difficult task. In such cases, another dimension is added to routing, i.e. transit switches are added. Figure 4.4 Interswitch connection If a transit switch (TS) were added to the network, it would look like Figure 4.5. All the traffic, both internally and externally generated, will be routed through the transit switch. As the transit switch has great importance in this arrangement, it is often advised to have a second redundant transit switch. If the cellular network is spread over a very large region and has many switches, then the number of transit switches increases also. Although the MSCs Figure 4.5 Switch network with transit switch CORE NETWORK PLANNING PROCESS are connected to the nearest transit switches, the network topology between the transits switches themselves is fully meshed. Next the routing plan is devised the flow of traffic between switches. One single route must not be overloaded with traffic; rather, the traffic needs to be evenly divided between various routes in the best possible way. The routing plan gives the direct path that is taken by traffic and, in case of failure of the primary route, the secondary path taken. The secondary path choice is dependent on the traffic it is already carrying. Thus, even in the initial days of the network, spare capacity planning is required SO as to route calls through the secondary paths when the primary ones are blocked. An example of traffic dimensioning is shown in Figure 4.6. There are four MSCs and an external network. The number of subscribers handled by each MSC is 50,000. With the traffic generated by each subscriber being 27 mErl (milli-Erlangs), the total traffic generated by each MSC is 1350 Erl. The percentage of calls originating within their own switch is 60%, of which 40% terminate in the same network. Calling interest and moving interest are 215 km and 50 km respectively. Input call int move int subscribers 50000 50000 50000 50000 tr/subs (mErl) total traffic subs originating subs terminating to own network to own NW of total to own NW o