test2-2_计算机网络-back-all

计算机网络

Communication between distributed communities of computers is required for many reasons．At a national level，for example，computers located in different parts of the country use public communication services to exchange electronic messages (mail) and to transfer files of information from one computer to another．Similarly，at a local level within，say，a single building or establishment．distributed communities of computer-based workstations use local communication networks to access expensive shared resources—for example，printers，copiers，disks，tapes，etc．—that are also managed by computers．Clearly，as the range of computer communication networks proliferate，computer to computer communication will expand rapidly and ultimately dominate the field of distributed systems．A communication network is a series of points interconnected by communication lines，located at the points are computers，switching devices，and user terminals．

由于多种原因，分布式的计算机群之间需要进行通信，例如，在全国范围内，位于不同地区的计算机使用公共通信服务设施 来交换电子信息(邮件)及从一台计算机传递文件信息到另一台。类似地，在局部（域）范围内，比如说在单独的大楼或建筑内，分散的计算机工作站可以利用局域网共享昂贵的资源，如打印机、复印机、磁盘、磁带等，这些（设备）也都是由计算机管理的。很显然，随着计算机通信网范围的扩大，计算机之间的通信将迅速增加，而且最终将主导整个由分散的计算机系统构成的领域。通信网是由通信线路互联在一起的一系列节点，位于节点上的是计算机、交换设备和用户终端。

In general a computer network，as we know today，may be said to have gotten its start with the ARPANET development in the late 1960s and early 1970s．The initial ARPANET design had a definite structure of network architecture and introduced another key concept：protocol layering． Much of our present knowledge about network is a direct result of the ARPANET project．

一般来讲，人们今天所说的计算机网络，可以说是从20世纪60年代末到70年代初 ARPANET网的产生（开发）开始的。最初的ARPANET网的设计就具有确切的网络体系结构，并提出了另一个关键的概念：协议分层。现在许多关于网络的知识都是ARPANET网项目的直接产物。

The ISO, in the late 1970s，formulated a reference model to provide a common basis for the coordination of standards developments and to allow existing and evolving standards activities to be placed into perspective with one another．The ultimate aim was to allow an application process in any computer

that supported the applicable standards to freely communicate with an application process in any other computer supporting the same standard，irrespective of its origin of manufacture．This model was termed the ISO reference model for open system Interconnection (as shown in Fig．12．1 )． It should be stressed，however，that this model is not concerned with specific applications of computer communication networks．Rather．it is concerned with the structuring of the communication software that is needed to provide a reliable，data transparent with the structuring of the communication software that is needed to provide a reliable，data transparent communication service (which is independent of any specific manufacturers equipment or conventions) to support a wide range of applications．

国际标准化组织ISO在20世纪70年代后期提出（发布）了一个参考模型，该模型为协调标准的开发提供了一个公共基准，并且使得现有的和发展中的标准相互作用协调，其最终目标是使支持这些适用的标准的任何计算机能与支持相同标准的其他计算机上的应用进程间随意通信，而不管使用何种厂家生产的计算机，这一模型称为ISO开放系统互连参考模型。需要强调的是，该模型与计算机通信网络专用的程序无关。然而，为支持更大范围的应用，它只与需要提供可靠的、数据透明的传输（通信）服务的通信软件结构有关，而与任何厂家的设备和规范无关。

A network is a series of points interconnected by communication lines，located at the points are computer，switching devices，and/or user terminals．A typical network includes the following components：host computers，nodes，terminal controllers，and terminals．

网络是由通信线路互联在一起的一系列节点，位于节点上的是计算机、交换设备与／或用户终端。一个典型的网络包括以下组件：计算机主机、节点、终端控制器和终端。

The benefit of network is that it can transmit information rapidly and easily among widely separated people；people can share hardware and software resources on the network；a computer network can also provide high reliability by having alternative source of supply．

网络的优点是它能够在相距很远的人们之间快速便捷地传递信息；大家可以在网络上共享软硬件资源；而且通过资源的冗余备份，计算机网络还可以提供更高的可靠性。