 is particularly striking. It is simply the product of 3.1.3 Kendall Queuing Notation the equilibrium distribution for each queue operating as if it were in isolation! As randomness plays such an important part in queuing perfor- Such models have been used to analyse the performance of packet mance queues are categorised accordingly. The notation A/B/m/S/P switched networks. In these networks, packets of information are is in common use, as in Table 3.1. transmitted from source to destination via a series of transmission links. These transmission links are interconnected by switching nodes which queue the packets in storage buffers until it is their turn Table 3.1 Notations in common use to be transmitted. (Packet switched networks are sometimes referred to as store and forward networks.) From these models it is known that the delays which packets Symbol experience and, indeed, the throughput of the network itself depends Interarrival distribution strongly on the routes which the packets take through it. But deter- Service distribution mining the best choice of routes using such models is not easy because the assignment of one particular route or set of routes to one Number of servers group of packets has an effect on the delays which other packets Storage capacity experience. However effective routing algorithms have been pro- posed based on models such as the ones mentioned above (Gallager, Customer population 1977). 3.1.2 Little's law Omission of either the storage or the population entry means they are infinite. The most often used interarrival and service distribu- There are very few completely general queuing results. However tions are given in Table 3.2. there is one, which is of considerable utility for the analysis of queues in equilibrium, Little's law. It even applies to systems which can hardly be thought of as queues at all. Suppose that customers arrive to the system (depicted as a black box in Figure 3.3) at a rate Table 3.2 Distribution symbols in use with Kendall queuing a and that the average so