立阶段,端节点s可以保存用于连接的资源。端节点s也可以协商,建立用于传输的某些准则,如一个用于TCP连接的窗口大小。资源保存在某些'拒绝服务攻击s'中被利用的做法中的一个。一个攻击系统可以送出许多用于建立连接的资源申请,但从不完成连接。这样受攻击计算机的大量资源'被冻结',使正常连接因为没有足够的资源而无法建立.
The data transfer phase occurs when the actual data is transmitted over the connection. During data transfer, most connection-oriented services will monitor for lost packets and handle resending them. The protocol is generally also responsible for putting the packets in the right sequence before passing the data up the protocol stack.
数据传输阶段发生于当实际数据在连接上被发送时。在数据传输期间,大多数面向连接的服务s将监视数据包丢失和重发。协议通常也有责任,在传送数据前,以正确的次序把数据包放入堆栈中。
When the transfer of data is complete, the end nodes terminate the connection and release resources reserved for the connection.
连接终止阶段:当传输数据完成后,端节点s终止连接,释放用于连接所保存的资源。
Connection-oriented network services have more overhead than connectionless ones. Connection-oriented services must negotiate a connection, transfer data, and tear down the connection, whereas a connectionless transfer can simply send the data without the added overhead of creating and tearing down a connection. Each has its place in internetworks.
面向连接的网络服务s比无连接网络要有更多的开销。
6. Internetwork Addressing 互联网络寻址
Internetwork addresses identify devices separately or as members of a group. Addressing schemes vary depending on the protocol family and the OSI layer. Three types of internetwork addresses are commonly used: data link layer addresses, Media Access Control (MAC) addresses, and network layer addresses.
互联网络的地址可以用来识别单个设备,或将一组设备作为单个成员进行识别。寻址策略是可变的,取决于协议族和所处OSI层。通常使用三类互联网络地址：数据链路层地址,MAC(介质访问控制)地址,和网络层地址。
6.1 Data Link Layer Addresses 数据链路层地址
A data link layer address uniquely identifies each physical network connection of a network device. Data-link addresses sometimes are referred to as physical or hardware addresses. Data-link addresses usually exist within a flat address space and have a pre-established and typically fixed relationship to a specific device. 一个数据链路层地址唯一地识别每个物理网络连接的一个网络设备。数据链路地址有时指的是物理或硬件地址。数据链路地址通常存在于一个扁平地址空间内,与一个预建的指定设备通常有固定关系。(为什么不能把唯一对应LAN接口的MAC地址按物理寻址排列的思路分层结构化？而把唯一可识别的固定关系的扁平地址—即URI—作为物理对象的识别,留给物联网使用。)
End systems generally have only one physical network connection and thus have only one data-link address. Routers and other internetworking devices typically have multiple physical network connections and therefore have multiple data-link addresses. 端系统通常只有一个物理网络连接,于是就只有一个数据链路地址。路由器和其他互联网络联网设备通常有多个物理网络连接,因此就有多个数据链路地址。
Figure: Each Interface on a Device Is Uniquely Identified by a Data-Link Address illustrates how each interface on a device is uniquely identified by a data-link address.
图6.1.1举例说明了每个接口如何用一个数据链路地址唯一地识别。
6.1.1 Figure: Each Interface on a Device Is Uniquely Identified by a Data-Link Address
6.2 MAC Addresses MAC层地址
Media Access Control (MAC) addresses consist of a subset of data link layer addresses. MAC addresses identify network entities in LANs that implement the IEEE MAC addresses of the data link layer. As with most data-link addresses, MAC addresses are unique for each LAN interface.
MAC地址组成了数据链路层地址的一个子集。MAC地址识别LAN中的网络实体,该实体实现了数据链路层的IEEE MAC地址。如同大多数数据链路地址一样,MAC地址唯一地用于每个LAN接口。
Figure 6.2.1: MAC Addresses, Data-Link Addresses, and the IEEE Sublayers of the Data Link Layer Are All Related illustrates the relationship between MAC addresses, data-link addresses, and the IEEE sublayers of the data link layer.
图6.2.1举例说明了MAC地址,数据链路地址,和数据链路层的IEEE子层之间的关系。
6.2.1 Figure: MAC Addresses, Data-Link Addresses, and the IEEE Sublayers of the Data Link Layer Are All Related
MAC地址,数据链路地址,和IEEE的数据链路层的各子层相互间的关系
MAC addresses are 48 bits in length and are expressed as 12 hexadecimal digits. The first 6 hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor and thus comprise the Organizationally Unique Identifier (OUI). The last 6 hexadecimal digits comprise the interface serial number, or another value administered by the specific vendor. MAC addresses sometimes are called burned-in addresses (BIAs) because they are burned into read-only memory (ROM) and are copied into random-access memory (RAM) when the interface card initializes.
MAC地址是48位长度,用12位十六进制数字表示。十六进制数字的前6位是由IEEE管理的,用来识别制造商或售主,构成OUI(唯一的机构识别符)；后6位十六进制数字组成接口串号,或其它的为专门售主所管理的值。MAC地址有时叫做BIA(烙印地址),因为它们被烧制在ROM内,并在接口卡启动时被拷贝到RAM中。.
Figure 6.2.2: The MAC Address Contains a Unique Format of Hexadecimal Digits illustrates the MAC address format.
图6.2.2列举了MAC地址格式
6.2.2 Figure: The MAC Address Contains a Unique Format of Hexadecimal Digits MAC地址包含了一个唯一的十六进制数字格式
6.3 Mapping Addresses 映射地址
Because internetworks generally use network addresses to route traffic around the network, there is a need to map network addresses to MAC addresses. When the network layer has determined the destination station's network address, it must forward the information over a physical network using a MAC address. Different protocol suites use different methods to perform this mapping, but the most popular is Address Resolution Protocol (ARP).
因为互联网络通常使用网络地址来路由围绕着网络的信流,于是就有一种需要将网络地址映射到MAC地址。当网络层已经确定了目的站网络地址,就必须在物理网络上使用MAC地址转发信息。不同协议族使用不同方法来执行这种映射,但大多数流行的是ARP(地址解析协议)。
Different protocol suites use different methods for determining the MAC address of a device. The following three methods are used most often. Address Resolution Protocol (ARP) maps network addresses to MAC addresses. The Hello protocol enables network devices to learn the MAC addresses of other network devices. MAC addresses either are embedded in the network layer address or are generated by an algorithm.
不同的协议族使用不同的方法来确定一个设备的MAC地址。下面是最常使用的三种方法。ARP映射网络地址到MAC地址；Hello协议使网络设备获悉其他网络设备的MAC地址；MAC地址或被嵌入到网络层地址中,或由一种算法来产生。
Address Resolution Protocol (ARP) is the method used in the TCP/IP suite. When a network device needs to send data to another device on the same network, it knows the source and destination network addresses for the data transfer. It must somehow map the destination address to a MAC address before forwarding the data. First, the sending station will check its ARP table to see if it has already discovered this destination station's MAC address. If it has not, it will send a broadcast on the network with the destination station's IP address contained in the broadcast. Every station on the network receives the broadcast and compares the embedded IP address to its own. Only the station with the matching IP address replies to the sending station with a packet containing the MAC address for the station. The first station then adds this information to its ARP table for future reference and proceeds to transfer the data.
ARP是一种用于TCP/IP族的方法。当一个网络设备需要把数据送到同一网络的另一个设备时,它知道用于数据传输的源和目的地址。它必须在转发数据前将目的地址以某种方法映射到MAC地址。首先,发送站检查ARP表,了解是否已经发现了目的站点的MAC地址。如果还没有,就在网络上发送一个含有目的站点IP地址的广播。网络上的每个站点都会接收该广播并将广播中嵌入的IP地址与自身的地址相比较；只有地址匹配的站点,才会向原发送站返回一个包含自身MAC地址的数据包；原发送站于是就把该信息加到它自己的ARP表中,用于将来引用和实现数据传输。
When the destination device lies on a remote network, one beyond a router, the process is the same except that the sending station sends the ARP request for the MAC address of its default gateway. It then forwards the information to that device. The default gateway will then forward the information over whatever networks necessary to deliver the packet to the network on which the destination device resides. The router on the destination device's network then uses ARP to obtain the MAC of the actual destination device and delivers the packet. 如果目的设备位于远处,超过一个路由器,的网络，其处理过程相同,但除了发送站的ARP申请是针对默认网关的MAC地址。(它含在目的地址的子网掩码中)
The Hello protocol is a network layer protocol that enables network devices to identify one another and indicate that they are still functional. When a new end system powers up, for example, it broadcasts hello messages onto the network. Devices on the network then return hello replies, and hello messages are also sent at specific intervals to indicate that they are still functional. Network devices can learn the MAC addresses of other devices by examining Hello protocol packets.
Hello协议是网络层协议,它能使网络设备彼此相认,并指示它们保持联系。例如,当一个新的端设备启动时,它就在网络上广播hello消息,网络上的设备于是返回hello回答；hello消息也送往指定区间s以指明它们保持联系。网络设备s通过检查Hello协议数据包就能获悉其它设备的MAC地址。
Three protocols use predictable MAC addresses. In these protocol suites, MAC addresses are predictable because the network layer either embeds the MAC address in the network layer address or uses an algorithm to determine the MAC address. The three protocols are Xerox Network Systems (XNS), Novell Internetwork Packet Exchange (IPX), and DECnet Phase IV. 第三种方法是,有三种协议都是用来获悉可预测MAC地址。在这些协议族中,MAC地址是可预测的,因为或者在网络层地址中嵌入了MAC地址,或者使用了一种算法以确定MAC地址。三种协议是XNS(Xerox网络系统),IPX(Novell互联网络包交换),和DECnet阶段IV。
6.4 Network Layer Addresses 网络层地址
A network layer address identifies an entity at the network layer of the OSI layers. Network addresses usually exist within a hierarchical address space and sometimes are called virtual or logical addresses.
一个网络层地址识别OSI网络层上的一个实体。网络地址s通常位于一个结构地址空间内,有时称为虚地址或逻辑地址s。
The relationship between a network address and a device is logical and unfixed; it typically is based either on physical network characteristics (the device is on a particular network segment) or on groupings that have no physical basis (the device is part of an AppleTalk zone). End systems require one network layer address for each network layer protocol that they support. (This assumes that the device has only one physical network connection.) Routers and other internetworking devices require one network layer address per physical network connection for each network layer protocol supported. For example, a router with three interfaces each running AppleTalk, TCP/IP, and OSI must have three network layer addresses for each interface. The router therefore has nine network layer addresses.
一个网络地址和一个设备间的关系是逻辑的,不固定的；它或基于物理网络特性(其设备在一个具体网络段上),或基于非物理基础的归类(其设备是AppleTalk地带的一部分)。端设备需要一个网络层地址用于它们所支持的每一个网络层协议。(假定设备只连接一个物理网络。)路由器和其它互联网络联网设备,对于每一个它所支持的网络层协议的物理网络连接,需要一个网络层地址。例如,一个有三个接口的路由器,每一个都运行AppleTalk,TCP/IP,和 OSI,那么每一个接口必须有三个网络层地址,这样,路由器就有九个网络层地址。
不应该把高层的地址功能放到网络层地址上来实现。这已经违反了七层协议‘分层包含,分字段,功能独立'的原则。实际上，网络层分组包的头部已经设有‘协议’字段,就是为了把两者区分开来而设置的。于是每个接口只对应一个网络层地址
而不是三个，大大缩小了网络层的地址空间；而且两者功能各自独立,升级、修改和增添都大为方便。
Figure: Each Network Interface Must Be Assigned a Network Address for Each Protocol Supported illustrates how each network interface must be assigned a network address for each protocol supported.
图6.4.1举例说明了每一个网络接口是如何必须指定一个网络地址给所支持的每一种协议。
6.4.1 Figure: Each Network Interface Must Be Assigned a Network Address for Each Protocol Supported
每个网络接口必须赋予一个网络地址用于每个所支持的协议
6.5 Hierarchical Versus Flat Address Space
结构式地址空间对比扁平地址空间
Internetwork address space typically takes one of two forms: hierarchical address space or flat address space. A hierarchical address space is organized into numerous subgroups, each successively narrowing an address until it points to a single device (in a manner similar to street addresses). A flat address space is organized into a single group (in a manner similar to U.S. Social Security numbers).
互联网络地址空间通常取两种形式之一：分层结构式地址空间或扁平地址空间。一个结构式地址空间组织层许多(嵌套的)子组,它们逐级缩小地址范围,直至子组只指向单一设备(用一种类似于街道地址的方法)。一个扁平地址空间则组织成单一组(用一种类似于美国社会安全号码s)。
Hierarchical addressing offers certain advantages over flat-addressing schemes. Address sorting and recall is simplified using comparison operations. For example, "Ireland" in a street address eliminates any other country as a possible location.
结构式寻址提供某些比扁平寻址更好的优点。它可以使用比较操作,简化了分拣和记忆。例如,一个街道地址中的"爱尔兰岛",排除了任何其它国家再把它作为一个可能的重复位置。
Figure 6.5.1: Hierarchical and Flat Address Spaces Differ in Comparison Operations illustrates the difference between hierarchical and flat address spaces.
图6.5.1举例说明了结构式地址空间和扁平地址空间的不同。
6.5.1 Figure: Hierarchical and Flat Address Spaces Differ in Comparison Operations
6.6 Address Assignments 地址安排
Addresses are assigned to devices as one of two types: static and dynamic. Static addresses are assigned by a network administrator according to a preconceived internetwork addressing plan. A static address does not change until the network administrator manually changes it. Dynamic addresses are obtained by devices when they attach to a network, by means of some protocol-specific process. A device using a dynamic address often has a different address each time that it connects to the network. Some networks use a server to assign addresses. Server-assigned addresses are recycled for reuse as devices disconnect. A device is therefore likely to have a different address each time that it connects to the network.
安排给设备的地址可取两种类型之一：静态的和动态的。静态地址是通过网络管理员按照已拟互联网络寻址计划安排的。一个静态地址不会改变直至网络管理员手动改变它。动态地址则通过附加到网络上的一个设备根据某些协议的安排过程来获得的。使用动态地址的设备通常在每次连接网络时有一个不同的地址。有些网络使用一个服务器来安排地址。服务器安排的地址,当设备断开连接后,能回收再利用。因此,一个设备当它每次连接到网络时,可能有不同的地址。
6.7 Addresses Versus Names 地址对照名字
Internetwork devices usually have both a name and an address associated with them. Internetwork names typically are location-independent and remain associated with a device wherever that device moves (for example, from one building to another). Internetwork addresses usually are location-dependent and change when a device is moved (although MAC addresses are an exception to this rule). As with network addresses being mapped to MAC addresses, names are usually mapped to network addresses through some protocol. The Internet uses Domain Name System (DNS) to map the name of a device to its IP address. For example, it's easier for you to remember www.cisco.com instead of some IP address. Therefore, you type www.cisco.com into your browser when you want to access Cisco's web site. Your computer performs a DNS lookup of the IP address for Cisco's web server and then communicates with it using the network address.
互联网络设备通常既有一个名字,又有一个与之相关的地址。互联网络的名字通常是位置独立的,而且永随设备,不论设备移动到何处(例如,从一座建筑物到另一座)。互联网络地址则通常是位置有关的,当设备移动时是变化的(虽然MAC地址是这一规则的一个例外)。正如网络地址映射到MAC地址,名字通常通过某种协议映射到网络地址。互联网使用DNS(域名系统)来将一个设备的名字映射到它的IP地址上。例如,用www.cisco.com来代替某种它的IP地址就易于记忆。因此,当你要访问Cisco网站时,就输入www.cisco.com。计算机通过DNS查找相应IP地址,然后与之通信。
7. Flow Control Basics 流量控制基本原理
Flow control is a function that prevents network congestion by ensuring that transmitting devices do not overwhelm receiving devices with data. A high-speed computer, for example, may generate traffic faster than the network can transfer it, or faster than the destination device can receive and process it. The three commonly used methods for handling network congestion are buffering, transmitting source-quench messages, and windowing.
流量控制是一种功能,通过保证发送设备发送的数据不淹没接收设备,从而阻止网络拥塞。例如,一个高速计算机可以产生比网络传送更快的信流,或者快于目的设备能够接收和处理它的速度。三种通常用于处理网络拥挤的方法是缓冲,发送源收紧,,和变更窗口。
Buffering is used by network devices to temporarily store bursts of excess data in memory until they can be processed. Occasional data bursts are easily handled by buffering. Excess data bursts can exhaust memory, however, forcing the device to discard any additional datagrams that arrive.
缓冲是网络设备用来将过量的突发数据暂存于存储器中,直到它们能被处理。数据偶然突发是容易使用缓冲来处理的。然而过量的数据突发可以耗尽存储器迫使设备丢弃到达的任何额外的数据报。
Source-quench messages are used by receiving devices to help prevent their buffers from overflowing. The receiving device sends source-quench messages to request that the source reduce its current rate of data transmission. First, the receiving device begins discarding received data due to overflowing buffers. Second, the receiving device begins sending source-quench messages to the transmitting device at the rate of one message for each packet dropped. The source device receives the source-quench messages and lowers the data rate until it stops receiving the messages. Finally, the source device then gradually increases the data rate as long as no further source-quench requests are received.
收紧发送源的消息是接收设备用来通知发送源,以帮助阻止缓存器溢出。接收设备向发送源发出发出发送源收紧通知的消息,申请降低当前数据的发送速率。开始,接收设备由于缓存器溢出而丢弃数据,接着,接收设备对于每个被丢失的数据包以单消息的速率向发送设备发回收紧消息。发送源收到后就不断降低数据速率直至停止接收到消息s。之后,发生源逐步增加数据速率,只要收不到申请消息。
Windowing is a flow-control scheme in which the source device requires an acknowledgment from the destination after a certain number of packets have been transmitted. With a window size of 3, the source requires an acknowledgment after sending three packets, as follows. First, the source device sends three packets to the destination device. Then, after receiving the three packets, the destination device sends an acknowledgment to the source. The source receives the acknowledgment and sends three more packets. If the destination does not receive one or more of the packets for some reason, such as overflowing buffers, it does not receive enough packets to send an acknowledgment. The source then retransmits the packets at a reduced transmission rate.
收发窗是一种流量控制策略,其中源设备在发送了指定数(例如,三个)的数据包后就等待接收数据的目的设备返回认可消息,然后再发(三个)。如果发送源(在一定的时间间隔内)收不到认可,发送源就会降低发送速率重发。
8. Error-Checking Basics错误检验基本原理
Error-checking schemes determine whether transmitted data has become corrupt or otherwise damaged while traveling from the source to the destination. Error checking is implemented at several of the OSI layers.
