s. Fewer MGWs makes easier management of the network configuration, interconnections and traffic routing. In a real project situation a pure decentralised or centralised architecture of the MGW is rare but a hybrid solution with some sites with heavy access traffic with a decentralised solution and some remote sites with low user traffic in a centralised solution is common. The selection of an architecture solution is therefore the prerogative of the network designer to ensure an optimum solution. As a general rule the distributed solution can be used when there is heavy 'local' traffic at a site. This often takes place when 3GPP Release 4 Circuit Core Network Backbone Figure 4.13 MGW centralised architecture the RNC/BSC site covers a large geographical area (rural area) with low user density. The POI for MOC to PSTN should be located close to the RNC/BSC sites and the transmission costs are high compared to costs of deploying and operating an MGW. Another situation where a distributed solution could be selected is in the replacement of outdated old switching equipment (e.g. a low capacity subrack MSC) with an A interface re-homing to MGW elements. In this case, placing one or more MGWs of the same capacity as the replaced MSCs in the same site allows minimal reconfiguration and enables reuse of existing transmission links to the BSCs. A centralised solution is the preferred solution when the inter-MGW traffic is high. The total number of MGW to meet the operator's requirement therefore depends on the selected user plane network architecture. Finally, the capacity statement for the MGW in certain SW (software) releases enables the total number of required MGW elements to be estimated. The capacity of the MGW depends on the following factors: call handling capacity, i.e. the total number of BH call attempts and simultaneous calls; port capacity, i.e. E1/T1 ports, ATM ports, IP ports; hardware configuration, i.e. signalling units, transcending units, etc. It is also important to take into account any resiliency req