ctions re- quired on each route between a pair of exchanges depends on the traffic between them. If a direct junction route is provided between every pair of exchanges, these routes form a fully interconnected mesh network, as shown in Figure 29.2(a). An alternative is to provide a junction route from each exchange to a central switching centre, to form a star network as shown in Figure 29.2(b). The exchanges to which subscribers' circuits are connected are called local exchanges and the central switching centre is called a tandem exchange. In a star junction network with a tandem exchange there are fewer junction routes than in a fully-interconnected mesh. However, each route incurs the additional cost of its associated switching equip- ment in the tandem exchange. Consequently, if there is sufficient traffic between a pair of local exchanges to ensure that direct junctions are heavily loaded and they are cheap because the distance is short, it is more economical to provide direct junctions between these exchanges than to route the traffic indirectly through a tandem exchange. Consequently, a multi-exchange area will often contain direct junction routes between nearby local exchanges which have a high community of interest, and traffic between the other ex- changes will be routed indirectly via a tandem exchange. The network thus consists of a mixture of a star and meshes, as shown in Figure 29.2(c). In a multi-exchange area, only a minority of calls generated at a local exchange will be to other subscribers served by that exchange. Figure 29.3 Effect of number of exchanges on length of Most traffic will be to or from other exchanges, either in the same subscribers' lines: (a) single-exchange area (b) area or outside it. Thus, the principal function of a local exchange multi-exchange area is not to carry traffic between its own subscribers, but to provide an interface between these subscribers and the rest of the network. A national telecommunication network links many towns and 3. Signalling systems. cities; it