错误检验策略确定被传送的数据,在传送过程中是否损毁或被损坏。错误检验可以在几个OSI层中实现。
One common error-checking scheme is the cyclic redundancy check (CRC), which detects and discards corrupted data. Error-correction functions (such as data retransmission) are left to higher-layer protocols. A CRC value is generated by a calculation that is performed at the source device. The destination device compares this value to its own calculation to determine whether errors occurred during transmission. First, the source device performs a predetermined set of calculations over the contents of the packet to be sent. Then, the source places the calculated value in the packet and sends the packet to the destination. The destination performs the same predetermined set of calculations over the contents of the packet and then compares its computed value with that contained in the packet. If the values are equal, the packet is considered valid. If the values are unequal, the packet contains errors and is discarded.
一个公共的错误检验策略是CRC(循环冗余校验)，它检测和丢弃被损坏的数据。错误纠正功能(如数据重发)则留给更高层的协议。CRC值是在源设备处通过计算产生。目的设备将此值与自己的计算值相比较,以确定在传输的过程中是否发生错误。······
9. Multiplexing Basics复用基本原理
Multiplexing is a process in which multiple data channels are combined into a single data or physical channel at the source. Multiplexing can be implemented at any of the OSI layers. Conversely, demultiplexing is the process of separating multiplexed data channels at the destination. One example of multiplexing is when data from multiple applications is multiplexed into a single lower-layer data packet.
复用是一种过程,将多个数据通道,在源端,组合到单一数据通道或物理通道中。复用可以在OSI任一层实现。解复用则是一个相反的过程,在目的端将复用的数据通道分解开来。复用的一个例子是来自多个应用的数据复用到低层单一数据分组包中。
Figure 9.1: Multiple Applications Can Be Multiplexed into a Single Lower-Layer Data Packet illustrates this example.
图9.1说明了这个例子。
9.1 Figure: Multiple Applications Can Be Multiplexed into a Single Lower-Layer Data Packet
多个应用可以复用到低层单一数据分组包
Another example of multiplexing is when data from multiple devices is combined into a single physical channel (using a device called a multiplexer).
复用的另一个例子是,来自多个设备的数据,组合到单一物理通道中(使用一个称为复用器的设备)。
Figure 9.2: Multiple Devices Can Be Multiplexed into a Single Physical Channel illustrates this example.
图9.2说明了这个例子。
9.2 Figure: Multiple Devices Can Be Multiplexed into a Single Physical Channel 多个设备可以复用到单一物理通道中
A multiplexer is a physical layer device that combines multiple data streams into one or more output channels at the source. Multiplexers demultiplex the channels into multiple data streams at the remote end and thus maximize the use of the bandwidth of the physical medium by enabling it to be shared by multiple traffic sources.
复用器是一种物理层设备,它在源端组合多个数据流到一个或更多的输出通道。在远程端的复用器解复用这些通道到多个数据流，于是,多个信流源共享同一介质,使之最大限度地使用物理介质的带宽。
Some methods used for multiplexing data are time-division multiplexing (TDM), asynchronous time-division multiplexing (ATDM), frequency-division multiplexing (FDM), and statistical multiplexing.
用于复用数据的一些方法是TDM(时分复用),ATDM(异步时分复用),FTDM(频分复用),和随机复用(码分复用)。
In TDM, information from each data channel is allocated bandwidth based on preassigned time slots, regardless of whether there is data to transmit. In ATDM, information from data channels is allocated bandwidth as needed by using dynamically assigned time slots. In FDM, information from each data channel is allocated bandwidth based on the signal frequency of the traffic. In statistical multiplexing, bandwidth is dynamically allocated to any data channels that have information to transmit.
在TDM中,来自每个数据通道的信息是基于预先指定时间槽来分配带宽的,不管有否数据发送。在ATDM中, 来自每个数据通道的信息是按需动态指定时间槽来分配带宽的。在FDM中, 来自每个数据通道的信息是基于信流的(载波)信号频率来分配带宽的。在随机复用中,带宽动态分配给有数据要发送的任何通道。
10. Standards Organizations标准s化的组织机构s
A wide variety of organizations contribute to internetworking standards by providing forums for discussion, turning informal discussion into formal specifications, and proliferating specifications after they are standardized.
各式各样的机构组织致力于互联网络组网的标准化。它们通过提供论坛来展开讨论,把非正式的讨论转化为正式的规范，并在标准化后推广和扩展规范。
Most standards organizations create formal standards by using specific processes: organizing ideas, discussing the approach, developing draft standards, voting on all or certain aspects of the standards, and then formally releasing the completed standard to the public.
大多数标准化组织是通过特别的程序产生标准的：规划概念,讨论方法,开发标准草案,就标准的全部或某方面进行投票表决,然后向公众发布完整的标准。
Some of the best-known standards organizations that contribute to internetworking standards include these:
一些著名的致力于互联网络联网标准的标准化组织有：
11. Summary总结
This article introduced the building blocks on which internetworks are built. Under-standing where complex pieces of internetworks fit into the OSI model will help you understand the concepts better. Internetworks are complex systems that, when viewed as a whole, are too much to understand. Only by breaking the network down into the conceptual pieces can it be easily understood. As you read and experience internetworks, try to think of them in terms of OSI layers and conceptual pieces.
本文介绍了建立互联网联网的积木块。能够了解复杂的互联网络条块相当于OSI模型的何处,将有助于更好地理解概念。互联网络是复杂系统,如果看作为一个整体,要了解的实在是太多了。只有将网络分成概念的条块,可能就易于理解。当你在了解和体验互联网络时,试着从OSI层次和概念条块的角度来思考它们。
Understanding the interaction between various layers and protocols makes designing, configuring, and diagnosing internetworks possible. Without understanding of the building blocks, you cannot understand the interaction between them.
理解了各层和各协议间的相互作用就有可能实现设计,配置,和诊断互联网络。不理解积木块,就无法理解它们间的相互作用。
12. Review Questions复习问答
Q - What are the layers of the OSI model?
A - Application, presentation, session, transport, network, data link, physical. Remember the sentence "All people seem to need data processing."
Q - Which layer determines path selection in an internetwork?
A - Layer 3, the network layer.
Q - What types of things are defined at the physical layer?
A - Voltage levels, time of voltage changes, physical data rates, maximum transmission distances, physical connectors, and type of media.
Q - What is one method of mapping network addresses to MAC addresses?
A - ARP, Hello, predictable.
Q - Which includes more overhead, connection-oriented or connectionless services?
A - Connection-oriented.
13. For More Information 了解更多信息
Cisco's web site (www.cisco.com) is a wonderful source for more information about these topics. The Documentation section includes in-depth discussions on many of the topics covered in this article.
Teare, Diane. Designing Cisco Networks. Indianapolis: Cisco Press, July 1999.
Retrieved from "http://docwiki.cisco.com/wiki/Internetworking_Basics"
Category:IOS Technology Handbook
Retrieved from "http://docwiki.cisco.com/wiki/Internetworking_Basics "
Category :IOS Technology Handbook分类 ：IOS技术手册
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二． Introduction to LAN Protocols 介绍LAN协议
联网技术手册 指导目录
Contents
http://docwiki.cisco.com/wiki/Introduction_to_LAN_Protocols
二 ． Introduction to LAN Protocols 介绍LAN(局域网)协议
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This article introduces the various media-access methods, transmission methods, topologies, and devices used in a local-area network (LAN). Topics addressed focus on the methods and devices used in Ethernet/IEEE 802.3, Token Ring/IEEE 802.5, and Fiber Distributed Data Interface (FDDI). Subsequent articles in LAN Protocols (Part 2) address specific protocols in more detail.
本文介绍了用于局域网(LAN)的各种媒体访问方法,传送方法,拓扑,和设备。所讲的题目集中在用于以太网/IEEE 802.3，令牌环/IEEE 802.5，和FDDI(光纤分布式数据接口)。在手册第二部分LAN协议各篇文章中将更详细地讲述具体的协议。
Figure 1: Three LAN Implementations Are Used Most Commonly illustrates the basic layout of these three implementations.
图1 三种最普遍使用的LAN实现 举例说明了这三种实现的基本布置。
Contents
[hide]
1 Figure: Three LAN Implementations Are Used Most Commonly
图1.：三种最常见的局域网实现
2 What Is a LAN? 什么是局域网？
3 LAN Protocols and the OSI Reference Model 局域网协议和OSI参考模型
3.1 Figure: Popular LAN Protocols Mapped to the OSI Reference Model
图3.1：流行的局域网协议与OSI参考模型的映射关系
4 LAN Media-Access Methods 局域网介质访问方法
5 LAN Transmission Methods 局域网传送方法
6 LAN Topologies 局域网拓扑
6.1 Figure: Some Networks Implement a Local Bus Topology
图6.1：某些网络实现本地总线拓扑
6.2 Figure: Some Networks Implement a Logical Ring Topology
图6.2：某些网络实现本地逻辑环网拓扑
6.3 Figure: A Logical Tree Topology Can Contain Multiple Nodes
图6.3：一个逻辑树形拓扑可以包含多个节点
7 LAN Devices 局域网设备
7.1 Figure: A Repeater Connects Two Network Segments
图7.1：一个中继器连接两段网络段
7.2 Figure: Multiple LAN Extenders Can Connect to the Host Router Through a WAN 图7.2：多个局域网扩展器可以通过广域网连接到主机路由器
8 Review Questions 复习问答
9 For More Information 参阅更多信息
1. Figure: Three LAN Implementations Are Used Most Commonly
图1 三种最普遍使用的LAN实现
2. What Is a LAN? LAN是什么？
A LAN is a high-speed data network that covers a relatively small geographic area. It typically connects workstations, personal computers, printers, servers, and other devices. LANs offer computer users many advantages, including shared access to devices and applications, file exchange between connected users, and communication between users via electronic mail and other applications.
LAN是一个高速数据网络,它覆盖了相对较小的地理区域。它通常连接工作站s,PC,打印机,服务器,和其它设备。
对计算机用户而言,LANs有很多优点,包括共享所访问的设备s和应用s,已连接用户间的文件交换,以及用户间经由电子邮件和其它(包括多媒体)应用s的通信。
3. LAN Protocols and the OSI Reference Model
LAN协议s及其OSI参考模型
LAN protocols function at the lowest two layers of the OSI reference model, as discussed in article Internetworking Basics between the physical layer and the data link layer.
LAN协议是在OSI参考模型的最低两层上起作用,如在第一章'互联网络联网的基本原理'中所讨论的,在物理层和数据链路层之间。
Figure: Popular LAN Protocols Mapped to the OSI Reference Model illustrates how several popular LAN protocols map to the OSI reference model.
Figure: Popular LAN Protocols Mapped to the OSI Reference Model
4. LAN Media-Access Methods LAN介质访问方法
Media contention occurs when two or more network devices have data to send at the same time. Because multiple devices cannot talk on the network simultaneously, some type of method must be used to allow one device access to the network media at a time. This is done in two main ways: carrier sense multiple access collision detect (CSMA/CD) and token passing.
介质抢占发生在两个或更多的设备,要在同一时间发送数据情况下。由于多个设备不能在网络上同时交谈,必须使用某种方法以允许在同一时间内只有一个设备访问网络介质。这可以用两种主要的方法来完成: CSMA/CD(载波感知多重访问/冲突检出)和令牌传送。
In networks using CSMA/CD technology such as Ethernet, network devices contend for the network media. When a device has data to send, it first listens to see if any other device is currently using the network. If not, it starts sending its data. After finishing its transmission, it listens again to see if a collision occurred. A collision occurs when two devices send data simultaneously. When a collision happens, each device waits a random length of time before resending its data. In most cases, a collision will not occur again between the two devices. Because of this type of network contention, the busier a network becomes, the more collisions occur. This is why performance of Ethernet degrades rapidly as the number of devices on a single network increases.
在使用CSMA/CD技术,如以太网,的网络中,网络设备为网络介质而争夺。当一个设备有数据要发送,它就要侦听是否有其它设备当前在使用网络。如果没有,它就开始发送数据。在完成发送后,它再次侦听是否有竞争发生。当两个设备同时发送数据时,竞争就发生了。此时,每个设备在发送它的数据前先等待一段随机时间间隔。在大多数的情况下,两个设备间的竞争不会再发生。然而这类网络越忙碌,竞争就越多,这就是为什么以太网在单一网络中随着设备数量的增加而性能迅速退化。
In token-passing networks such as Token Ring and FDDI, a special network frame called a token is passed around the network from device to device. When a device has data to send, it must wait until it has the token and then sends its data. When the data transmission is complete, the token is released so that other devices may use the network media. The main advantage of token-passing networks is that they are deterministic. In other words, it is easy to calculate the maximum time that will pass before a device has the opportunity to send data. This explains the popularity of token-passing networks in some real-time environments such as factories, where machinery must be capable of communicating at a determinable interval.
在令牌传送网络,如令牌环和FDDI,一种特殊的帧叫令牌帧,围绕网络,从一个设备到另一个设备,被传递着。当一个设备有数据要传送,它必须等待令牌到来,然后发送数据,完成后,释放令牌,以便其它设备使用。令牌传递网络是确定性的。换句话说,在设备有机会发送数据前,它易于计算传递的最长时间。这就解释了令牌传递网络s在某些实时环境中为何流行,如在工厂里,装置必须能够在可确定的时间区间内进行通信。
For CSMA/CD networks, switches segment the network into multiple collision domains. This reduces the number of devices per network segment that must contend for the media. By creating smaller collision domains, the performance of a network can be increased significantly without requiring addressing changes.
网络交换机将网络分段成几个区段。
对于CSMA/CD,这就减少了一个区段中必须为使用媒体而竞争的设备数目。通过创建较小的竞争区段,一个网络的性能可以显著地增加而不需要改变寻址；(并可引进全双工)。
Normally CSMA/CD networks are half-duplex, meaning that while a device sends information, it cannot receive at the time. While that device is talking, it is incapable of also listening for other traffic. This is much like a walkie-talkie. When one person wants to talk, he presses the transmit button and begins speaking. While he is talking, no one else on the same frequency can talk. When the sending person is finished, he releases the transmit button and the frequency is available to others.
通常CSMA/CD是半双工的,即当一个设备发送信息时它无法同时接收信息。它在说话时也无法侦听其它信流。这很像步话机。
When switches are introduced, full-duplex operation is possible. Full-duplex works much like a telephone-you can listen as well as talk at the same time. When a network device is attached directly to the port of a network switch, the two devices may be capable of operating in full-duplex mode. In full-duplex mode, performance can be increased, but not quite as much as some like to claim. A 100-Mbps Ethernet segment is capable of transmitting 200 Mbps of data, but only 100 Mbps can travel in one direction at a time. Because most data connections are asymmetric (with more data traveling in one direction than the other), the gain is not as great as many claim. However, full-duplex operation does increase the throughput of most applications because the network media is no longer shared. Two devices on a full-duplex connection can send data as soon as it is ready.
当引进交换机以后,全双工就可能了。全双工十分像电话,可以同时听和说。当一个网络设备直接加到一个网络交换机接口时,两个设备就能运行在全双工模式下。尽管全双工能提高性能,但没有声称的那么多。···两个全双工连接的设备一旦准备好就能立刻发送。
Token-passing networks such as Token Ring can also benefit from network switches. In large networks, the delay between turns to transmit may be significant because the token is passed around the network.
令牌传递网络也能从网络交换机中得益。在大型网络中，轮转发送由于令牌围绕着网络传递而可能延迟显著。(交换机可显著降低令牌传递延迟。)
5. LAN Transmission Methods LAN发送方法
LAN data transmissions fall into three classifications: unicast, multicast, and broadcast. In each type of transmission, a single packet is sent to one or more nodes.
LAN数据发送分成三类: 单播,组播,和广播。在每类传送中,单个数据包被送到一个或更多节点。
In a unicast transmission, a single packet is sent from the source to a destination on a network. First, the source node addresses the packet by using the address of the destination node. The package is then sent onto the network, and finally, the network passes the packet to its destination.
单播传送是在一个网络中将单个数据包从信源送到信宿。首先,源节点使用目的节点地址作为数据包的地址；然后,数据包被送到网络上；最后,网络传递数据包到目的地。
A multicast transmission consists of a single data packet that is copied and sent to a specific subset of nodes on the network. First, the source node addresses the packet by using a multicast address. The packet is then sent into the network, which makes copies of the packet and sends a copy to each node that is part of the multicast address. 一个组播传送是将单一数据包复制并送往网络的一组指定节点组成的子集。首先,源节点使用组播地址作为数据包地址；然后,数据包被送往网络,实现数据包复制,并将拷贝送到属于组播地址范围内的每个节点上。
A broadcast transmission consists of a single data packet that is copied and sent to all nodes on the network. In these types of transmissions, the source node addresses the packet by using the broadcast address. The packet is then sent on to the network, which makes copies of the packet and sends a copy to every node on the network. 一个广播传送是将单一数据包复制并送到网络的所有节点上。源节点使用广播地址作为数据包地址,然后将包送向网络,实现数据包复制,并将拷贝送到网络的每个节点上。
6. LAN Topologies LAN拓扑
LAN topologies define the manner in which network devices are organized. Four common LAN topologies exist: bus, ring, star, and tree. These topologies are logical architectures, but the actual devices need not be physically organized in these configurations. Logical bus and ring topologies, for example, are commonly organized physically as a star. A bus topology is a linear LAN architecture in which transmissions from network stations propagate the length of the medium and are received by all other stations. Of the three most widely used LAN implementations, Ethernet/IEEE 802.3 networks-including 100BaseT-implement a bus topology, which is illustrated inFigure 6.1: Some Networks Implement a Local Bus Topology.
LAN拓扑定义了网络设备逻辑上如何组织起来的方法。这些拓扑是逻辑结构,实际的设备在物理上并不需要按照这些配置来组织。有四种LAN拓扑：总线,环形,星形,和树形。一个总线拓扑是一种线性LAN结构,从网络站发送,沿着介质的长度方向传播,并为所有其它站点所接收。三种最广泛使用的LAN实现之一,以太网/IEEE 802.3网络---包括100BaseT---实现了一种总线拓扑,它举例说明在下图6.1中。
Figure 6.1: Some Networks Implement a Local Bus Topology
A ring topology is a LAN architecture that consists of a series of devices connected to one another by unidirectional transmission links to form a single closed loop. Both Token Ring/IEEE 802.5 and FDDI networks implement a ring topology.
