 thus contains many of these multi-exchange areas. There is thus a need for a network of long-distance routes between them; this These systems are discussed in more detail in Chapters 30, 31 and constitutes the trunk network. It is obviously uneconomic for the 43, respectively. trunk network to interconnect all the local exchanges. However, Network architecture Total network If each subscriber's station in a telecommunication system must be able to communicate with every other station, one possibility would be to provide a circuit between each pair of stations, as shown in Figure 29.2(a). If there are n stations, the number of circuits required is 1/2n(n-1). Thus, the cost is approximately proportional to If n Junction line is large, such a fully- interconnected mesh network would obviously be enormously expensive. Instead, each station is provided with a circuit to a central switching system, or exchange, as shown in Exchange cost Figure 29.2(b). The number of circuits required is then only n. Clearly, the star network of Figure 29.2(b) is much cheaper than the Subscriber line cost mesh network of Figure 29.2(a), unless n is very small. Site & building If an exchange area is large and contains a large number of stations, many of the subscribers' circuits will be long and therefore Optimum Number of exchanges expensive, as shown in Figure 29.3(a). If the area is divided into number several separate areas, each with its own exchange, as shown in exchanges Figure 29.3(b), subscribers' circuits are shorter and their total cost is less. However, the costs of switching equipment and its accom- modation increase with the number of exchanges. Thus, as shown in Figure 29.4, there is an optimum number of exchanges for which the total cost of the network is a minimum. Figure 29.4 Variation of network cost with number of exchanges Network architecture 29/5 Satellite links Submarine cables International gateway exchange (Centre de Transit 1) International network National tandem exchanges (tertiary trunk switching centres) T