Overview

In Chapters 4, 5, and 6, we introduced many of the concepts and hardware components related to layers 1 and 2 of the OSI model. Our discussion in these chapters also was relative to local area networks. In this chapter, we extend our previous discussions to wide area links. As part of this presentation we introduce the concept of internetworking from a WAN perspective and discuss the third layer of the OSI model the network layer.

1. In Chapter 1, you said a WAN can be thought of as a collection of interconnected, geographically separated LANs. How are LANs interconnected to form a WAN?

   Wide area networks employ a point-to-point topology. In the simplest WAN one that interconnects two LANs a typical scenario involves two remote bridges or routers (one at each end), interconnected via a WAN data communication circuit. (See Figure 7-1.) WANs use either circuit switching or packet switching techniques. (See Chapter 2 and Table 2-1 for more information about packet and circuit switching.) In a circuit-switched WAN, a fixed connection is established between source and destination nodes prior to transmission, each packet takes the same path, and all packets arrive in sequence. ISDN (see Chapter 11) is one example of a circuit-switched WAN. In a packet-switched WAN, connections are established during the transmission process. Thus, packets do not necessarily travel the same route and they might arrive out of sequence at the destination node. Examples of a packet switched WAN include frame relay (see Chapter 12), SMDS (see Chapter 13),...