WIDE AREA NETWORK 广域网
A wide area network，or WAN，spans a large geographical area，often a country or continent．It contains a collection of machines intended for running user（i. e. ，application）programs．We will follow traditional usage and call these machines hosts． The term end system is sometimes also used in the literature．The hosts are connecte by a communication subnet，or just subnet for short．The job of the subnet is to carry messages from host to host，just as the telephone system carries words from speaker to listener．By separating the pure communication aspects of the network（the subnet）from the application aspects（the hosts），the complete network design is greatly simplified．Relation between hosts and the subnet is shown in Fig. 14-4．
广域网跨越地域很大，通常是一个国家或一个大陆。这种网络中汇集了运行各种用户程序（即应用程序）的计算机。本文还是沿用传统的称呼把它们叫做主机，在一些文献里有时也称其为端系统。这些主机是通过通信子网（有时简称子网）连接起来的。子网的任务是把报文从一个主机传送到另一个主机，就像电话系统把话音从讲话人传到收听者一样：把网络（子网）的纯通信任务与主机的应用任务分开以后，整个网络的设计工作就得到了很大的简化。图14-4展示了主机与通信子网之间的关系。
One of many methods that can be used to categorize wide area networks is with respect to the flow of information on a transmission facility．If we use this method to categorize wide area networks，we can group them into three basic types：circuit switched，leased line and packet switched．
广域网的分类方法很多，其中一种方法是按传输设备中的信息流分类。如果使用这种方法，可将广域网分成3种基本类型：电路交换、专线和分组交换。
1．CIRCUIT SWITCHED NETWORKS 电路交换网
The most popular type of network and the one almost all readers use on a daily basis is a circuit switched network．The public switched telephone network，however，is not limited to the telephone company．By purchasing appropriate switching equipment，any organization can construct their own internal circuit switched network and，if desired，provide one or more interfaces to the public switched network to allow voice and data transmission to flow between the public network and their private internal network．
电路交换网是最常用的并且是几乎所有用户每天都要用的网络类型。然而，公用电话交换网不限于电话公司。任何单位只要购买了合适的交换设备，就可以建立自己的内部电路交换网，而且如果需要，还可配置一至多个与公用交换网相连的接口，在公用网和他们的专用内部网之间进行话音和数据传输。