环形拓扑是一种LAN结构,它是由一串设备按单向传送链路彼此连接构成单个封闭回路。令牌环/IEEE 802.5和FDDI网络实现了一种环形拓扑。图2-4说明了一种逻辑环形拓扑。,,
Figure: Some Networks Implement a Logical Ring Topology depicts a logical ring topology.
Figure: Some Networks Implement a Logical Ring Topology
A star topology is a LAN architecture in which the endpoints on a network are connected to a common central hub, or switch, by dedicated links. Logical bus and ring topologies are often implemented physically in a star topology, which is illustrated in the following figure.
星形拓扑是一种LAN结构,其中网络的各端点都通过专有链路s连向一个公共的中心集线器,或交换机。逻辑总线拓扑和环形拓扑被用于星形拓扑的物理实现，图2-5作了举例说明。
A tree topology is a LAN architecture that is identical to the bus topology, except that branches with multiple nodes are possible in this case.
树形拓扑是一种与总线拓扑一样的拓扑,唯其可以带有多节点的分叉。图2-5举例说明了一种逻辑树形拓扑。
Figure: A Logical Tree Topology Can Contain Multiple Nodes illustrates a logical tree topology.
Figure: A Logical Tree Topology Can Contain Multiple Nodes
7. LAN Devices LAN设备
Devices commonly used in LANs include repeaters, hubs, LAN extenders, bridges, LAN switches, and routers.
通常用于LAN的设备包括中继器,集线器,LAN扩展器,网桥,交换机,和路由器。
Note:
Repeaters, hubs, and LAN extenders are discussed briefly in this section. The function and operation of bridges, switches, and routers are discussed generally in "Bridging and Switching Basics" and "Routing Basics."
中继器,集线器,和LAN扩展器在本节中作简单的讨论。网桥,交换机,和路由器在第四章,'桥接和交换的基本原理',作一般性的讨论。
A repeater is a physical layer device used to interconnect the media segments of an extended network. A repeater essentially enables a series of cable segments to be treated as a single cable. Repeaters receive signals from one network segment and amplify, retime, and retransmit those signals to another network segment. These actions prevent signal deterioration caused by long cable lengths and large numbers of connected devices. Repeaters are incapable of performing complex filtering and other traffic processing. In addition, all electrical signals, including electrical disturbances and other errors, are repeated and amplified. The total number of repeaters and network segments that can be connected is limited due to timing and other issues.
中继器是物理层的设备,用于一个扩展网络介质段的互连.中继器本质上使一连串缆线段处理成像一条缆线。中继器从一个网络段接收信号,放大,再定时,再发送这些信号到另一个网络段。通过这些作用,阻止了信号由于缆线太长和连接设备太多而引起的退化。中继器不可能执行复杂的过滤和其他的信流处理。另外,所有电气信号,包括电气干扰和其他错误,都被中继和放大。用中继器连接起来的网络段总数和中继器总数,由于时间同步引进其他方面的问题,是有限制的。
Figure: A Repeater Connects Two Network Segments illustrates a repeater connecting two network segments.
下图 举例说明了一个中继器连接了两段网络段。
Figure: A Repeater Connects Two Network Segments
A hub is a physical layer device that connects multiple user stations, each via a dedicated cable. Electrical interconnections are established inside the hub. Hubs are used to create a physical star network while maintaining the logical bus or ring configuration of the LAN. In some respects, a hub functions as a multiport repeater.
集线器是物理层设备,它连接着多个用户站,每一个都连上一条专用线缆。电气交换建立在集线器内部。集线器用于建立一个物理的星形网络,维持着LAN的逻辑总线或环网的配置。在某些方面,一个集线器的功能,就像是一个多口中继器。
A LAN extender is a remote-access multilayer switch that connects to a host router. LAN extenders forward traffic from all the standard network layer protocols (such as IP, IPX, and AppleTalk) and filter traffic based on the MAC address or network layer protocol type. LAN extenders scale well because the host router filters out unwanted broadcasts and multicasts. However, LAN extenders are not capable of segmenting traffic or creating security firewalls.
LAN扩展器是一个远端访问的多层交换机,它在逻辑上连接着一个主机路由器。LAN
扩展器根据所有标准网络层协议(如IP,IPX,和AppleTalk)转发信流,过滤出基于MAC地址或网络层协议类型的信流。LAN扩展器能很好地缩放,因为主机路由器能过滤出不需要的广播和组播。然而,不能分段信流或建立安全的防火墙。
Figure: Multiple LAN Extenders Can Connect to the Host Router Through a WAN illustrates multiple LAN extenders connected to the host router through a WAN.
下图 举例说明了通过一个WAN连接了主机路由器的多个LAN扩展器。
Figure: Multiple LAN Extenders Can Connect to the Host Router Through a WAN 多个LAN扩展器可以通过一个广域网连接到主机路由器
8. Review Questions 复习问答s
Q - Describe the type of media access used by Ethernet.
叙述以太网所使用的介质访问的类型。
A - Ethernet uses carrier sense multiple access collision detect (CSMA/CD). Each network station listens before and after transmitting data. If a collision is detected, both stations wait a random time before trying to resend.
以太网使用CSMA/CD。每个网站在发送数据前,后都在侦听。如果碰撞被检出,各发送网站就各自等待一段随机时间后再发送。
Q - Describe the type of media access used by Token Ring.
叙述令牌环网所使用的介质访问的类型。
A - Token Ring passes a special type of packet called a token around the network. If a network device has data to send, it must wait until it has the token to send it. After the data has been sent, the token is released back on the network.
令牌环网传递一个特殊类型的数据包,称为令牌,顺次在网络设备上环绕。如果一个网络设备有数据要发送,它必须等待,直到它有了送至该设备的令牌。数据发送后,令牌就被释放回网络。
Q - Describe unicast, multicast, and broadcast transmissions.
说明单播,组播,和广播传送。
A - A unicast is a transmission from one source to one destination. A multicast is a transmission from one source to many stations that register to receive the traffic. A broadcast is a transmission from one source to every station on the local network segment.
单播是一种从一个信源到信宿的传送。组播是一种从一个信源到许多注册要接收信流的站点的传送。广播是一种从一个信源到本地网段每个站点的传送。
9. For More Information了解更多的信息
Cisco's web site (www.cisco.com) is a wonderful source for more information about these topics. The Documentation section includes in-depth discussions on many of the topics covered in this article.
Cisco网站(www.cisco.com)是一个极好的,作为有关这些论题方面更多信息的来源的网站。其文档部分包含了在本章中所覆盖的许多论题的深入讨论。
Teare, Diane. Designing Cisco Networks. Indianapolis: Cisco Press, July 1999.
Retrieved from "http://docwiki.cisco.com/wiki/Introduction_to_LAN_Protocols"
Category:IOS Technology Handbook
====================================================================
三．Introduction to WAN Technologies介绍广域网技术
联网技术手册 指导目录
Contents
http://docwiki.cisco.com/wiki/Introduction_to_WAN_Technologies
This page was last modified on 17 December 2009, at 21:54
三．Introduction to WAN Technologies介绍WAN技术
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This article introduces the various protocols and technologies used in wide-area network (WAN) environments. Topics summarized here include point-to-point links, circuit switching, packet switching, virtual circuits, dialup services, and WAN devices. Chapters in Part 3, “WAN Protocols,” address specific technologies in more detail.
本章介绍了用于WAN(广域网)环境的各种协议和技术。在这里概述的题目包括点对点链路，电路交换，分组包交换，虚电路，拨号服务，和WAN设备。在手册第3部分,‘WAN协议’,的各章里将更详细地讲述专门技术。
Contents
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1 What Is a WAN? 什么是广域网？
1.1 Figure: WAN Technologies Operate at the Lowest Levels of the OSI Model 图1.1：广域网技术运行在OSI模型的最低三层
2 Point-to-Point Links 点对点链路
2.1 Figure: A Typical Point-to-Point Link Operates Through a WAN to a Remote Network
图2.1：一个通过广域网到远程网的,典型的点对点链路的运行
3 Circuit Switching 电路交换
3.1 Figure: A Circuit-Switched WAN Undergoes a Process Similar to That Used for a Telephone Call
图3.1：一个电路交换的广域网经历了类似于用于电话呼叫的过程
4 Packet Switching 分组交换
4.1 Figure: Packet Switching Transfers Packets Across a Carrier Network
图4.1：分组交换将数据包传输通越一个承载网络
5 WAN Virtual Circuits 广域网虚电路
6 WAN Dialup Services 广域网拨号电路
7 WAN Devices 广域网设备
7.1 WAN Switch 广域网交换机
7.1.1 Figure: Two Routers at Remote Ends of a WAN Can Be Connected by WAN Switches
图7.1.1广域网远程两端的两个路由器可以通过广域网交换机连接起来
7.2 Access Server 接入服务器
7.2.1 Figure: An Access Server Concentrates Dial-Out Connections into a WAN 图7.2.1：一个接入服务器将各拨出连接连到一个广域网内
7.3 Modem 调制解调器
7.3.1 Figure: A Modem Connection Through a WAN Handles Analog and Digital Signals
图7.3.1：一个通过广域网的调制解调器连接,处理模拟和数字信号
7.4 CSU/DSU通道服务单元/数字服务单元
7.4.1 Figure: The CSU/DSU Stands Between the Switch and the Terminal
图7.4.1：CSU/DSU置于交换机和终端之间
7.5 ISDN Terminal Adapter ISDN终端适配器
7.5.1 Figure: The Terminal Adapter Connects the ISDN Terminal Adapter to Other Interfaces
图7.5.1：终端适配器将ISDN终端适配器连接到其他接口
8 Review Questions复习问答
9 For More Information提供更多的信息
1. What Is a WAN? 什么是WAN(广域网)?
A WAN is a data communications network that covers a relatively broad geographic area and that often uses transmission facilities provided by common carriers, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer.Figure: WAN Technologies Operate at the Lowest Levels of the OSI Model illustrates the relationship between the common WAN technologies and the OSI model.
WAN(广域网)是一种数据通信网络,覆盖了一个相对广阔的地理区域,并通常使用由公共承运商,如电话公司,提供的传送设施。WAN技术通常在OSI参考模型的低三层起作用：物理层,数据链路层,和网络层。图1.1举例说明了常用WAN技术和OAI模型之间的关系。
（本章讲述了一大堆的术语和具体设备的名字，应该是互联网络联网长期进展的历史过程中形成的产物。似乎予人以堆积、少章、杂乱的感觉。既然基本原理是一种概念性的思路，可否这样来归纳一下：······）
1.1 Figure: WAN Technologies Operate at the Lowest Levels of the OSI Model
图1.1：广域网技术在OSI模型的最低三层上起作用
2. Point-to-Point Links 点对点链路
A point-to-point link provides a single, pre-established WAN communications path from the customer premises through a carrier network, such as a telephone company, to a remote network. Point-to-point lines are usually leased from a carrier and thus are often called leased lines. For a point-to-point line, the carrier allocates pairs of wire and facility hardware to your line only. These circuits are generally priced based on bandwidth required and distance between the two connected points. Point-to-point links are generally more expensive than shared services such as Frame Relay.Figure: A Typical Point-to-Point Link Operates Through a WAN to a Remote Network illustrates a typical point-to-point link through a WAN.
点对点链路提供了单一的预建的WAN通信通路,从用户附加的设备,通过一个承运商网络,如电话公司,到一个远端网络。点对点线路通常从一个承运商处租借而来,因而通常称为租借线路。对于一个租借线路,承运商只给你分配线对和设施硬件。这些电路,通常是基于所需要的带宽和两个连接点间的距离来计价。点对点链路s通常要比共享服务,如帧中继,更贵。
图2.1举例说明了一个典型的通过一个WAN的点对点链路。
2.1 Figure: A Typical Point-to-Point Link Operates Through a WAN to a Remote Network
3. Circuit Switching 电路交换
Switched circuits allow data connections that can be initiated when needed and terminated when communication is complete. This works much like a normal telephone line works for voice communication. Integrated Services Digital Network (ISDN) is a good example of circuit switching. When a router has data for a remote site, the switched circuit is initiated with the circuit number of the remote network. In the case of ISDN circuits, the device actually places a call to the telephone number of the remote ISDN circuit. When the two networks are connected and authenticated, they can transfer data. When the data transmission is complete, the call can be terminated.Figure: A Circuit-Switched WAN Undergoes a Process Similar to That Used for a Telephone Call illustrates an example of this type of circuit.
交换式电路允许数据连接在需要时启动(接通连接),通信完成后终止(断开连接)。这十分像用于声音通信的正常电话。ISDN(综合业务数字网)是电路交换的一个很好的例子。当一个路由器有用于远处的数据时,交换电路就被启动,并使用远方网络的电路号码。在ISDN电路的情况下,设备实际上对远方ISDN电路的电话号码安排一次呼叫。当两个网络连接并生效时,它们就可以传输数据。当数据传送完成了,呼叫就可以结束。
图3.1举例说明了这类电路的一个例子。
3.1 Figure: A Circuit-Switched WAN Undergoes a Process Similar to That Used for a Telephone Call
4. Packet Switching 分组包交换
Packet switching is a WAN technology in which users share common carrier resources. Because this allows the carrier to make more efficient use of its infrastructure, the cost to the customer is generally much better than with point-to-point lines. In a packet switching setup, networks have connections into the carrier's network, and many customers share the carrier's network. The carrier can then create virtual circuits between customers' sites by which packets of data are delivered from one to the other through the network. The section of the carrier's network that is shared is often referred to as a cloud.
分组包交换是一种WAN技术,在那里用户共享公共的承运商资源。由于这能使承运商更有效地使用其基础结构,用户所付出的代价会优于点对点线路。在分组交换建立中,网络已经连入承运商网络,并且许多用户共享了承运商的网络。承运商于是可以在用户所在地之间创建虚电路,从而分组包数据就可以通过网络从一处被交付到到另一处。承运商的被共享的那部分网络通常被称之为一片云。
Some examples of packet-switching networks include Asynchronous Transfer Mode (ATM), Frame Relay, Switched Multimegabit Data Services (SMDS), and X.25.Figure: Packet Switching Transfers Packets Across a Carrier Network shows an example packet-switched circuit. 分组交换网络的一些例子包括ATM(异步传输模式),FR(帧中继),SMDS(交换式多兆位数据服务),和X.25。图4.1示明了分组交换电路的一个例子。
4.1 Figure: Packet Switching Transfers Packets Across a Carrier Network图4.1 分组交换传输数据包通过承运商网络
5. WAN Virtual Circuits WAN虚电路
The virtual connections between customer sites are often referred to as a virtual circuit.
用户所在地之间的虚连接通常称之为一个虚电路。
A virtual circuit is a logical circuit created within a shared network between two network devices. Two types of virtual circuits exist: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).
一个虚电路是一个在共享的网络内两个网络设备之间产生的逻辑电路。有两类虚电路：SVC(交换式虚电路)和PVC(永久性虚电路)。
SVCs are virtual circuits that are dynamically established on demand and terminated when transmission is complete. Communication over an SVC consists of three phases: circuit establishment, data transfer, and circuit termination. The establishment phase involves creating the virtual circuit between the source and destination devices. Data transfer involves transmitting data between the devices over the virtual circuit, and the circuit termination phase involves tearing down the virtual circuit between the source and destination devices. SVCs are used in situations in which data transmission between devices is sporadic, largely because SVCs increase bandwidth used due to the circuit establishment and termination phases, but they decrease the cost associated with constant virtual circuit availability.
SVC是在需要时动态建立的,而在传送完成后被终止。在一个SVC上承载的通信由三个阶段组成：电路建立,数据传输,和电路终止。电路建立阶段涉及在源设备和目的设备之间创建虚电路。数据传输涉及在虚电路上的两个设备间传送数据,而电路终止阶段则涉及在源和目的设备间关闭虚电路。 SVCs用于设备间偶有数据传输的情况。
PVC is a permanently established virtual circuit that consists of one mode: data transfer. PVCs are used in situations in which data transfer between devices is constant. PVCs decrease the bandwidth use associated with the establishment and termination of virtual circuits, but they increase costs due to constant virtual circuit availability. PVCs are generally configured by the service provider when an order is placed for service.
PVC是一种永久建立的电路,它只有一种模式：数据传输模式,而且保持设备间进行传输的情况不变。PVCs减少了SVCs的使用带宽,却由于PVCs保持不变的可用性而增加了用户的代价。PVCs通常是由服务提供商根据用户要求服务的订单配置的。
6. WAN Dialup Services WAN拨号服务
Dialup services offer cost-effective methods for connectivity across WANs. Two popular dialup implementations are dial-on-demand routing (DDR) and dial backup. 拨号服务提供了一类用于WANs范围内有效连接计价的方法。拨号有两种流行的实现：DDR(需要时拨号路由)和拨号后备。
DDR is a technique whereby a router can dynamically initiate a call on a switched circuit when it needs to send data. In a DDR setup, the router is configured to initiate the call when certain criteria are met, such as a particular type of network traffic needing to be transmitted. When the connection is made, traffic passes over the line. The router configuration specifies an idle timer that tells the router to drop the connection when the circuit has remained idle for a certain period.
DDR是一种技术,当需要发送数据时,路由器可以动态地在一个交换电路上发起一个呼叫。在DDR建立期间,当某种条件满足时,例如需要发送一种具体类型的网络信流,就配置路由器,以发起呼叫。连接完成后,信流就在线路上传递。如果呼叫时线路空置,在超过指定时间后路由器就放弃连接。,
Dial backup is another way of configuring DDR. However, in dial backup, the switched circuit is used to provide backup service for another type of circuit, such as point-to-point or packet switching. The router is configured so that when a failure is detected on the primary circuit, the dial backup line is initiated. The dial backup line then supports the WAN connection until the primary circuit is restored. When this occurs, the dial backup connection is terminated.
拨号后备是另外一种配置DDR的方法,通常作为交换电路的后备,如点对点或分组交换。当发现交换电路失效时,路由器就启动拨号备用线路,以支持WAN连接,直至交换电路恢复。于是拨号后备连接终止。
7. WAN Devices WAN设备
WANs use numerous types of devices that are specific to WAN environments. WAN switches, access servers, modems, CSU/DSUs, and ISDN terminal adapters are discussed in the following sections. Other devices found in WAN environments that are used in WAN implementations include routers, ATM switches, and multiplexers.
