说明：   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。
( 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. )