MGWs but only to one MSS; therefore the division of subscribers among MSSs is based on the RNC's size. The initial plan should consider the coverage area of one MSS as the union of the coverage areas of the controlled MGWs and consider the effect of virtual MGWs only when consistent unused capacity is available in a given MSS. For instance, three MGWs with a capacity of 400k BHCAs can be controlled by three MSSs partially loaded (3 X 400k BHCAs) or by two MSSs at the maximum capacity (2 X 600k BHCAs), if this is allowed by the subscriber distribution between the RNCs. The number of GCSs is calculated based on the MGWs dedicated to the PSTN interconnection and their traffic load in terms of BHCAs (usually calculated after the user plane is routed). In the case of new delivery stand-alone MSSs/GCSs, as they are handling only control plane traffic, the only constraint on the physical location of the elements is the availability of IP connectivity for the signalling traffic. In the case of integrated or upgraded MSSs/GCSs, their location is determined by the fact that the element provides interworking functionality for CS data or not. Signalling Network Dimensioning In mobile networks, signalling network dimensioning can be considered in two major areas: Call-related signalling. In call-related signalling, all SS7 signalling messaged required to establish, maintain and tear down a call would be considered. Non-call-related signalling. In non-call-related signalling other signalling services for mobility man- agement, SMS, IN, etc., would be considered. The message flows for SS7 signalling are defined by the 3GPP standards. Average message sizes depend on the message signal unit (MSU) payload sizes and OH introduced by underlying SS7 protocol layers. The signalling messages can be analysed per service and signalling interface. SIGTRAN - SS7 Transport over IP SIGTRAN is a standardised (IETF) method of transporting CCS7 signalling over IP with the option of using one of three solutions of user adaptation layers, i.e. SU