有很多种用于WAN环境的设备。WAN交换机,接入服务器,调制解调器,CSU/DSU,和ISDN终端适配器，将在以下各节中讨论。其它WAN中用于WAN实现的设备,还包括路由器,ATM交换机,和复用器。
7.1 WAN Switch WAN交换机
A WAN switch is a multiport internetworking device used in carrier networks. These devices typically switch such traffic as Frame Relay, X.25, and SMDS, and operate at the data link layer of the OSI reference model.Figure: Two Routers at Remote Ends of a WAN Can Be Connected by WAN Switches illustrates two routers at remote ends of a WAN that are connected by WAN switches.
WAN交换机是一个用于承运商网络的多口网络互联设备。这些设备,通常用来交换如帧中继,X.25,和SMDS等的信流,它们运行在OSI参考模型的数据链路层上。图7.1.1举例说明了两个相距很远的WAN路由器用WAN交换机连接起来。
7.1.1 Figure: Two Routers at Remote Ends of a WAN Can Be Connected by WAN Switches
7.2 Access Server 接入服务器
An access server acts as a concentration point for dial-in and dial-out connections.Figure: An Access Server Concentrates Dial-Out Connections into a WAN illustrates an access server concentrating dial-out connections into a WAN.
一个接入服务器作为用于拨入拨出连接的一个集中点。图7.2.1举例说明了一个接入服务器是如何将拨出连接集中进入WAN的。
7.2.1 Figure: An Access Server Concentrates Dial-Out Connections into a WAN 图7.2.1 一个接入服务器将拨出连接集中进入WAN
7.3 Modem 调制解调器
A modem is a device that interprets digital and analog signals, enabling data to be transmitted over voice-grade telephone lines. At the source, digital signals are converted to a form suitable for transmission over analog communication facilities. At the destination, these analog signals are returned to their digital form.Figure: A Modem Connection Through a WAN Handles Analog and Digital Signals illustrates a simple modem-to-modem connection through a WAN.
调制解调器是一种设备,用来转换数字和模拟信号,使能在声音级电话线上传送数据。在源端,数字信号转换成适合于模拟通信设施上传送的形式;而在目的端,则将模拟信号转回数字信号。图7.3.1举例说明了一个调制解调器,通过一个WAN,连接到另一个调制解调器上。
7.3.1 Figure: A Modem Connection Through a WAN Handles Analog and Digital Signals
7.4 CSU/DSU
A channel service unit/digital service unit (CSU/DSU) is a digital-interface device used to connect a router to a digital circuit like a T1. The CSU/DSU also provides signal timing for communication between these devices.Figure: The CSU/DSU Stands Between the Switch and the Terminal illustrates the placement of the CSU/DSU in a WAN implementation.
CSU/DSU(通道服务单元/数字服务单元)是一种数字接口单元,用于路由器和数字电路,如T1,相连。CSU/DSU也提供信号同步用于设备间的通信。图7.4.1举例说明了在一个WAN实现中CSU/DSU的位置。
7.4.1 Figure: The CSU/DSU Stands Between the Switch and the Terminal
图7.4.1 在交换机和终端间CSU/DSU的位置
7.5 ISDN Terminal Adapter ISDN终端适配器
An ISDN terminal adapter is a device used to connect ISDN Basic Rate Interface (BRI) connections to other interfaces, such as EIA/TIA-232 on a router. A terminal adapter is essentially an ISDN modem, although it is called a terminal adapter because it does not actually convert analog to digital signals.Figure: The Terminal Adapter Connects the ISDN Terminal Adapter to Other Interfaces illustrates the placement of the terminal adapter in an ISDN environment.
ISDN终端适配器是一种用于将ISDN的BRI(基本速率接口)连接到其它的接口上,如路由器上的EIA/TIA-232。一个终端适配器本质上是一个ISDN调制解调器,但它不属于A/D-D/A的那一种。图7.5.1举例说明了ISDN环境中的终端适配器的位置。
7.5.1 Figure: The Terminal Adapter Connects the ISDN Terminal Adapter to Other Interfaces 图7.5.1 终端适配器用于将ISDN终端适配器的BRI(基本速率接口)连接到其他接口上
8. Review Questions 复习问答
Q - What are some types of WAN circuits?
WAN电路有哪些类型？
A - Point-to-point, packet-switched, and circuit-switched.
点对点,分组交换和电路交换。
Q - What is DDR, and how is it different from dial backup?
什么是DDR?它与拨号备用有何不同？
A - DDR is dial-on-demand routing. DDR dials up the remote site when traffic needs to be transmitted. Dial backup uses the same type of services, but for backup to a primary circuit. When the primary circuit fails, the dial backup line is initiated until the primary circuit is restored. 当信流需要传送时,DDR是按拨号需要路由。拨号是主电路的一种后备,它使用被备用的同一类服务。当主电路失效时,拨号备用线路启动,直到主电路恢复。
Q - What is a CSU/DSU used for?
CSU/DSU的作用是什么？
A - A CSU/DSU interfaces a router with a digital line such as a T1.
CSU/DSU接口s是带一条数字线路的路由器,如T1.
Q - What is the difference between a modem and an ISDN terminal adapter?
调制解调器与ISDN终端适配器有何不同?
A - A modem converts digital signals into analog for transmission over a telephone line. Because ISDN circuits are digital, the conversion from digital to analog is not required.
调制解调器将数字信号转换为模拟信号,以便在电话线路上进行传送。而ISDN电路是数字的,因而没有必要进行数模转换。
9. For More Information 了解更多信息
Mahler, Kevin. CCNA Training Guide. Indianapolis: New Riders, 1999.
Cisco IOS Dial Solutions. Indianapolis: Cisco Press, 1998.
Cisco IOS Wide Area Networking Solutions. Indianapolis: Cisco Press, 1999.
Retrieved from "http://docwiki.cisco.com/wiki/Introduction_to_WAN_Technologies"
Category:IOS Technology Handbook
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四．Bridging and Switching Basics 桥接和交换的基本原理
Contents联网技术手册 指导目录
http://docwiki.cisco.com/wiki/Bridging_and_Switching_Basics
四．Bridging and Switching Basics 桥接和交换的基本原理
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This article introduces the technologies employed in devices loosely referred to as bridges and switches. Topics summarized here include general link layer device operations, local and remote bridging, ATM switching, and LAN switching. Chapters in Part 5, “Bridging and Switching,” address specific technologies in more detail.
本章介绍用于,不严格地称为桥和交换机的设备,的技术。综述在这里的论题包括一般链路层设备的操作，本地和远程的桥接，ATM交换，和LAN交换。在手册第5部分"桥接和交换"的各章中将更详细地讲述具体技术。
Contents
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1 What Are Bridges and Switches? 什么是网桥和交换机？
2 Link Layer Device Overview 链路层设备概述
3 Types of Bridges 网桥的类型
3.1 Figure: Local and Remote Bridges Connect LAN Segments in Specific Areas 图3.1 本地和远程网桥在指定区连接局域网段
3.2 Figure: A MAC-Layer Bridge Connects the IEEE 802.3 and IEEE 802.5 Networks 图3.2 MAV层网桥连接IEEE 802.3和IEEE 802.5网络
4 Types of Switches 交换机的类型
4.1 ATM Switch ATM交换机
4.1.1 Figure: Multi-LAN Networks Can Use an ATM-Based Backbone When Switching Cells
图4.1.1 多局域网交换信元时可以使用基于ATM的骨干网
4.2 LAN Switch 局域网交换机
4.2.1 Figure: A LAN Switch Can Link 10-Mbps and 100-Mbps Ethernet Segments
图4.2.1 局域网交换机可以连接10-Mbps和100-Mbps以太网段
5 Review Questions 复习问答
6 For More Information 参阅更多信息
1. What Are Bridges and Switches? 什么是桥和交换机?
Bridges and switches are data communications devices that operate principally at Layer 2 of the OSI reference model. As such, they are widely referred to as data link layer devices.
桥和交换机都是通信设备,它们主要是在OSI参考模型的第二层上操作,所以它们被广泛地称之为数据链路层设备。
Bridges became commercially available in the early 1980s. At the time of their introduction, bridges connected and enabled packet forwarding between homogeneous networks. More recently, bridging between different networks has also been defined and standardized.
商业上使用桥始于1980年初。当时,桥用来在同质的网络间连接并实现数据包转发。最近,在不同网络间的桥接,已经被定义和标准化了。
Several kinds of bridging have proven important as internetworking devices. Transparent bridging is found primarily in Ethernet environments, while source-route bridging occurs primarily in Token Ring environments. Translational bridging provides translation between the formats and transit principles of different media types (usually Ethernet and Token Ring). Finally, source-route transparent bridging combines the algorithms of transparent bridging and source-route bridging to enable communication in mixed Ethernet/Token Ring environments.
有几种桥接已经被证实很重要,就像互联网络联网设备那样。透明的桥接主要在以太网环境中能找到,而源路由桥接则主要发生在令牌环网的环境中。透明的桥接提供格式间的转换和不同媒体类型(通常是以太网和令牌环网)的转送规则。源路由透明桥接则是组合了上述两种桥接的算法,以实现在以太网/令牌环网的混合环境中进行通信。
Today, switching technology has emerged as the evolutionary heir to bridging-based internetworking solutions. Switching implementations now dominate applications in which bridging technologies were implemented in prior network designs. Superior throughput performance, higher port density, lower per-port cost, and greater flexibility have contributed to the emergence of switches as replacement technology for bridges and as complements to routing technology.
今天,交换技术已经显露成为基于桥接的互联网络联网解案的革命性的继承技术。交换实现现在主要的应用是在那些以前的网络设计中曾用桥接技术实现的地方。超级的吞吐性能,较高的接口密度,更低的单口代价,和更大的灵活性等,由于交换机s作为桥接的替代技术和路由技术的补充而出现了。
2. Link Layer Device Overview 链路层设备概述
Bridging and switching occur at the link layer, which controls data flow, handles transmission errors, provides physical (as opposed to logical) addressing, and manages access to the physical medium. Bridges provide these functions by using various link layer protocols that dictate specific flow control, error handling, addressing, and media-access algorithms. Examples of popular link layer protocols include Ethernet, Token Ring, and FDDI.
桥接和交换发生在链路层。链路层控制数据流,处理传送错误,提供物理寻址,以及管理对物理介质的访问。桥提供这些功能是通过使用各种链路层协议来实现的。协议安排特殊的流量控制,错误处理,寻址,和媒体访问算法。流行的链路层协议的例子包括以太网,令牌环网,和FDDI。,
Bridges and switches are not complicated devices. They analyze incoming frames, make forwarding decisions based on information contained in the frames, and forward the frames toward the destination. In some cases, such as source-route bridging, the entire path to the destination is contained in each frame. In other cases, such as transparent bridging, frames are forwarded one hop at a time toward the destination.
桥和交换机并非复杂设备。它们解析收入的帧,根据包含在帧中的信息,作出转发决策,并将帧向目的地转发。在某些情况下,如源路由桥接,到目的地的整个通路都包含在每个帧中。在其他情况下,如透明桥接,帧是向着目的地,一次转发一跳。
Upper-layer protocol transparency is a primary advantage of both bridging and switching. Because both device types operate at the link layer, they are not required to examine upper-layer information. This means that they can rapidly forward traffic representing any network layer protocol. It is not uncommon for a bridge to move AppleTalk, DECnet, TCP/IP, XNS, and other traffic between two or more networks. 对上层协议的透明性是桥接和交换的主要优点。由于两种类型的设备都操作在链路层,它们并不需要检查上层信息。这意味着它们可以迅速转发表达任何网络层协议的信流。对于一个桥而言,在两个或更多的网络间移动AppleTalk, DECnet, TCP/IP, XNS, 和其它信流并不罕见。
Bridges are capable of filtering frames based on any Layer 2 fields. For example, a bridge can be programmed to reject (not forward) all frames sourced from a particular network. Because link layer information often includes a reference to an upper-layer protocol, bridges usually can filter on this parameter. Furthermore, filters can be helpful in dealing with unnecessary broadcast and multicast packets.
桥有能力过滤基于任何层2字段的帧。例如,一个桥可以编程以拒绝(不转发)源自一个具体网络的所有帧。由于链路层信息通常包括一个对一个上层协议的引用,桥常可过滤该参数。此外,过滤器可以帮助处理不必要广播和组播数据包。
By dividing large networks into self-contained units, bridges and switches provide several advantages. Because only a certain percentage of traffic is forwarded, a bridge or switch diminishes the traffic experienced by devices on all connected segments. The bridge or switch will act as a firewall for some potentially damaging network errors and will accommodate communication between a larger number of devices than would be supported on any single LAN connected to the bridge. Bridges and switches extend the effective length of a LAN, permitting the attachment of distant stations that was not previously permitted.
通过把大网络划分成自含单元,桥和交换机就能提供若干优点。因为只有部分信流被转发,一个桥或交换机就减少了在所有连接的网段上的设备不必要再历经信流。桥和交换机可以作为防火墙用于防止某些潜在地损坏网络的错误,容许大量设备间的通信,而不必对连接到桥的单个LAN,一个一个地支持。桥和交换机扩展了LAN的有效长度,允许附加以前不允许的远处站点。
Although bridges and switches share most relevant attributes, several distinctions differentiate these technologies. Bridges are generally used to segment a LAN into a couple of smaller segments. Switches are generally used to segment a large LAN into many smaller segments. Bridges generally have only a few ports for LAN connectivity, whereas switches generally have many. Small switches such as the Cisco Catalyst 2924XL have 24 ports capable of creating 24 different network segments for a LAN. Larger switches such as the Cisco Catalyst 6500 can have hundreds of ports. Switches can also be used to connect LANs with different media-for example, a 10-Mbps Ethernet LAN and a 100-Mbps Ethernet LAN can be connected using a switch. Some switches support cut-through switching, which reduces latency and delays in the network, while bridges support only store-and-forward traffic switching. Finally, switches reduce collisions on network segments because they provide dedicated bandwidth to each network segment.
虽然桥和交换机大多数有关的属性,若干特帧区分了这些技术。桥通常用于将LAN分段成几个较小段。桥一般只有几个端口用于LAN连接,而交换机通常有很多。小交换机如2924XL有24端口,对一个LAN有能力产生24段不同的网络段。大交换机如6500可有百数端口。交换机也可以用于将LAN与不同的媒体相连接---例如,一个10Mbps和一个100Mbps以太网LANs可以使用一个交换机连接起来。某些交换机支持交换切断,减少了网络中的潜伏和延迟,而桥只支持存储-和-转发信流交换。最后,交换机减少了网络段上的碰撞,因为它们提供了专用的带宽给每个网络段。
3. Types of Bridges 桥的种类
Bridges can be grouped into categories based on various product characteristics. Using one popular classification scheme, bridges are either local or remote. Local bridges provide a direct connection between multiple LAN segments in the same area. Remote bridges connect multiple LAN segments in different areas, usually over telecommunications lines.
桥可以根据各种产品的特性组成类别。使用一个流行的分类方案,桥可以分成本地的或远处的。本地桥s在同一域的多个LAN段之间提供一个直接连接。远处桥在不同域中,通常在电信线路上,连接多个LAN段。图4-1举例说明了这两种配置。
Figure: Local and Remote Bridges Connect LAN Segments in Specific Areas illustrates these two configurations.
Figure: Local and Remote Bridges Connect LAN Segments in Specific Areas
Remote bridging presents several unique internetworking challenges, one of which is the difference between LAN and WAN speeds. Although several fast WAN technologies now are establishing a presence in geographically dispersed internetworks, LAN speeds are often much faster than WAN speeds. Vast differences in LAN and WAN speeds can prevent users from running delay-sensitive LAN applications over the WAN.
远程桥接存在若干少见的互联网络联网的挑战,其中之一是LAN和WAN间速度不同。虽然在地理上分布的互联网络s中,若干快速WAN技术现在正在出现,然而LAN的速度通常总是十分快于WAN速度。LAN与WAN在速度上的巨大差别可能阻止用户在WAN上运行LAN的应用。
Remote bridges cannot improve WAN speeds, but they can compensate for speed discrepancies through a sufficient buffering capability. If a LAN device capable of a 3-Mbps transmission rate wants to communicate with a device on a remote LAN, the local bridge must regulate the 3-Mbps data stream so that it does not overwhelm the 64-kbps serial link. This is done by storing the incoming data in onboard buffers and sending it over the serial link at a rate that the serial link can accommodate. This buffering can be achieved only for short bursts of data that do not overwhelm the bridge's buffering capability.
远程桥不能改善WAN的速度,然而它们通过其足够的缓冲能力来补偿速度上的(巨大)差别。例如,,,
The Institute of Electrical and Electronic Engineers (IEEE) differentiates the OSI link layer into two separate sublayers: the Media Access Control (MAC) sublayer and the Logical Link Control (LLC) sublayer. The MAC sublayer permits and orchestrates media access, such as contention and token passing, while the LLC sublayer deals with framing, flow control, error control, and MAC sublayer addressing.
IEEE将OSI链路层分成两个子层：MAC(媒体访问控制)子层和LLC子层(逻辑链路控制)。MAC子层允许和安排媒体访问,如竞争和令牌传递,而LLC子层处理成帧,流量控制,错误控制,和MAC子层寻址。
Some bridges are MAC-layer bridges, which bridge between homogeneous networks (for example, IEEE 802.3 and IEEE 802.3), while other bridges can translate between different link layer protocols (for example, IEEE 802.3 and IEEE 802.5).
某些桥是MAC子层桥,处于同质网络之间(例如IEEE 802.3和IEEE 802.3),而其它桥则处在不同链路层协议之间,(例如,IEEE 802.3和IEEE 802.5),桥中可以实现转换。这样的一种转换的基本机制图示在图4.2中。
The basic mechanics of such a translation are shown inFigure: A MAC-Layer Bridge Connects the IEEE 802.3 and IEEE 802.5 Networks.
Figure: A MAC-Layer Bridge Connects the IEEE 802.3 and IEEE 802.5 Networks
The above figure illustrates an IEEE 802.3 host (Host A) formulating a packet that contains application information and encapsulating the packet in an IEEE 802.3-compatible frame for transit over the IEEE 802.3 medium to the bridge. At the bridge, the frame is stripped of its IEEE 802.3 header at the MAC sublayer of the link layer and is subsequently passed up to the LLC sublayer for further processing. After this processing, the packet is passed back down to an IEEE 802.5 implementation, which encapsulates the packet in an IEEE 802.5 header for transmission on the IEEE 802.5 network to the IEEE 802.5 host (Host B).
