 services. This is used within C7 signalling networks. Point codes are used to There are various types of numbers used within a PCN, as de- allow signalling information to be routed through the signalling scribed in the following sections. network. 48.6.4.1 International mobile subscriber identity (IMSI) 48.7 Mobility The IMSI is associated with the PCN customer by being utilised on The philosophy behind the PCN service is 'Phones for people not the Subscriber Identity Module (SIM). This is the plug card which, places'. One must assume that the customer will move around with when combined with the handset, forms the mobile station. his telephone, unlike the fixed network where the telephone is The IMSI is the number which the network uses to identify the always at the end of the line. If customers can move, the network mobile. must always have information concerning the location of the cus- tomer to enable calls to be delivered. Furthermore the customer must also be able to change location when engaged in a call. 48.6.4.2 Mobile station ISDN number (MSISDN) For the purpose of mobility, the network can be considered as depicted in Figure 48.6. The network is split into a number of The MSISDN is the 'telephone number' of the customer. This location areas (LA1 to LA8). The size of these areas is dependent number conforms to CCITT recommendation E.164 (E.163), e.g. upon the balance of signalling load; if the location area is small then +44 71 492 2426 for international and 071 492 2426 for national. the amount of location updating will be high. However, if the The HLR provides a mapping between the MSISDN and IMSI. location area is large then the number of cells that have to be paged over to find the mobile will be high. RSS is the radio subsystem and incorporates BTS and BCS. 48.6.4.3 Roaming number 48.7.1 Location updating This is used when it is necessary to route a call to a customer within a different service area. For instance where the customer has moved Each PCN cell has a Broadcast Common Control Channel (BC