INTEGRATED SERVICES DIGITAL NETWORK（ISDN） 综合业务数字网
A good example of a circuit-switched network is the Integrated Services Digital Network（ISDN），an evolving communications network standard that provides universal end-to-end connectivity over digital lines．ISDN services are still being introduced and are not yet available everywhere[1]．Once ISDN is fully implemented，however，today’s disparate transmission and switching services will be accessible internationally via a single set of interface standards．
电路交换网的一个成功的例子是综合业务数字网（ISDN），它是一种正在发展中的、在数字通路上提供多种端到端连接的通信网络标准。ISDN业务仍在推广中，尚未普遍应用。不过，一旦ISDN完全实现，则通过一组单一接口标准可以实现当今国际间异类传输和交换业务。
As a worldwide telecommunications network standard，ISDN will pull together a wide variety of consumer services into a single access package．An ISDN allow users to send data， voice，image，facsimile，and so on over the same wires and to have ready access to digital service．
作为世界范围的电信网络标准，ISDN将把各式各样的客户服务综合成一种单一访问的数据包。ISDN允许用户在同一组线路上发送数据、话音、图像、传真等，也可使用户随时访问数字业务。
1．ISDN System Architecture ISDN系统结构
It is now time to look at the ISDN architecture in detail，particularly the customer’s equipment and the interface between the customer and the telephone company or PTT．
现在来详细地看一下ISDN的结构，特别是客户设备和客户与电话公司或邮电局之间的接口。
In Fig. 14-9 we see the normal configuration for a home or small business．The carrier places a network terminating device NT1 on the customer’s premises and connects it to the ISDN exchange in the carrier’s office，several kilometers away，using the twisted pair that was previously used to connect to the telephone[2]．The NTl box has a connector on it into which a passive bus cable can be inserted．Up to eight ISDN telephones，terminals，alarms，and other devices can be connected to the cable，similar to the way devices are connected to a LAN．From the customer’s point of view，the network boundary is the connector on NTl．
图14-9展示了一种家庭或小型企业的通常配置。通信公司把网络终端设备NT1放在客户处所，并通过以前用于连接电话的双绞线将它连至几千米远的通信公司内部的ISDN交换机上。NTI盒子上有一个可以插入无源总线电缆的连接器，最多可以有8个ISDN电话、终端、警铃和其他设备连在该电缆上，这与在局域网上连接设备的方法一样。从客户的观点来看，该网络的边界是NT1上的连接器。
