satellite. communication links hired from a PTT or common carrier. Their Network topology sometimes plays a role in the selection of the operating speed is usually restricted by these links. multiple access technique. Figure 13.1 shows the most commonly Metropolitan Area Networks (MANs) fall in-between LANs and WANs in geographical area and characteristics. Although multiple access techniques can be used irrespective of the transmission technology, their performance is often different. For example radio networks have a relatively low propagation time, of the order of a few milliseconds. They are largely omnidirectional and are usually sent as a broadcast to all receivers, although some may be out of reach or screened from the source. Satellite based radio systems broadcast their information, but can be made to have a relatively small footprint on the earth. They suffer from long delays, of the order of a quarter of a second (up-link and down-link), which would seriously hamper the performance of some multiple access techniques. 13.2 Queuing theory Queuing theory is covered in Chapter 3 and in Kleinrock (1976). The basic parameters are introduced in this section. Queuing analysis usually refers to queues by a notation such as A/B/m. This applies for an infinite population of users, which is usually the assumption made when analysing multiple access tech- niques. In this notation A represents the interarrival time probability density, i.e. the time between consecutive arrivals into the queue. B is the service time probability density, i.e. the time required to serve an arrival. The letter m represents the number of servers which are present to accommodate the arrivals. Values of A and B vary from M (representing Markov or exponen- tial probability density); D (representing Deterministic i.e. all arri- vals have the same value); and G (representing General or arbitrary probability density). The most common is the M/M/1 queue, and this is used in the Figure 13.1 Popular network topologies: analysis in this chapter. T