图4-2举例说明了一个连接主机sIEEE 802.3和IEEE 802.5的MAC层桥的工作过程。主机(主机A)将包含应用信息的数据包进行安排,用IEEE 802.3兼容的帧格式包装数据包,然后经由IEEE 802.3介质送至桥处。在桥的MAC子层,帧
IEEE 802.3头部被剥离,并上传到LLC子层作进一步处理。处理后,数据包回传到IEEE 802.5实现,封装IEEE 802.5头部,出桥,再经802.5介质,传送到802.5主机B。
A bridge's translation between networks of different types is never perfect because one network likely will support certain frame fields and protocol functions not supported by the other network.
桥在不同类型网络间的转换是不会完美的,因为一个网络支持某种帧字段s和协议功能s,但不一定为其它网络所支持。
4. Types of Switches 交换机s的种类
Switches are data link layer devices that, like bridges, enable multiple physical LAN segments to be interconnected into a single larger network. Similar to bridges, switches forward and flood traffic based on MAC addresses. Any network device will create some latency. Switches can use different forwarding techniques-two of these are store-and-forward switching and cut-through switching.
交换机s是数据链路层设备s,像桥s那样,能使多个物理LAN段连成单一大网络。也类似于桥s,能按照MAC地址s转发信流,也会淹没信流。任何网络设备都会产生某些潜伏。交换机s可以使用不同的转发技术---其中的两个是转发交换和穿越交换。
In store-and-forward switching, an entire frame must be received before it is forwarded. This means that the latency through the switch is relative to the frame size-the larger the frame size, the longer the delay through the switch. Cut-through switching allows the switch to begin forwarding the frame when enough of the frame is received to make a forwarding decision. This reduces the latency through the switch. Store-and-forward switching gives the switch the opportunity to evaluate the frame for errors before forwarding it. This capability to not forward frames containing errors is one of the advantages of switches over hubs. Cut-through switching does not offer this advantage, so the switch might forward frames containing errors. Many types of switches exist, including ATM switches, LAN switches, and various types of WAN switches.
在存储--转发交换中,必须接收到完整的一帧才能转发。这意味着通过交换机的潜伏时间,帧越大,延迟就越长。穿越交换则允许交换机在接收到部分的,但已达到指定大小的帧时,就按转发决策转发。这样就减少了通过交换机的潜伏时间。
存储---转发交换使交换机有机会在转发前检查帧的错误。比较集线器s,这是交换机s的优点之一。,,, 穿越交换则不提供这个优点,所以交换机可能转发包含错误的帧s。有很多种类的交换机s,包括ATM交换机s,LAN交换机s,以及各种WAN交换机s。
ATM Switch ATM交换机
Asynchronous Transfer Mode (ATM) switches provide high-speed switching and scalable bandwidths in the workgroup, the enterprise network backbone, and the wide area. ATM switches support voice, video, and data applications, and are designed to switch fixed-size information units called cells, which are used in ATM communications. ATM(异步传输模式)交换机s提供高速交换和可缩放带宽予工作组,企业网络骨干,和广域网。ATM交换机s支持声音,视频,和数据应用s。ATM设计了固定尺寸的信息单元,叫做信元,用于ATM通信s中作为交换的基本单元。图4-3举例说明了一个企业网络,由多个LAN互联,通越一个ATM骨干网组成。
Figure: Multi-LAN Networks Can Use an ATM-Based Backbone When Switching Cells illustrates an enterprise network comprised of multiple LANs interconnected across an ATM backbone.
Figure: Multi-LAN Networks Can Use an ATM-Based Backbone When Switching Cells
LAN Switch LAN交换机
LAN switches are used to interconnect multiple LAN segments. LAN switching provides dedicated, collision-free communication between network devices, with support for multiple simultaneous conversations. LAN switches are designed to switch data frames at high speeds.
LAN交换机s用于互联多个LAN段。LAN交换提供网络设备间中专用的无碰撞通信,支持多个同时对话。LAN交换机s设计用来以高速度交换数据帧s。图4-4举例说明了一个简单的网络,其中,一个LAN交换机将10Mbps的LAN和
100Mbps的LAN互联起来。
Figure: A LAN Switch Can Link 10-Mbps and 100-Mbps Ethernet Segments illustrates a simple network in which a LAN switch interconnects a 10-Mbps and a 100-Mbps Ethernet LAN.
Figure: A LAN Switch Can Link 10-Mbps and 100-Mbps Ethernet Segments
5. Review Questions 复习问答
Q - What layer of the OSI reference model to bridges and switches operate.
OSI参考模型那一层操作桥s和交换机s?
A - Bridges and switches are data communications devices that operate principally at Layer 2 of the OSI reference model. As such, they are widely referred to as data link-layer devices.
桥s和交换机s是数据通信设备。主要操作在OSI参考模型的第二层,因此,它们被称之为链路层设备s。,
Q - What is controlled at the link layer?
在链路层上受控制的是什么？
A - Bridging and switching occur at the link layer, which controls data flow, handles transmission errors, provides physical (as opposed to logical) addressing, and manages access to the physical medium.
桥接和交换发生在链路层,该层控制数据流,处理传送错误,提供物理(而不是逻辑)寻址,以及管理物理介质。
Q - Under one popular classification scheme what are bridges classified as?
在一个流行分类策略下对桥s如何分类?
A - Local or Remote: Local bridges provide a direct connection between multiple LAN segments in the same area. Remote bridges connect multiple LAN segments in different areas, usually over telecommunications lines.
本地和远程：本地桥s提供在同一域的多个LAN段间的一个直接连接。远程桥s在不同域,通常在电信线路s上,连接多个LAN段。
Q - What is a switch?
交换机是什么?
A - Switches are data link-layer devices that, like bridges, enable multiple physical LAN segments to be interconnected into a single larger network.
交换机s是数据链路层设备s,像桥s那样,能将多个网络LAN段连成单一较大的网络。
6. For More Information 了解更多信息
Cisco's web site (www.cisco.com) is a wonderful source for more information about these topics. The Documentation section includes in-depth discussions on many of the topics covered in this article.
Cisco的网站是了解更多信息的极好来源，其文档部分包含了许多深度的讨论，其中不少文章覆盖了本文中的论题。
Clark, Kennedy, and Kevin Hamilton. Cisco LAN Switching. Indianapolis: Cisco Press, August 1999.
Cisco Systems, Inc. Cisco IOS Bridging and IBM Network Solutions. Indianapolis: Cisco Press, June 1998.
Retrieved from "http://docwiki.cisco.com/wiki/Bridging_and_Switching_Basics"
Category:IOS Technology Handbook
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五．Routing Basics 路由基本原理
指导目录
Contents联网技术手册 指导目录
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五．Routing Basics 路由基本原理
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This article introduces the underlying concepts widely used in routing protocols. Topics summarized here include routing protocol components and algorithms. In addition, the role of routing protocols is briefly contrasted with the role of routed or network protocols.
本章介绍广泛用于路由协议的基本概念。在这里概述的论题包括路由协议的组成部分和算法。另外,将路由协议的作用与被路由或网络协议的作用作一简短的对比。在第VII部分的章节"路由协议s,"里,会更详细地讲述特殊的路由协议s,而使用路由协议s的网络协议,是在第VI部分,"网络协议s",中讨论。
Contents
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1 What Is Routing?
2 Routing Components
2.1 Path Determination
2.1.1 Figure: Destination/Next Hop Associations Determine the Data’s Optimal Path
2.2 Switching
2.2.1 Figure: Numerous Routers May Come into Play During the Switching Process
3 Routing Algorithms
3.1 Design Goals
3.1.1 Figure: Slow Convergence and Routing Loops Can Hinder Progress
3.2 Algorithm Types
3.2.1 Static Versus Dynamic
3.2.2 Single-Path Versus Multipath
3.2.3 Flat Versus Hierarchical
3.2.4 Host-Intelligent Versus Router-Intelligent
3.2.5 Intradomain Versus Interdomain
3.2.6 Link-State Versus Distance Vector
3.2.7 Routing Metrics
4 Network Protocols
5 Review Questions
1. What Is Routing? 什么是路由？
Routing is the act of moving information across an internetwork from a source to a destination. Along the way, at least one intermediate node typically is encountered. Routing is often contrasted with bridging, which might seem to accomplish precisely the same thing to the casual observer. The primary difference between the two is that bridging occurs at Layer 2 (the link layer) of the OSI reference model, whereas routing occurs at Layer 3 (the network layer). This distinction provides routing and bridging with different information to use in the process of moving information from source to destination, so the two functions accomplish their tasks in different ways. 路由就是一种作用,将信息从源经过互联网络移动到目的地。沿着这条通路,通常会遇到至少一个中间节点。路由通常与桥接相对照,在陌生的旁观者看来,它们精确地完成同一件事情。两者主要的不同在于,桥接发生在OSI参考模型第二层(链路层),而路由则发生在第三层(网络层)。这种不同提供给路由和桥接用不同的信息,来处理从源到目的信息移动的过程，所以这两种功能(路由和桥接)是用不同的方法来完成它们的任务的。
The topic of routing has been covered in computer science literature for more than two decades, but routing achieved commercial popularity as late as the mid-1980s. The primary reason for this time lag is that networks in the 1970s were simple, homogeneous environments. Only relatively recently has large-scale internetworking become popular.
路由这个论题,出现在计算机科学文献里,已经有多于20年的时间,然而路由实现在商业上流行,则是迟至80年代中期。其原因在于70年代的网络s还是简单,同质的环境s。大规模互联网络联网的流行出现较晚。
2. Routing Components 路由的组成部分
Routing involves two basic activities: determining optimal routing paths and transporting information groups (typically called packets) through an internetwork. In the context of the routing process, the latter of these is referred to as packet switching. Although packet switching is relatively straightforward, path determination can be very complex.
路由涉及两种活动:确定优化路由通路s,和传输通过互联网络的信息组s(通常叫数据包s)。,在路由过程的上下文中,后一活动称为包交换。虽然包交换相对直观,通路确定却可能十分复杂。
Path Determination 通路确定
Routing protocols use metrics to evaluate what path will be the best for a packet to travel. A metric is a standard of measurement, such as path bandwidth, that is used by routing algorithms to determine the optimal path to a destination. To aid the process of path determination, routing algorithms initialize and maintain routing tables, which contain route information. Route information varies depending on the routing algorithm used.
路由协议使用度量来评价,这样使通路能最好地传输数据包。一次度量就是一次标准测量,如通路带宽,路由算法s使用它来确定到达一个目标的最佳路径。为了辅助路径确定的过程,路由算法s启动和维持包含路由信息的路由表s。路由信息的变化取决于所用的路由算法。
Routing algorithms fill routing tables with a variety of information. Destination/next hop associations tell a router that a particular destination can be reached optimally by sending the packet to a particular router representing the "next hop" on the way to the final destination. When a router receives an incoming packet, it checks the destination address and attempts to associate this address with a next hop.
路由算法s将各种信息填入路由表s。目标/下一跳组合告诉路由器,一个具体的目标可以优化地到达,只要将数据包,沿着通向最终目标的通路,送到表示'下一跳'的具体路由器。当路由器接收到一个数据包时,它就检查目标地址,并在目标地址上结合下一跳相关地址。图5-1说明了一个例样目标/效益跳路由表。
Figure: Destination/Next Hop Associations Determine the Data’s Optimal Path depicts a sample destination/next hop routing table.
Figure: Destination/Next Hop Associations Determine the Data’s Optimal Path
Routing tables also can contain other information, such as data about the desirability of a path. Routers compare metrics to determine optimal routes, and these metrics differ depending on the design of the routing algorithm used. A variety of common metrics will be introduced and described later in this article.
路由表s也可以包含其他信息,如有关所希望通路的数据。路由器s比较度量值来确定优化由路,而这些度量值则取决于所使用的路由算法的设计,而有所不同。各种通用的度量s将被介绍,并在本章后面予以说明。
Routers communicate with one another and maintain their routing tables through the transmission of a variety of messages. The routing update message is one such message that generally consists of all or a portion of a routing table. By analyzing routing updates from all other routers, a router can build a detailed picture of network topology. A link-state advertisement, another example of a message sent between routers, informs other routers of the state of the sender's links. Link information also can be used to build a complete picture of network topology to enable routers to determine optimal routes to network destinations.
路由器s彼此通信,并通过传输各种消息来维持它们的路由表s。路由更新消息是一种这样的消息,它们构成了路由表的全部或部分。通过分析来自所有其它路由器的路由更新消息,一个路由器就可以建立一张详细的网络投票图。一个链路状态通告,路由器间发送消息的另一种例子,通知其它路由器,告知发送者链路s的状态。链路信息也可以用来建立一张完整的网络拓扑图,以使路由器s能确定由路到网络目标s的优化通路。
Switching 交换
Switching algorithms is relatively simple; it is the same for most routing protocols. In most cases, a host determines that it must send a packet to another host. Having acquired a router's address by some means, the source host sends a packet addressed specifically to a router's physical (Media Access Control [MAC]-layer) address, this time with the protocol (network layer) address of the destination host.
交换算法相对简单;对于大多数路由协议s都是一样的。在大多数的情况下,一个主机确定它必须将一个数据包传送给另外一个主机。通过某些方法,得到所需要的路由器地址,源主机就将一个带有目的主机协议(网络层)地址的数据包,针对寻址路由器的物理(MAC(介质访问控制)层)地址发送出去。
As it examines the packet's destination protocol address, the router determines that it either knows or does not know how to forward the packet to the next hop. If the router does not know how to forward the packet, it typically drops the packet. If the router knows how to forward the packet, however, it changes the destination physical address to that of the next hop and transmits the packet. 当路由器检查数据包的协议目的地址时,如果路由器不知道如何转发数据包,通常就丢弃它;如果知道,就改变目的物理地址到下一跳,并传送该数据包。
The next hop may be the ultimate destination host. If not, the next hop is usually another router, which executes the same switching decision process. As the packet moves through the internetwork, its physical address changes, but its protocol address remains constant, as illustrated inFigure: Numerous Routers May Come into Play During the Switching Process.
这下一跳可能是最后目的主机。如果不是,下一跳通常是另一个路由器,它执行同样的交换决策过程。当数据包移动通过互联网络时,它的物理地址就改变了,但它的协议地址保持不变,如在图5-2中举例说明的那样。
The preceding discussion describes switching between a source and a destination end system. The International Organization for Standardization (ISO) has developed a hierarchical terminology that is useful in describing this process. Using this terminology, network devices without the capability to forward packets between subnetworks are called end systems (ESs), whereas network devices with these capabilities are called intermediate systems (ISs). ISs are further divided into those that can communicate within routing domains (intradomain ISs) and those that communicate both within and between routing domains (interdomain ISs). A routing domain generally is considered a portion of an internetwork under common administrative authority that is regulated by a particular set of administrative guidelines. Routing domains are also called autonomous systems. With certain protocols, routing domains can be divided into routing areas, but intradomain routing protocols are still used for switching both within and between areas.
前面的讨论叙述了一个源和一个目的端系统间的交换。ISO已经开发了一类层次的术语,用来描述这个过程。按此类术语,一种网络设备,如果在子网间没有转发数据包的能力,该种设备就叫做ES(端系统);有此能力的,叫IS(中间系统)。ISs进一步分为域内ISs(在路由域内可以通信),和域间ISs(在路由域内和域间)可以通信。一个路由域通常认为是一个互联网络的一部分,该互联网络在公共的管理授权下,受一组特殊的管理原则指导。路由域也叫ASs(自治系统s)。在某些协议中,路由域s可以细分成路由区s,域内路由协议s仍可用于区内交换和区间交换。
Figure: Numerous Routers May Come into Play During the Switching Process
3. Routing Algorithms 路由算法
Routing algorithms can be differentiated based on several key characteristics. First, the particular goals of the algorithm designer affect the operation of the resulting routing protocol. Second, various types of routing algorithms exist, and each algorithm has a different impact on network and router resources. Finally, routing algorithms use a variety of metrics that affect calculation of optimal routes. The following sections analyze these routing algorithm attributes.
路由算法s可以根据若干关键特征来区分。其一,算法设计者的具体目标s影响到最终路由协议的操作;其二,有各种类型的路由算法,每一种算法对于网络和路由资源有不同的影响;其三,路由算法使用各种影响优化由路计算的度量。下面各节分析这些路由算法的属性。
Design Goals 设计目标
Routing algorithms often have one or more of the following design goals:
路由算法通常使用下列一个或多个设计目标：
Optimality 优化性
Simplicity and low overhead 简单性和低开销
Robustness and stability 稳固性和稳定性
Rapid convergence 快速收敛
Flexibility 灵活性
Optimality refers to the capability of the routing algorithm to select the best route, which depends on the metrics and metric weightings used to make the calculation. For example, one routing algorithm may use a number of hops and delays, but it may weigh delay more heavily in the calculation. Naturally, routing protocols must define their metric calculation algorithms strictly.
优化性是路由算法选择最佳由路的能力。这取决于用于计算的度量和度量加权。例如,一种路由算法可以使用若干跳和若干延迟,可以在计算中对延迟更重地加权。当然,路由协议s必须严格地定义它们的度量计算算法。
Routing algorithms also are designed to be as simple as possible. In other words, the routing algorithm must offer its functionality efficiently, with a minimum of software and utilization overhead. Efficiency is particularly important when the software implementing the routing algorithm must run on a computer with limited physical resources.
路由算法也应该设计成尽可能地简单。换句话说,路由算法必须以最少的软件开销和使用开销,提供其有效的功能性。效率是特别重要的,如果路由算法必须采用采用软件实现,而计算机上的物理资源又很有限的话。
Routing algorithms must be robust, which means that they should perform correctly in the face of unusual or unforeseen circumstances, such as hardware failures, high load conditions, and incorrect implementations. Because routers are located at network junction points, they can cause considerable problems when they fail. The best routing algorithms are often those that have withstood the test of time and that have proven stable under a variety of network conditions.
路由算法必须稳固,这意味着在面向不长用或想不到的环境中仍能准确地执行,如硬件失效,重载状况,和不正确地实现。由于路由器s位于网络连接点s,它们的失效可能引发可观的问题s。最好的路由算法通常容许有测试的时间,以及在各种网络状况下能保证其稳定性。
In addition, routing algorithms must converge rapidly. Convergence is the process of agreement, by all routers, on optimal routes. When a network event causes routes to either go down or become available, routers distribute routing update messages that permeate networks, stimulating recalculation of optimal routes and eventually causing all routers to agree on these routes. Routing algorithms that converge slowly can cause routing loops or network outages.
