e may therefore be dimensioned to carry the total load (i.e. 100 %) but the secondary route should also be dimensioned to carry some of the load during failures. For example, the secondary route may be dimensioned for 20 % spare capacity. It is therefore important to determine the number of internal routes and the routing strategy. The applied routing strategy should be established. Capacity calculations may vary considerably de- pending on whether the operator applies load sharing or alternative routing. Generally in load-sharing of calls, voice traffic is shared equally among the routes while the alternative routing allows for dispro- portional distribution of the traffic load. Often, for load-sharing, a 50-50 % ratio is applied for primary and secondary routes. For example, alternative routing of 100-20 % sharing of traffic is allowed. This implies that an extra 20 % capacity is required on the alternative route and in the case of link failure 20 % capacity will be supported on the additional route. The network environment in relation to other telecommunication networks existing in service area of the operator should be considered. A study of the percentage of outgoing traffic destined for each of the existing networks should be established in order to determine the most suitable external routing strategy. Increasing traffic between two operators presents the need to open direct POI to avoid routing failures and additional transit costs, but if the amount of traffic from an MSC area to another operator is relatively small then the traffic may be routed to the nearest POI with the operator or when it is even smaller to Core Network Planning and Optimisation the nearest PSTN connection. The last option should, however, only be considered if there are no formal restrictions for transit traffic through the PSTN. Signalling Network Plan Review The signalling system can be used with different types of signalling network structures. The choice be- tween different types of signalling network structures may be influenc