ount of resources required to handle a given type of call (i.e. UE-UE, UE-MS, MS-MS, UE-PSTN, MS-PSTN, etc.). The definition of the coverage areas for MGWs is quite flexible as the constraints are on the maximum size but not the optimal size. There are therefore two possible solutions, i.e. distributed and centralised architectures. Distributed Architecture of MGWs In a distributed architecture, the MGWs are located close to the RNC/BSC, which is the major source of traffic in the network (Figure 4.12). This enables switching of user traffic close to the traffic source, thus reducing transmission costs. Traffic generated by the RNCs/BSCs is connected directly to a given MGW and terminated in the RNCs/BSCs connected to the same MGW and can be switched directly close to the originating site with no need to carry it to the core site. In order to maximise the transmission savings, traffic terminating in other networks (POIs) should be switched locally at the MGWs. Therefore the operator should consider providing a POI to the external network (i.e. PSTN/PLMN) at the host MGW. The interconnect design for the outgoing traffic should allow local PSTN/PLMN traffic to be emptied directly at the closest POI. The distributed architecture of the MGW reduces the required transmission costs but at a cost because it increases the number of MGWs as more MGWs are required to collocate with the RNC/BSC. In sites with high user traffic, this solution is cost-effective but in sites with low user traffic the transmission savings are too low to justify the increased number of MGWs. Centralised Architecture of MGWs The centralised architecture of MGWs is more suitable for access sites with low user traffic (Figure 4.13). The MGWs can be located in a centralised core site while the traffic from the access site is carried to the core site using transmission lines. The result is a reduced number of MGW elements in a few selected core sites, where traffic is collected and switched to/from all RNCs/BSCs in the network and to external network