另外,路由算法必须快速收敛。收敛是所有路由器,关于由路优化,都一致认可的过程。当一个网络事件引发由路关闭或成为可用时,路由器s发布路由更改消息遍布网络s,激发重新计算优化由路,并实际上引起所有路由器都要认可这些由路。收敛很慢的路由算法可能引起路由成环或网络断开。
In the routing loop displayed inFigure: Slow Convergence and Routing Loops Can Hinder Progress, a packet arrives at Router 1 at time t1. Router 1 already has been updated and thus knows that the optimal route to the destination calls for Router 2 to be the next stop. Router 1 therefore forwards the packet to Router 2, but because this router has not yet been updated, it believes that the optimal next hop is Router 1. Router 2 therefore forwards the packet back to Router 1, and the packet continues to bounce back and forth between the two routers until Router 2 receives its routing update or until the packet has been switched the maximum number of times allowed.
Figure: Slow Convergence and Routing Loops Can Hinder Progress
Routing algorithms should also be flexible, which means that they should quickly and accurately adapt to a variety of network circumstances. Assume, for example, that a network segment has gone down. As many routing algorithms become aware of the problem, they will quickly select the next-best path for all routes normally using that segment. Routing algorithms can be programmed to adapt to changes in network bandwidth, router queue size, and network delay, among other variables.
路由算法也应该是灵活的,意思是它们应当快而准确地适应各种网络环境s。例如,假定一个网络段已经断开了。当许多路由算法知道了问题,它们就迅速地从所有的由路中选择下一个最好的通路,以能正常地使用该网段。可以编程路由算法,以适应在网络带宽,路由器队列大小,和网络延迟,还有其它可变的因素等,而引起的变化。
Algorithm Types 路由算法种类
Routing algorithms can be classified by type. Key differentiators include these:
Static versus dynamic
Single-path versus multipath
Flat versus hierarchical
Host-intelligent versus router-intelligent
Intradomain versus interdomain
Link-state versus distance vector
Static Versus Dynamic 对照静态与动态
Static routing algorithms are hardly algorithms at all, but are table mappings established by the network administrator before the beginning of routing. These mappings do not change unless the network administrator alters them. Algorithms that use static routes are simple to design and work well in environments where network traffic is relatively predictable and where network design is relatively simple. 静态算法s毕竟是很难的算法s,是由网络管理员在路由开始前建立的表映射s。这些映射s不会变化,除非管理员改变它们。使用静态路由的算法,设计简单,在网络信流相对可预测,网络设计相对简单的的环境s里,工作得很好。
Because static routing systems cannot react to network changes, they generally are considered unsuitable for today's large, constantly changing networks. Most of the dominant routing algorithms today are dynamic routing algorithms, which adjust to changing network circumstances by analyzing incoming routing update messages. If the message indicates that a network change has occurred, the routing software recalculates routes and sends out new routing update messages. These messages permeate the network, stimulating routers to rerun their algorithms and change their routing tables accordingly.
由于静态路由系统不能反映网络的变化,它们通常被认为不适合当今大型常变的网络s。如今大多数主流路由算法是动态路由算法s,它们通过分析收入的路由
变更消息,调整变化的网络环境s。如果消息指示,一个网络的变化已经发生,路由软件就重新计算,并送出新的路由更改消息。这些消息遍布网络,激发路由器重新运行它们的算法s,并改变它们相应的路由表。,,
Dynamic routing algorithms can be supplemented with static routes where appropriate. A router of last resort (a router to which all unroutable packets are sent), for example, can be designated to act as a repository for all unroutable packets, ensuring that all messages are at least handled in some way.
动态路由算法可以被静态由路适当地补充。例如,一个最后求助的路由器(一个路由器,其所有不可路由的数据包都已被送出的路由器),可以被指定当作储藏所有不可路由的储藏室,以保证所有的消息,至少用某种方法,都能得到处理。
Single-Path Versus Multipath 对照单通路与多通路
Some sophisticated routing protocols support multiple paths to the same destination. Unlike single-path algorithms, these multipath algorithms permit traffic multiplexing over multiple lines. The advantages of multipath algorithms are obvious: They can provide substantially better throughput and reliability. This is generally called load sharing. 某些高级的路由协议支持指向同一信宿的多通路。不像单通路的算法,多通路算法在多条线路上执行信流复用。多通路算法的优点很明显:它们可以提供大得多的吞吐能力和可靠性。这通常称为负载共享。
Flat Versus Hierarchical 对照扁平与分层
Some routing algorithms operate in a flat space, while others use routing hierarchies. In a flat routing system, the routers are peers of all others. In a hierarchical routing system, some routers form what amounts to a routing backbone. Packets from nonbackbone routers travel to the backbone routers, where they are sent through the backbone until they reach the general area of the destination. At this point, they travel from the last backbone router through one or more nonbackbone routers to the final destination.
某些路由算法操作在扁平空间,另一些则使用分层路由。在扁平路由系统中,所有路由器都是对等的。在分层路由系统中,一些路由器构成一个路由骨干网。数据包从非骨干路由器s传输到骨干路由器s,经由路由骨干网,直到它们到达目的地的一般区域。在那里,它们从离得最近的骨干路由器,经过一个或多个非骨干路由器,到达最终目的地。
Routing systems often designate logical groups of nodes, called domains, autonomous systems, or areas. In hierarchical systems, some routers in a domain can communicate with routers in other domains, while others can communicate only with routers within their domain. In very large networks, additional hierarchical levels may exist, with routers at the highest hierarchical level forming the routing backbone. 路由系统s通常将一组节点命名为域s,ASs(自治系统s),或区。在分层系统s中,某些在域中的路由器,可以与其它域的路由器通信;而其它的,就只能与它们域内的路由器通信。在非常大的网络s中,可以有附加的分层级s,其最高层的路由器s构成路由骨干网。
The primary advantage of hierarchical routing is that it mimics the organization of most companies and therefore supports their traffic patterns well. Most network communication occurs within small company groups (domains). Because intradomain routers need to know only about other routers within their domain, their routing algorithms can be simplified, and, depending on the routing algorithm being used, routing update traffic can be reduced accordingly. 分层路由的主要优点是它模拟大多数公司的组织机构,因而能很好地支持它们的信流模式。大多数网络通信发生在小公司组s(域s)内。由于域内路由器s只需要知道有关它们自己域内的路由器s,它们的路由算法就可以简化,这取决于所使用的具体的路由算法;更新信流的路由可以得到相应的简化。
Host-Intelligent Versus Router-Intelligent 对照主机智能与路由器智能
Some routing algorithms assume that the source end node will determine the entire route. This is usually referred to as source routing. In source-routing systems, routers merely act as store-and-forward devices, mindlessly sending the packet to the next stop. 某些路由算法假定源端节点能确定整个路由。它们常被称之为源路由系统s。在源路由系统s中,路由器只作为存储-转发设备s,不关心数据包送向的下一站。
Other algorithms assume that hosts know nothing about routes. In these algorithms, routers determine the path through the internetwork based on their own calculations. In the first system, the hosts have the routing intelligence. In the latter system, routers have the routing intelligence.
其它算法s则假定主机s对路由器s一无所知。在这些算法中,路由器s根据它们自己的计算,确定通过互联网络的通路。一开始,主机s具有路由智能,之后,路由器s具有路由智能。
Intradomain Versus Interdomain 对照域内与域间
Some routing algorithms work only within domains; others work within and between domains. The nature of these two algorithm types is different. It stands to reason, therefore, that an optimal intradomain-routing algorithm would not necessarily be an optimal interdomain-routing algorithm.
某些路由算法只工作在域内;其它的,工作在域内和域外。这两种算法类型的本性是不同的。很明显,一种优化域内路由的算法,并不需要优化域间路由；(反之亦然。)
Link-State Versus Distance Vector 对照链路状态与距离向量
Link-state algorithms (also known as shortest path first algorithms) flood routing information to all nodes in the internetwork. Each router, however, sends only the portion of the routing table that describes the state of its own links. In link-state algorithms, each router builds a picture of the entire network in its routing tables. Distance vector algorithms (also known as Bellman-Ford algorithms) call for each router to send all or some portion of its routing table, but only to its neighbors. In essence, link-state algorithms send small updates everywhere, while distance vector algorithms send larger updates only to neighboring routers. Distance vector algorithms know only about their neighbors.
链路状态算法s(也称最短路径首选算法s),洪泛路由信息到互联网络所有的节点。然而,每个路由器只送路由表中只含它自己链路状态的那一部分。在链路状态算法s中,每个路由器在其路由表s中建立一张全网络图。距离向量算法s(也称Bellman-Ford算法s)调用每个路由器送出其路由表所有或部分表项,但只给它的邻居s。一般说来,链路状态算法s到处送小更新信息,而距离向量算法s则只向邻居s发送较大的更新信息。
Because they converge more quickly, link-state algorithms are somewhat less prone to routing loops than distance vector algorithms. On the other hand, link-state algorithms require more CPU power and memory than distance vector algorithms. Link-state algorithms, therefore, can be more expensive to implement and support. Link-state protocols are generally more scalable than distance vector protocols.
由于链路状态算法s比距离向量算法s收敛更快,距离向量算法s有点更倾向用于路由环路。另一方面,链路状态算法s需要更多的CPU能力和存储量,因此,其实现和支持的代价也更大。链路状态协议s通常比距离向量协议s更具有伸缩性。
Routing Metrics 路由度量
Routing tables contain information used by switching software to select the best route. But how, specifically, are routing tables built? What is the specific nature of the information that they contain? How do routing algorithms determine that one route is preferable to others? 路由表s包含了交换软件用来选择最佳路由的信息。然而,路由表s是如何建立的?什么是它们所包含信息的本性?如何确定路由算法s所得的一种由路比其它的更好？
Routing algorithms have used many different metrics to determine the best route. Sophisticated routing algorithms can base route selection on multiple metrics, combining them in a single (hybrid) metric. All the following metrics have been used: 路由算法s已经使用了许多不同种类的度量来确定最佳由路。高级路由算法s
可以根据多种度量,将它们组合成一种单一的度量,来进行由路选择。所有下面的度量都是已经使用了：
Path length 通路长度
Reliability可靠性
Delay路由延迟
Bandwidth 带宽
Load 载荷
Communication cost 通信代价
Path length is the most common routing metric. Some routing protocols allow network administrators to assign arbitrary costs to each network link. In this case, path length is the sum of the costs associated with each link traversed. Other routing protocols define hop count, a metric that specifies the number of passes through internetworking products, such as routers, that a packet must take en route from a source to a destination.
通路长度是最普通的路由度量。某些路由协议s允许网络管理员给每个网络链路指定任意的代价。在这种情况下,通路长度就是与每个链路相联系的代价的总和。其它路由协议s定义跳步数。跳步数是一种由路度量,用来指定在由路中,传递通过互联网络联网产品s,如路由器s,的数量,而这种由路是一个数据包从源到目的所必须取道的。
Reliability, in the context of routing algorithms, refers to the dependability (usually described in terms of the bit-error rate) of each network link. Some network links might go down more often than others. After a network fails, certain network links might be repaired more easily or more quickly than other links. Any reliability factors can be taken into account in the assignment of the reliability ratings, which are arbitrary numeric values usually assigned to network links by network administrators. 可靠性。在路由算法s的上下文中,指的是每个网络链路的可信度(通常指的是误位率)。一些网络链路s可能比另一些更容易断开。一旦网络失效,某种网络链路比其它的更易于修复或修复得更快。任何可靠性因素s都可以在指定的可靠性范围内计及。这是一个任意的数值,它通常由网络管理员指定给网络链路s。
Routing delay refers to the length of time required to move a packet from source to destination through the internetwork. Delay depends on many factors, including the bandwidth of intermediate network links, the port queues at each router along the way, network congestion on all intermediate network links, and the physical distance to be traveled. Because delay is a conglomeration of several important variables, it is a common and useful metric.
路由延迟指的是时间长短,用来度量一个数据包,从源通过互联网络,到目的所需要的时间。延迟取决于许多因素,包括中间网络链路的带宽,沿着通路的每个路由器上的端口排列,在所有中间网络链路的网络拥塞,以及传送的物理距离。因为延迟是若干重要变数的组合,因而它是一种普通而又有用的度量。
Bandwidth refers to the available traffic capacity of a link. All other things being equal, a 10-Mbps Ethernet link would be preferable to a 64-kbps leased line. Although bandwidth is a rating of the maximum attainable throughput on a link, routes through links with greater bandwidth do not necessarily provide better routes than routes through slower links. For example, if a faster link is busier, the actual time required to send a packet to the destination could be greater.
带宽指的是一条链路可用的信流能力。如果其它前提都一样,10Mbps以太网链路要比一条64kbps的租借线路更可取。虽然带宽是一种在一条链路上最值得
注意的吞吐能力的额定值,但通过有较大带宽链路s的由路s,不一定需要提供比通过较慢链路s的由路s更好的由路s。例如,如果一条较快的链路很忙,需要用来发送一个数据包到目的地的实际时间可能更大。
Load refers to the degree to which a network resource, such as a router, is busy. Load can be calculated in a variety of ways, including CPU utilization and packets processed per second. Monitoring these parameters on a continual basis can be resource-intensive itself. 载荷指的是一种网络资源,如路由器,忙碌的程度。载荷可以用各种方法计算,包括CPU使用率和每秒处理的数据包s数。不间断地监视这些参数可以改善资源本身。
Communication cost is another important metric, especially because some companies may not care about performance as much as they care about operating expenditures. Although line delay may be longer, they will send packets over their own lines rather than through the public lines that cost money for usage time.
通信代价是另外一种重要的度量,尤其是,有些公司可能在关注性能方面不如关注操作费用那样多。他们情愿将数据包在
他们自己的租借线路s上发送;虽然延迟可能更长,也不愿意通过公共线路s。在那里是按使用时间付钱的。
4. Network Protocols 网络协议s
Routed protocols are transported by routing protocols across an internetwork. In general, routed protocols in this context also are referred to as network protocols. These network protocols perform a variety of functions required for communication between user applications in source and destination devices, and these functions can differ widely among protocol suites. Network protocols occur at the upper five layers of the OSI reference model: the network layer, the transport layer, the session layer, the presentation layer, and the application layer.
网路协议s(也叫网络协议s)由路由协议s传送穿越一个互联网络。一般说来,网路协议s在这种上下文中也叫网络协议s;这些网络协议s执行各种功能;这些功能是为实现源和目的设备s中,用户应用程序s之间的通信所必需;它们普遍地包含在协议族s中,各不相同。网络协议发生在OSI参考模型的上五层。
Confusion about the terms routed protocol and routing protocol is common. Routed protocols are protocols that are routed over an internetwork. Examples of such protocols are the Internet Protocol (IP), DECnet, AppleTalk, Novell NetWare, OSI, Banyan VINES, and Xerox Network System (XNS). Routing protocols, on the other hand, are protocols that implement routing algorithms. Put simply, routing protocols are used by intermediate systems to build tables used in determining path selection of routed protocols. Examples of these protocols include Interior Gateway Routing Protocol (IGRP), Enhanced Interior Gateway Routing Protocol (Enhanced IGRP), Open Shortest Path First (OSPF), Exterior Gateway Protocol (EGP), Border Gateway Protocol (BGP), Intermediate System-to-Intermediate System (IS-IS), and Routing Information Protocol (RIP). Routed and routing protocols are discussed in detail later in this book.
关于术语网路协议和路由协议的混淆是很普遍的。网路协议s是在一个互联网络上被路由的协议。这样协议的例子是IP(互联网协议),DECnet, AppleTalk,
Novell NetWare, OSI, Banyan VINES,和XNS(Xerox网络系统)。而路由协议s是执行路由算法s的协议。简而言之,路由协议s用于ISs(中间系统s)建立用来确定被路由协议s的通路选择。这些协议s的例子包括IGRP(内部网关路由协议),增强的IGRP,OSPF(开放最短路径首选),EGP(外部网关协议),BGP(边界网关协议),IS-IS(中间系统至中间系统),和RIP(路由信息协议)。
5. Review Questions 复习问答
Q - Describe the process of routing packets.
说明数据包s的路由过程。
A - Routing is the act of moving information across an internetwork from a source to a destination.
路由是一种行为,它将信息从一个源端通过一个互联网络移动到一个目的端。
Q - What are some routing algorithm types?
路由算法有哪些类型？
A - Static, dynamic, flat, hierarchical, host-intelligent, router-intelligent, intradomain, interdomain, link-state, and distance vector.
静态,动态,扁平,层次,主机智能,路由器智能,域内,域间,链路状态,和距离向量。
Q - Describe the difference between static and dynamic routing.
说明静态路由和动态路由有何不同。
A - Static routing is configured by the network administrator and is not capable of adjusting to changes in the network without network administrator intervention. Dynamic routing adjusts to changing network circumstances by analyzing incoming routing update messages without administrator intervention.
静态路由由网络管理员配置,没有网络管理员的干预不能调整网络中的变化。动态路由调整网络环境中的变化是经由解析收入的路由变更消息而实现的,没有管理员的干预。
Q - What are some of the metrics used by routing protocols?
用于路由协议s的一些度量s是什么？
A - Path length, reliability, delay, bandwidth, load, and communication cost. 通路长度,可靠性,延迟,带宽,载荷,和通信代价。
Retrieved from "http://docwiki.cisco.com/wiki/Routing_Basics"
Category:IOS Technology Handbook
====================================================================
六．Network Management Basics 网络管理基本原理
指导目录
联网技术手册 指导目录
http://docwiki.cisco.com/wiki/Network_Management_Basics
六．Network Management Basics 网络管理基本原理
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This article describes functions common to most network-management architectures and protocols. It also presents the five conceptual areas of management as defined by the International Organization for Standardization (ISO). Subsequent articles in Network Management address specific network management technologies, protocols, and platforms in more detail.
本章叙述了管理功能,它们对于大多数网络管理体系结构和协议s都是共同的。它也介绍了,如ISO所定义的,五种概念区域的管理。在第IX部分的各章,“网络管理”更详细地讲述具体的网络管理技术s,协议s,和平台s。
Contents
[hide]
1 What Is Network Management?
1.1 A Historical Perspective
2 Network Management Architecture
2.1 Figure: A Typical Network Management Architecture Maintains Many Relationships
3 ISO Network Management Model
3.1 Performance Management
3.2 Configuration Management
3.3 Accounting Management
3.4 Fault Management
3.5 Security Management
4 Review Questions
1. What Is Network Management? 什么是网络管理？
Network management means different things to different people. In some cases, it involves a solitary network consultant monitoring network activity with an outdated protocol analyzer. In other cases, network management involves a distributed database, autopolling of network devices, and high-end workstations generating real-time graphical views of network topology changes and traffic. In general, network management is a service that employs a variety of tools, applications, and devices to assist human network managers in monitoring and maintaining networks. 网络管理对于不同的人们意味着不同的事情。在某些情况下,它有一个独立的网络监护,用一个保持更新的协议分析器,监视着网络的活动。在其它的情况下,网络管理包含了一个分布式数据库,自动轮询网络设备s,并在高端工作站s上,产生网络拓扑变化和信流的实时图形视图。一般说来,网络管理是一种服务,它使用各种工具,应用程序s,和设备s,帮助网络管理者的人们来监督和维护网络s。
A Historical Perspective 历史透视
The early 1980s saw tremendous expansion in the area of network deployment. As companies realized the cost benefits and productivity gains created by network technology, they began to add networks and expand existing networks almost as rapidly as new network technologies and products were introduced. By the mid-1980s, certain companies were experiencing growing pains from deploying many different (and sometimes incompatible) network technologies.
