nstant service time). No queue builds up variation in the backlog of packets awaiting retransmission. How- until the system is at full load. When the occupancy is less than 1 it ever, adjusting the retransmission probability according to an esti- is said that the queue is stable and an equilibrium will then exist, as mate of this backlog does perform adequately. Each packet is already mentioned. retransmitted with high probability when the backlog is estimated In most queuing situations arrivals and services do not take place to be low and with low probability when the estimated backlog is in a deterministic fashion and SO it is the former graph which is more high. 3/4 Queuing theory In queuing network problems there is more than one queue and remain in the system. There are two equivalent ways in which the customers make a series of visits from one queue to the next. In such money can be collected. The first is to collect the appropriate networks there are interactions between the individual queues amount from each customer as they leave. Since the average dura- within it. For example the pattern of arrivals at one queue is tion within the system is E[D] and the mean departure rate must be determined by the departure patterns from the queues which have a the rate at which money is being collected is a E[D]. The other customers feeding into it (together with the new arrivals to the way to collect the money is for the customers to pay continuously network). Since these departure patterns themselves depend on the as they remain in the system. Since the average number of cus- arrivals to those queues analysis can prove very difficult. tomers within the system is E[N] the rate at which money is being It is interesting to learn, therefore, that for one very wide class of collected is also E[N]. By altering the rates of payment one may networks it is relatively easy to determine the equilibrium distribu- obtain other similar identities. tion of the customers within the network. Furthermore the form of this distribution