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A communication protocol is a set of rules to ensure the successful transfer of data between two terminals. These rules are analogous with the rules governing human conversation, e.g. only one person should speak at one time, things not understood can be repeated, etc. Data link protocols are one type of communication protocol and govern the transfer of data over both point-to-point links and multi-drop links. A point-to-point link is analogous to a conversation between two people, whilst a multi-drop link, between e.g. a mainframe computer and many terminals, is analogous to the way a TV presenter takes questions from an audience for a panel of experts.

The previous chapter concentrated on point-to-point links. In this chapter we use these links to build a Wide Area Network (WAN). WANs are to be contrasted with Local Area Networks (LANs), which are discussed in the next chapter. A simple distinction between WANs and LANs is that WANs are designed to cover a large geographical area, whilst LANs are designed for a restricted area, e.g. an office building or factory. Another way of distinguishing between WANs and LANs, this time in terms of the network parameters t p (end-to-end propagation delay) and H (holding time), is that the factor a = t p /H is, say, ≥ 10 for WANs and < 10 for LANs, as discussed in chapter 7 on latency.

This chapter considers a data communication channel, using the example of a modem link across the telephone network to connect two computers. The channel can be considered in two parts: from the computers to the modems, and between the modems. In both cases we need a transmission path and some means of coding the data to ensure that it can be transmitted over the link.

LANs such as Ethernet and Token Ring provide a high speed connection compared to WANs such as X.25; they allow workstations to communicate at high speed with little delay, creating a seamless environment where the end users are unaware of the supporting network. The challenge is to extend this seamless environment by providing high speed connections over a wide area, i.e. over WANs and MANs. Fast packet switching is the generic name for new high speed WAN and MAN services and is the topic of this chapter. The protocols considered are: frame relay, which is an access protocol for a WAN; ATM (Asynchronous Transfer Mode), which is a cell relay based WAN protocol; and DQDB (Distributed Queue Dual Bus) which is a MAN protocol. They are not mutually exclusive, indeed both Frame Relay and DQDB can be used to access an ATM network. However, before looking at these protocols we need to consider the topic of congestion, as it is crucial to the design of WANs and WAN protocols.

Local Area Networks (LANs) were originally a way of sharing a resource, such as a printer or modem, amongst several workstations. The solution chosen was to provide a common medium between the workstations and the server, e.g. the printer server. Only one station can use the medium at any one time but it provides a high speed channel, e.g. 10 Mbps in some versions of Ethernet. The alternative way of doing this would have been to use a company’s digital PABX (Private Automatic Branch eXchange) to switch the data over telephone lines. However, although switching means many channels can be simultaneously supported, the data rates available then (typically 64 kbps) were too low to offer a satisfactory solution.