八十年代早期,已经看到了网络部署区域极大的扩充。当公司s意识到了由于网络技术所产生的价值利益和生产收益,他们就开始增加网络s和扩充现有的网络s,几乎与新技术s出现的速度和产品的引进一样迅速。到了八十年代中期,某些公司从部署许多不同的(有时是不兼容的)网络技术中,已经体验到增长的苦涩。
The problems associated with network expansion affect both day-to-day network operation management and strategic network growth planning. Each new network technology requires its own set of experts. In the early 1980s, the staffing requirements alone for managing large, heterogeneous networks created a crisis for many organizations. An urgent need arose for automated network management (including what is typically called network capacity planning) integrated across diverse environments.
伴随网络扩充而带来的问题s,同时影响着日常网络操作管理和战略上的网络增长规划。每一种新的网络技术需要它自己的一组专家。在八十年代早期,对于许多机构而言,仅有管理大型,同质网络s的职能需求,就产生了一种危机。一种紧急的需要,要求自动管理综合了含盖各种环境的网络s(包括通常称为网络容量规划),出现了。
2. Network Management Architecture 网络管理的体系结构
Most network management architectures use the same basic structure and set of relationships. End stations (managed devices), such as computer systems and other network devices, run software that enables them to send alerts when they recognize problems (for example, when one or more user-determined thresholds are exceeded). Upon receiving these alerts, management entities are programmed to react by executing one, several, or a group of actions, including operator notification, event logging, system shutdown, and automatic attempts at system repair
大多数网络管理体系结构s使用同一种基本结构和同一组关系。端站(受管理的设备s),如计算机系统s和其它网络设备s,运行软件,当它们确认有问题s时(例如
,当超过一个或多个用户确定的门槛时),使它们发出警报s。在接收到这些警报后,管理实体s被编程,用执行一个,几个或一组动作,包括操作者提醒,事件登记,系统关闭,以及自动尝试系统修复,来作出反应。.
Management entities also can poll end stations to check the values of certain variables. Polling can be automatic or user-initiated, but agents in the managed devices respond to all polls. Agents are software modules that first compile information about the managed devices in which they reside, then store this information in a management database, and finally provide it (proactively or reactively) to management entities within network management systems (NMSs) via a network management protocol. Well-known network management protocols include the Simple Network Management Protocol (SNMP) and Common Management Information Protocol (CMIP). Management proxies are entities that provide management information on behalf of other entities.
管理实体也可以轮询端站s来检查某些变量的值。轮询可以自动启动或用户启动,而由在被管理设备s中的代理s来响应所有的轮询。代理s是软件模块s,居于管理设备s中。它们首先编译有关管理设备s的信息,然后,把这些信息存储在一个管理数据库中,最后经由一个网络管理协议,把信息提供给(或主动或被动)NMSs(网络管理系统s)内的实体s。已知的网络管理协议s包括SNMP(简单网络管理协议),和CMIP(公用管理信息协议)。管理代理s是实体s,代表其它实体s提供管理信息。图6-1说明了一个典型的网络管理体系结构.
Figure: A Typical Network Management Architecture Maintains Many Relationships depicts a typical network management architecture.
Figure: A Typical Network Management Architecture Maintains Many Relationships
3. ISO Network Management Model ISO网络管理模型
The ISO has contributed a great deal to network standardization. Its network management model is the primary means for understanding the major functions of network management systems. This model consists of five conceptual areas, as discussed in the next sections. ISO已经为网络标准化做了大量的贡献。它的网络管理模型是用于理解网络管理系统s主要功能s的主要工具。该模型
如在下一节讨论中指出的,由五个概念区域组成。
3.1 Performance Management 性能管理域
The goal of performance management is to measure and make available various aspects of network performance so that internetwork performance can be maintained at an acceptable level. Examples of performance variables that might be provided include network throughput, user response times, and line utilization.
性能管理的目标是测量和实现网络性能可用的各个方面,以使互联网络性能能持在可接受的水平。可以提供的性能变量s的例子s,包括网络吞吐量,用户响应时间,和线路利用。
Performance management involves three main steps. First, performance data is gathered on variables of interest to network administrators. Second, the data is analyzed to determine normal (baseline) levels. Finally, appropriate performance thresholds are determined for each important variable so that exceeding these thresholds indicates a network problem worthy of attention. 性能管理包括三个主要步骤。首先,
收集信息数据,并把它放到网络管理员
感兴趣的变量s上。其次,分析数据以确定正常水平。最后,对每个重要变量,确认适当的性能门槛,超过门槛,就指示一个值得注意的网络问题。
Management entities continually monitor performance variables. When a performance threshold is exceeded, an alert is generated and sent to the network management system.
管理实体不断监视性能变量s。当一个性能门槛被超过,就产生一个报警,并送到网络管理系统。
Each of the steps just described is part of the process to set up a reactive system. When performance becomes unacceptable because of an exceeded user-defined threshold, the system reacts by sending a message. Performance management also permits proactive methods: For example, network simulation can be used to project how network growth will affect performance metrics. Such simulation can alert administrators to impending problems so that counteractive measures can be taken. 上面所叙述的每一步,是建立一个反应系统的部分过程。当性能由于超过用户定义的门槛,而变得不可接受时,系统就通过发送一个消息来反应。性能管理也允许前摄的方法s:例如,网络仿真可以用于规划,如何将影响网络性能的度量s。这样的仿真可以向管理员报警即将发生的问题s,以及时采取预防措施。
3.2 Configuration Management 配置管理域
The goal of configuration management is to monitor network and system configuration information so that the effects on network operation of various versions of hardware and software elements can be tracked and managed.
配置管理的目标是监督网络和系统配置信息,以使能对影响网络操作的各种型式的软件和硬件组件,进行跟踪和管理。
Each network device has a variety of version information associated with it. An engineering workstation, for example, may be configured as follows:
每个网络设备都有各种与它有关的版本信息。例如,一个工程工作站可以配置如下：
Operating system, Version 3.2
Ethernet interface, Version 5.4
TCP/IP software, Version 2.0
NetWare software, Version 4.1
NFS software, Version 5.1
Serial communications controller, Version 1.1
X.25 software, Version 1.0
SNMP software, Version 3.1
Configuration management subsystems store this information in a database for easy access. When a problem occurs, this database can be searched for clues that may help solve the problem. 配置管理子系统s将该信息存储在一个数据库中,以便于访问。当一个问题发生时,就可以搜索该数据库,以求取线索,帮助解决问题。
3.3 Accounting Management 审计管理域
The goal of accounting management is to measure network utilization parameters so that individual or group uses on the network can be regulated appropriately. Such regulation minimizes network problems (because network resources can be apportioned based on resource capacities) and maximizes the fairness of network access across all users.
审计管理的目标是,测量网络使用参数,以使网络上的单体或成组可以适当地调整。这样的调整会使网络问题s最小化,(因为网络资源可以根据资源能力进行分摊),并使访问网络的所有用户能得到最大的公正性。
As with performance management, the first step toward appropriate accounting management is to measure utilization of all important network resources. Analysis of the results provides insight into current usage patterns, and usage quotas can be set at this point. Some correction, of course, will be required to reach optimal access practices. From this point, ongoing measurement of resource use can yield billing information as well as information used to assess continued fair and optimal resource utilization.
与性能管理结合在一起,进行适当的审计管理的第一步,是测量所有重要网络资源的利用度。进而分析其结果s,弄清当前用法的模式s,并设置用途的指标。当然,某些修正是必要的,以达到优化访问的实现。从这一点出发,继续测量资源使用就能得到信息清单和信息,用以访问利用度持续良好和优化的资源。
3.4 Fault Management 故障管理域
The goal of fault management is to detect, log, notify users of, and (to the extent possible) automatically fix network problems to keep the network running effectively. Because faults can cause downtime or unacceptable network degradation, fault management is perhaps the most widely implemented of the ISO network management elements. 故障管理的目标是检测,登记,通知用户s,和(尽可能)修复网络问题s以有效地保持网络运行。因为故障能够引起停机或不可接受的网络退化,故障管理或许是ISO网络管理部件s中实现最为广泛的部件s。
Fault management involves first determining symptoms and isolating the problem. Then the problem is fixed and the solution is tested on all-important subsystems. Finally, the detection and resolution of the problem is recorded.
故障管理包括确定症状和隔离问题;然后修复问题,并在所有重要的子系统上进行测试,取得结果;最后,记录被测出和被解决的问题。
3.5 Security Management 安全管理域
The goal of security management is to control access to network resources according to local guidelines so that the network cannot be sabotaged (intentionally or unintentionally) and sensitive information cannot be accessed by those without appropriate authorization. A security management subsystem, for example, can monitor users logging on to a network resource and can refuse access to those who enter inappropriate access codes.
安全管理的目标是按照局限的原则s,有控制地访问网络资源,以使网络不会被破坏(有意或无意),或敏感信息没有适当的授权就不能被访问。例如,一个安全管理子系统可以监督用户s对网络资源的使用,可以拒绝用不恰当代码访问的用户s进入。
Security management subsystems work by partitioning network resources into authorized and unauthorized areas. For some users, access to any network resource is inappropriate, mostly because such users are usually company outsiders. For other (internal) network users, access to information originating from a particular department is inappropriate. Access to Human Resource files, for example, is inappropriate for most users outside the Human Resources department.
安全管理子系统把网络资源按在授权区和不授权区分开。对于某些用户s,访问任何网络资源都是不适当的,这通常指的是公司外部人员。对于其他(内部)网络
用户s,访问源于一个具体部门的信息,是不适当的。例如,访问人事资源文件s,对于人事资源部门外部的大多数用户,都是不适当的。
Security management subsystems perform several functions. They identify sensitive network resources (including systems, files, and other entities) and determine mappings between sensitive network resources and user sets. They also monitor access points to sensitive network resources and log inappropriate access to sensitive network resources.
安全管理子系统执行若干功能。它们识别敏感网络资源s(包括系统s,文件s,和其他实体s),确定敏感网络资源和用户组的映射关系。它们也监视敏感网络资源的入口s,以及登录不适当地对敏感网络资源访问的事件s。
4. Review Questions 复习问答
Q - Name the different areas of network management.
网络管理不同区域的名字。
A - Configuration, accounting, fault, security, and performance.
配置,审计,故障,安全,和性能。
Q - What are the goals of performance management?
性能管理的目标s是什么？
A - Measure and make available various aspects of network performance so that internetwork performance can be maintained at an acceptable level.
测量并尽可能实现网络性能的各个方面,使网络性能维持在可接受的水平。
Q - What are the goals of configuration management?
配置管理的目标s是什么？
A - Monitor network and system configuration information so that the effects on network operation of various versions of hardware and software elements can be tracked and managed.
监督网络和系统配置信息,以便跟踪和管理各种硬件部分和软件部分在网络操作上的效能。
Q - What are the goals of accounting management?
审计管理的目标s是什么？
A - Measure network utilization parameters so that individual or group uses on the network can be regulated appropriately.
测量反映网络利用情况的参数,据此可以对个别或成联网络资源,进行适当调整。
Q - What are the goals of fault management?
故障管理的目标s是什么？
A - Detect, log, notify users of, and automatically fix network problems to keep the network running effectively.
检测,登记,通知用户,和自动修复网络问题,以有效保持网络运行。
Q - What are the goals of security management?
安全管理的目标s是什么？
A - Control access to network resources according to local guidelines so that the network cannot be sabotaged and so that sensitive information cannot be accessed by those without appropriate authorization.
按照就地的原则s对网络资源的访问进行控制,以免网络被破坏,并使敏感信息不会被没有适当授权的用户所访问。
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Category:IOS Technology Handbook
===================================================================
七．Open System Interconnection Protocols 开放系统互连协议
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联网技术手册 指导目录
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七．Open System Interconnection Protocols 开放系统互连协议
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The Open System Interconnection (OSI) protocol suite is comprised of numerous standard protocols that are based on the OSI reference model. These protocols are part of an international program to develop data-networking protocols and other standards that facilitate multivendor equipment interoperability. The OSI program grew out of a need for international networking standards and is designed to facilitate communication between hardware and software systems despite differences in underlying architectures.
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开放系统互连（OSI）协议栈由许多标准协议组成,它们都基于OSI参考模型。这些协议是，一项国际安排,用来开发数据联网协议s,以及促进多厂商设备互操作性的其它标准s,的一部分。OSI的安排是出于一种对国际联网标准化的需要，设计成便于硬件系统和软件系统之间通信,而不管基础结构的差异。
The OSI specifications were conceived and implemented by two international standards organizations: the International Organization for Standardization (ISO) and the International Telecommunication Union-Telecommunications Standards Sector (ITU-T). This article provides a summary of the OSI protocol suite and illustrates its mapping to the general OSI reference model.
OSI规范s是由两个国际标准化组织构思和组织实施的：国际标准化组织（ISO）和国际电信联盟电信标准化部（ITU - T）。本文提供了一个OSI协议栈的汇总,举例说明了它与一般OSI参考模型的映射关系。
Contents
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1 OSI Networking Protocols
1.1 Figure: The OSI Protocol Suite Maps to All Layers of the OSI Reference Model
1.2 OSI Physical and Data Link layers
1.3 OSI Network Layer
1.3.1 OSI Layer Standards
1.3.2 OSI Connectionless Network Service
1.3.3 OSI Connection-Oriented Network Service
1.3.4 Network Layer Addressing
1.3.5 OSI Protocols Transport Layer
1.3.6 OSI Protocols Session Layer
1.3.7 OSI Protocols Presentation Layer
1.3.8 OSI Protocols Application Layer
2 Review Questions
目录
[hide]
1 OSI网络协议
1.1图：OSI协议参考模型套件映射到所有图层的OSI
1.2 OSI的物理和数据链路层
1.3 OSI网络层
1.3.1 OSI的层标准
1.3.2 OSI无连接网络服务
1.3.3 OSI的面向连接的网络服务
1.3.4 网络层寻址
1.4 传输层OSI协议
1.5 会话层OSI协议
1.6 表示层OSI协议
1.7 应用层OSI协议
2 Review Questions2 复习题
1. OSI Networking Protocols
Figure: The OSI Protocol Suite Maps to All Layers of the OSI Reference Model illustrates the entire OSI protocol suite and its relation to the layers of the OSI reference model. Each component of this protocol suite is discussed briefly in this article.
图：OSI协议栈映射到OSI参考模型所有层 说明了整个OSI协议栈,以及与OSI参考模型层的关系。该协议族的每个部分在本文中都作了简短的讨论。
Figure: The OSI Protocol Suite Maps to All Layers of the OSI Reference Model
1.1 图：OSI协议族映射到OSI参考模型的所有层
1.2 OSI Physical and Data Link layers
The OSI protocol suite supports numerous standard media-access protocols at the physical and data link layers. The wide variety of media-access protocols supported in the OSI protocol suite allows other protocol suites to exist easily alongside OSI on the same network media. Supported media-access protocols include IEEE 802.2 LLC, IEEE 802.3, Token Ring/IEEE 802.5, Fiber Distributed Data Interface (FDDI), and X.25.
1.2 OSI物理和数据链路层
OSI协议族支持物理层和数据链路层上的多种标准媒体访问协议。在OSI协议栈中的大量媒体访问协议,允许其他多种协议与ISO一道,共存在同一个网络媒体上。所支持的媒体访问协议,包括IEEE 802.2 LLC,IEEE 802.3,令牌环/IEEE 802.5,光纤分布式数据接口（FDDI）和X.25。
1.3 OSI Network Layer
The OSI protocol suite specifies two routing protocols at the network layer: End System-to-Intermediate System (ES-IS) and Intermediate System-to-Intermediate System (IS-IS). In addition, the OSI suite implements two types of network services: connectionless service and connection-oriented service.
OSI网络层
OSI协议栈在网络层上指定了两个路由协议：端系统到中间系统（ES - IS）和中间系统到中间系统（IS - IS）。此外， OSI栈实现了两类网络服务：无连接服务和面向连接服务。
1.3.1 OSI Layer Standards
In addition to the standards specifying the OSI network layer protocols and services, the following documents describe other OSI network layer specifications:
1.3.1 OSI的层标准s
除了指定OSI网络层协议和服务的那些标准外，下列文档还描述了其他OSI网络层规范：
1.3.2 OSI Connectionless Network Service
OSI connectionless network service is implemented by using the Connectionless Network Protocol (CLNP) and Connectionless Network Service (CLNS). CLNP and CLNS are described in the ISO 8473 standard. CLNP is an OSI network layer protocol that carries upper-layer data and error indications over connectionless links. CLNP provides the interface between the Connectionless Network Service (CLNS) and upper layers. CLNS provides network layer services to the transport layer via CLNP. CLNS does not perform connection setup or termination because paths are determined independently for each packet that is transmitted through a network. This contrasts with Connection-Mode Network Service (CMNS). In addition, CLNS provides best-effort delivery, which means that no guarantee exists that data will not be lost, corrupted, misordered, or duplicated. CLNS relies on transport layer protocols to perform error detection and correction.
1.3.2 OSI无连接网络服务
OSI无连接网络服务是通过使用无连接网络协议（CLNP）和无连接网络服务（CLNS）来实现的。CLNP和CLNS是在ISO 8473标准中描述的。CLNP是一个OSI网络层协议,承载着上一层的数据,以及在无连接链路s以上的错误指示。CLNP提供了CLNS(无连接网络服务)和上层s之间的接口。CLNS通过CLNP,为传输层提供了网络层服务。CLNS并不执行连接的建立或终止，因为路径s是独立地用于每一个通过网络传送的数据包。这不同于连接模式的网络服务（CMNS）。此外，CLNS提供尽力传送，这意味着，不能保证数据不会丢失，损坏，乱序，或重复。 CLNS依靠传输层协议，来执行错误检测和纠错。
1.3.3 OSI Connection-Oriented Network Service
OSI connection-oriented network service is implemented by using the Connection-Oriented Network Protocol (CONP) and Connection-Mode Network Service (CMNS). CONP is an OSI network layer protocol that carries upper-layer data and error indications over connection-oriented links. CONP is based on the X.25 Packet-Layer Protocol (PLP) and is described in the ISO 8208 standard, "X.25 Packet-Layer Protocol for DTE." CONP provides the interface between CMNS and upper layers. It is a network layer service that acts as the interface between the transport layer and CONP, and it is described in the ISO 8878 standard. CMNS performs functions related to the explicit establishment of paths between communicating transport layer entities. These functions include connection setup, maintenance, and termination. CMNS also provides a mechanism for requesting a specific quality of service (QoS). This contrasts with CLNS.
