le access protocols, is models can be used to determine all the usual key performance a case in point. Such a channel will be occupied not only with the measures: delay, queue lengths etc. Based on this, the results here transmission of users' information but also with reservation periods are confined to equilibrium models. where the users actually obtain rights to transmit. Some time is used Typical delay performance for a single server queue in equili- in transmissions, some in reservation but how much time is spent in brium with a single customer class is shown in the steadily increas- each task? This depends on the nature of the protocols themselves. ing graph depicted in Figure 3.1. The horizontal axis is the For example, consider the operation of slotted ALOHA depicted in Figure 3.2. Packets of information are all the same length and a Average Delay Station Collision Success Figure 3.2 The slotted ALOHA protocol Occupancy packet is received successfully if it is the only one to be transmitted in a particular slot. If two or more are transmitted there is a collision Figure 3.1 Delay performance of a single server queue and none are successfully received, such packets are retransmitted at random in later slots. But there are many ways to choose a slot for retransmission at random and this choice is critical. It determines occupancy (arrival rate/service rate), which is the proportion of time how successful the protocol is in avoiding collisions and how many the customers keep the server busy. The vertical axis is average packets can be eventually transmitted. A poor protocol choice may delay. As the graph shows, there is an increasing delay penalty for give very little useful throughput, or, worse still, cause the channel using the system closer and closer to full capacity (occupancy =1). to jam. In this case it turns out that using fixed retransmission Also shown is the graph for a deterministic system (constant time probabilities does not work very well. It makes no allowance for the between arrivals and co