1.3.3 OSI的面向连接的网络服务
OSI面向连接的网络服务是通过使用CONP（面向连接的网络协议）和CMNS（连接模式网络服务）来实现的。CONP是一个OSI网络层协议，它承载着上一层的数据,以及在面向连接链路s以上的错误指示。CONP基于X.25分组层协议（PLP），在ISO 8208标准“X.25分组层协议的DTE”中描述。CONP提供CMNS及其上层s间的接口。这是一个网络层服务，起着传输层和CONP之间接口的作用，它在ISO 8878标准中描述。CMNS执行的功能，跟明确建立,在传输层实体间通信的通路,有关。这些功能包括连接的建立，维持和终止。CMNS还提供了一个用于申请特殊的服务质量（QoS）的机制。这不同于CLNS。
1.3.4 Network Layer Addressing
OSI network layer addressing is implemented by using two types of hierarchical addresses: network service access point addresses and network entity titles. A network service access point (NSAP) is a conceptual point on the boundary between the network and the transport layers. The NSAP is the location at which OSI network services are provided to the transport layer. Each transport layer entity is assigned a single NSAP, which is individually addressed in an OSI internetwork using NSAP addresses.Figure: The OSI NSAP Address Is Assigned to Each Transport Layer Entity illustrates the format of the OSI NSAP address, which identifies individual NSAPs.
1.3.4 网络层寻址
OSI网络层寻址,是通过使用两种类型的分层地址来实现的：网络服务访问点(NSAP)地址和网络实体标题s一个网络服务访问点（NSAP）是一个概念点,位于网络层与传输层的边界处。NSAP处于将网络服务提供给传输层的位置上。每个传输层实体被指定给单一的NSAP，它在使用NSAP地址的OSI互联网络中是独立寻址的。图：OSI的NSAP地址分配给每个传输层实体说明了用来识别单个NSAP的 OSI NSAP地址格式。
图： OSI的NSAP地址分配给每个传输层实体
Figure: The OSI NSAP Address Is Assigned to Each Transport Layer Entity
NSAP Address Fields
Two NSAP Address fields exist: the initial domain part (IDP) and the domain-specific part (DSP). The IDP field is divided into two parts: the authority format identifier (AFI) and the initial domain identifier (IDI). The AFI provides information about the structure and content of the IDI and DSP fields, such as whether the IDI is of variable length and whether the DSP uses decimal or binary notation. The IDI specifies the entity that can assign values to the DSP portion of the NSAP address. The DSP is subdivided into four parts by the authority responsible for its administration. The Address Administration fields allow for the further administration of addressing by adding a second authority identifier and by delegating address administration to subauthorities. The Area field identifies the specific area within a domain and is used for routing purposes. The Station field identifies a specific station within an area and also is used for routing purposes. The Selector field provides the specific n-selector within a station and, much like the other fields, is used for routing purposes. The reserved n-selector 00 identifies the address as a network entity title (NET).
NSAP地址字段s
（NSAP地址字段的这种划分是基于ES-IS和IS-IS系统的结构。为便于理解，请先了解它们。）
有两个NSAP地址的字段：IDP（初始域部分）和DSP（指定域部分）的。IDP字段分为两部分：AFI（授权格式标识符）和IDI（初始域标识符）。 AFI提供了有关IDI和DSP字段的信息,如IDI是否是变长的,DSP使用的表示法是十进制还是二进制。IDI指定的实体用于将值分配到NSAP地址的DSP部分。DSP根据对它管理的授权责任,再细分为四个部分。地址管理address administration字段允许,通过加入第二个授权标识符,来解决进一步管理，并下放地址管理部分授权。区Area字段识别一个域内的指定区域,系用于路由目的。站Station字段识别一个域内的指定站,也是用于路由目的。选择器Selector字段提供了一个站内指定的n-选择器，很像其他字段，也是用于路由目的。保留的n-选择器 00 识别为网络实体标题(NET)的地址。
End-System NSAPs
An OSI end system (ES) often has multiple NSAP addresses, one for each transport entity that it contains. If this is the case, the NSAP address for each transport entity usually differs only in the last byte (called the n-selector).
端点系统的NSAPs
一个OSI的端点系统(ES)通常有多个NSAP地址，每一个NSAP地址对应一个传输层实体。如果是这种情况，每个传输实体NSAP地址通常只有在最后一个字节才是不同的(称为n-选择器)。
图：NSAP提供了一个在运输层实体,NSAP,和网络服务之间的链路说明了运输实体,NSAP,和网络服务间的关系。
图： NSAP提供了一个在传输实体和一个网络服务之间的关系
Figure: The NSAP Provides a Link Between a Transport Entity and a Network Service illustrates the relationship between a transport entity, the NSAP, and the network service.
Figure: The NSAP Provides a Link Between a Transport Entity and a Network Service
A network entity title (NET) is used to identify the network layer of a system without associating that system with a specific transport layer entity (as an NSAP address does). NETs are useful for addressing intermediate systems (ISs), such as routers, that do not interface with the transport layer. An IS can have a single NET or multiple NETs, if it participates in multiple areas or domains.
一个NAT(网络实体标题)是用来识别一种网络层,该网络层所在系统并没有指定的传输层实体(即没有相应的NSAP地址)。NAT对于寻址ISs(中间系统s),如路由器s,是有用的，因为它们没有与传输层的接口。一个IS可以有一个，或多个NAT,如果它参与在多个区或域内。
1.4 OSI Protocols Transport Layer
The OSI protocol suite implements two types of services at the transport layer: connection-oriented transport service and connectionless transport service. Five connection-oriented transport layer protocols exist in the OSI suite, ranging from Transport Protocol Class 0 through Transport Protocol Class 4. Connectionless transport service is supported only by Transport Protocol Class 4. Transport Protocol Class 0 (TP0), the simplest OSI transport protocol, performs segmentation and reassembly functions. TP0 requires connection-oriented network service. Transport Protocol Class 1 (TP1) performs segmentation and reassembly, and offers basic error recovery. TP1 sequences protocol data units (PDUs) and will retransmit PDUs or reinitiate the connection if an excessive number of PDUs are unacknowledged. TP1 requires connection-oriented network service. Transport Protocol Class 2 (TP2) performs segmentation and reassembly, as well as multiplexing and demultiplexing of data streams over a single virtual circuit. TP2 requires connection-oriented network service. Transport Protocol Class 3 (TP3) offers basic error recovery and performs segmentation and reassembly, in addition to multiplexing and demultiplexing of data streams over a single virtual circuit. TP3 also sequences PDUs and retransmits them or reinitiates the connection if an excessive number are unacknowledged. TP3 requires connection-oriented network service. Transport Protocol Class 4 (TP4) offers basic error recovery, performs segmentation and reassembly, and supplies multiplexing and demultiplexing of data streams over a single virtual circuit. TP4 sequences PDUs and retransmits them or reinitiates the connection if an excessive number are unacknowledged. TP4 provides reliable transport service and functions with either connection-oriented or connectionless network service. It is based on the Transmission Control Protocol (TCP) in the Internet Protocols suite and is the only OSI protocol class that supports connectionless network service.
1.4 OSI协议栈的传输层
在传输层,OSI协议栈实现两类服务：面向连接的传输服务和无连接的传输服务。在OSI协议栈中,有五种面向连接的传输层协议s，范围从传输协议类0到类4(TP0到TP4)。无连接传输服务只支持TP4(传输协议类4)。TP0 (第0类传输协议),一种最简单的OSI传输协议,执行分段和重组功能。TP0需要面向连接的网络服务。TP1（第1类传输协议）除了执行分段和重组，还提供基本的错误恢复。TP1还以协议数据单元（PDU）为单位进行排序, 如果过多的PDU不被认可,就重新发送PDUs或重新启动连接。TP1需要面向连接的网络服务。TP2（第2类传输协议）执行分段和重组，以及在一个单一的虚电路上复用和解复用。TP2上要求面向连接的网络服务。TP3（第3类传输协议）除了在一个单一的虚电路上数据流的复用和解复用外,还提供基本的错误恢复和执行分割及重组。TP3还以协议数据单元（PDU）为单位进行排序, 如果过多的PDU不被认可,就重新发送PDUs或重新启动连接。TP3需要面向连接的网络服务。TP4（第四类传输协议）提供了基本的错误恢复，执行分段和重组，并提供在一个单一的虚电路上数据流的复用和解复用。TP4还以协议数据单元（PDU）为单位进行排序, 如果过多的PDU不被认可,就重新发送PDUs或重新启动连接。TP4提供可靠的传输服务和功能,无论是面向连接还是无连接网络服务。这是基于互联网协议栈的TCP（传输控制协议）,是唯一支持无连接网络服务的OSI传输协议类。
1.5 OSI Protocols Session Layer
The session layer implementation of the OSI protocol suite consists of a session protocol and a session service. The session protocol allows session-service users (SS-users) to communicate with the session service. An SS-user is an entity that requests the services of the session layer. Such requests are made at session-service access points (SSAPs), and SS-users are uniquely identified by using an SSAP address. Session service provides four basic services to SS-users. First, it establishes and terminates connections between SS-users and synchronizes the data exchange between them. Second, it performs various negotiations for the use of session layer tokens, which the SS-user must possess to begin communicating. Third, it inserts synchronization points in transmitted data that allow the session to be recovered in the event of errors or interruptions. Finally, it enables SS-users to interrupt a session and resume it later at a specific point.
1.5 OSI Protocols Session Layer OSI协议栈的会话层
OSI协议栈的会话层实现,包含了一个会话协议和一个会话服务。会话协议允许会话服务用户（SS-用户s）与会话服务相通信。一个SS-用户是一个实体,他向会话层申请服务s。这样的申请是在SSAPs(会话服务访问点s)提出的，通过使用SSAP地址,唯一地识别SS-用户。会话服务为SS-用户s提供了四种基本服务。第一种基本服务，它建立和终止SS -用户间的连接，同步它们之间的数据交换。第二种基本服务，因为SS-用户必须占有令牌才能进行通信，为了使用会话层令牌s而必须进行各种协商。第三种基本服务，在发生错误或中断事件时,于被发送的数据中插入同步点,从而允许恢复会话。最后一种基本服务，它使SS-用户s中断一次会话,并在以后在指定处重新开始。
图：会话层功能,经由SSAP,提供服务给表示层功能显示了SS-用户s,SSAP,会话协议和会话服务之间的关系。
图：会话层功能,经由SSAP,提供服务给表示层功能
Figure: Session Layer Functions Provide Service to Presentation Layer Functions via an SSAP shows the relationship between the SS-user, the SSAP, the session protocol, and the session service.
Figure: Session Layer Functions Provide Service to Presentation Layer Functions via an SSAP
Session service is defined in the ISO 8306 standard and in the ITU-T X.215 recommendation. The session protocol is defined in the ISO 8307 standard and in the ITU-T X.225 recommendation. A connectionless version of the session protocol is specified in the ISO 9548 standard.
会话服务定义在ISO 8306标准中,以及在ITU - T的X.215建议中。会话协议定义在ISO 8307标准中,以及在ITU - T的X.225建议中。一个会话协议的无连接版本被指定在ISO 9548标准中。
1.6 OSI Protocols Presentation Layer
The presentation layer implementation of the OSI protocol suite consists of a presentation protocol and a presentation service. The presentation protocol enables presentation-service users (PS-users) to communicate with the presentation service. A PS-user is an entity that requests the services of the presentation layer. Such requests are made at presentation-service access points (PSAPs). PS-users are uniquely identified by using PSAP addresses. Presentation service negotiates transfer syntax and translates data to and from the transfer syntax for PS-users, which represent data using different syntaxes. The presentation service is used by two PS-users to agree upon the transfer syntax that will be used. When a transfer syntax is agreed upon, presentation-service entities must translate the data from the PS-user to the correct transfer syntax. The OSI presentation layer service is defined in the ISO 8822 standard and in the ITU-T X.216 recommendation. The OSI presentation protocol is defined in the ISO 8823 standard and in the ITU-T X.226 recommendation. A connectionless version of the presentation protocol is specified in the ISO 9576 standard.
1.6 OSI协议栈的表示层
OSI协议栈的表示层实现,包含一个表示协议和一个表示服务。表示协议使表示服务用户s（PS-用户s）与表示服务通信。一个PS用户是一个实体,它申请表示层服务。(PSAPs).这样的申请是在PSAPs（表示服务访问点s）完成的。 PS-用户是通过使用PSAP地址来唯一标识的。表示服务协商传输语法,并根据用于PS-用户s的传输语法来转换数据，这说明,使用不同的语法来表示数据。表示服务实现两个PS-用户经协商确认,它们将要使用的传送语法。当一个传输语法商定了，表示服务实体s必须把数据从PS -用户转换到正确的传输语法。OSI表示层服务定义在ISO 8822标准和ITU - T的X.216建议中。OSI表示协议定义在ISO 8823标准和ITU - T的X.226建议中。一个表示协议的无连接版本被指定在ISO 9576标准中。
1.7 OSI Protocols Application Layer
The application layer implementation of the OSI protocol suite consists of various application entities. An application entity is the part of an application process that is relevant to the operation of the OSI protocol suite. An application entity is composed of the user element and the application service element (ASE). The user element is the part of an application entity that uses ASEs to satisfy the communication needs of the application process. The ASE is the part of an application entity that provides services to user elements and, therefore, to application processes. ASEs also provide interfaces to the lower OSI layers.
1.7 OSI协议应用层
在OSI协议栈的应用层实现包括各种应用实体。应用程序实体是,与OSI协议栈操作有关的,应用程序进程的一部分。一个应用程序的实体是由用户组件和ASE（应用服务组件）组成。用户组件是 应用程序实体的一部分，它使用的ASEs来满足应用程序进程的通信需要。ASE是一个应用程序实体的一部分,它向用户组件提供服务,因而也就是向应用程序进程提供了服务。ASEs也向OSI低层提供了接口。
图：一个应用程序进程依赖于PSAP和表示服务描述了单一应用进程的构成（由应用实体,用户组件,和ASEs组成）以及与PSAP和表示服务的关系。
图：一个应用程序进程依赖于PSAP和表示服务
Figure: An Application Process Relies on the PSAP and Presentation Service portrays the composition of a single application process (composed of the application entity, the user element, and the ASEs) and its relation to the PSAP and presentation service.
Figure: An Application Process Relies on the PSAP and Presentation Service
ASEs fall into one of the two following classifications: common-application service elements (CASEs) and specific-application service elements (SASEs). Both of these might be present in a single application entity.
ASEs分属下面两种类别之一：CASEs(公共应用服务组件)和SASEs(特殊应用服务组件)。两者都可以在单一应用实体中存在。
Common-Application Service Elements
Common-application service elements (CASEs) are ASEs that provide services used by a wide variety of application processes. In many cases, multiple CASEs are used by a single application entity. The following four CASEs are defined in the OSI specification:
公共应用服务组件
CASEs(公共应用服务组件)是一种ASEs,各种各样的应用程序进程使用它们提供的服务。在许多情况下，单一应用实体使用多个CASE。在OSI规范中定义了下面四种CASE：
Specific-Application Service Elements
Specific-application service elements (SASEs) are ASEs that provide services used only by a specific application process, such as file transfer, database access, and order entry, among others.
特殊应用服务组件
特殊应用服务组件(SASEs)是一种ASEs,只提供服务给一个特殊应用进程,如文件传输,数据库访问,和订单输入,以及其它等。
OSI Protocols Application Processes
An application process is the element of an application that provides the interface between the application itself and the OSI application layer. Some of the standard OSI application processes include the following:
OSI协议应用进程s
一个应用进程是一个应用的组成部分,它提供应用本身和OSI应用层之间的接口。某些标准应用进程s包括以下几个部分：
2. Review Questions
Q - What are the two routing protocols specified in the OSI suite?
A - End System-to-Intermediate System (ES-IS) and Intermediate System-to-Intermediate System (IS-IS).
Q - Describe the OSI connectionless network protocol.
A - OSI connectionless network service is implemented by using the Connectionless Network Protocol (CLNP) and Connectionless Network Service (CLNS). CLNP and CLNS are described in the ISO 8473 standard.
Q - Describe the OSI connection-oriented network protocol.
A - OSI connection-oriented network service is implemented by using the Connection-Oriented Network Protocol (CONP) and Connection-Mode Network Service (CMNS).
Q - How are requests to services at the session layer made within OSI protocols?
A - Requests are made at session-service access points (SSAPs), and SS-users are uniquely identified by using an SSAP address.
Q - Describe common-application service elements (CASEs).
A - Common-application service elements (CASEs) are ASEs that provide services used by a wide variety of application processes. In many cases, multiple CASEs are used by a single application entity.
Q - Name some of the media types that the OSI protocol suite supports.
A - IEEE 802.2 LLC, IEEE 802.3, Token Ring/IEEE 802.5, Fiber Distributed Data Interface (FDDI), and X.25.
Q - Why was the OSI protocol suite created?
A - The OSI specifications were conceived and implemented by two international standards organizations: the International Organization for Standardization (ISO) and the International Telecommunication Union-Telecommunications Standards Sector (ITU-T).
Q - Describe the session layer protocols within the OSI protocol suite.
A - The session layer implementation of the OSI protocol suite consists of a session protocol and a session service. The session protocol enables session-service users (SS-users) to communicate with the session service. An SS-user is an entity that requests the services of the session layer. Such requests are made at session-service access points (SSAPs), and SS-users are uniquely identified by using an SSAP address.
Q - Describe the presentation layer protocols of the OSI protocol suite.
A - The presentation layer implementation of the OSI protocol suite consists of a presentation protocol and a presentation service. The presentation protocol enables presentation-service users (PS-users) to communicate with the presentation service.
Q - What are the two types of ASEs?
A - ASEs fall into one of the two following classifications: common-application service elements (CASEs) and specific-application service elements (SASEs). Both of these might be present in a single application entity.
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2. Review Questions 复习问答
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