7 Network Functions

The whole GSM service area of every GSM network worldwide is built from areas of different operators (Public Land Mobile Area, PLMN Area) that can be subdivided in areas served by one Mobile Switching Center (MSC) (cf. Chapter 5). Each MSC area or at least each network can be further subdivided into location areas (LA), most of them consisting of several base station areas that themselves contain several cells.

The UMTS network is also divided into smaller parts, location areas (LAs) as in GSM for circuit switched networks and routing areas (RAs) for packet switched networks. Every RA only is part of one LA, but one LA can contain several RAs. If a user equipment is called, it is searched via paging only in the corresponding LA/RA. The user equipment updates its LA/RA with every crossing of a LA/RA border and informs the core network. The size of the LA/RA is given by the trade-off between signaling for location update and maximum load of the paging channels.

Before a call reaches the MS the cell in which it moves has to be determined by paging. Therefore, the VLR uses the location area (LA), which the MS is registered with. If the user is called, paging in this LA is initiated, e.g. every BS in this area send a call notification to all MSs in its area. The signaling traffic is kept low, as not the complete network but only the LA is involved. All BSCs that control cells of this LA are contacted via a message including TMSI or IMSI of the MS, a list of the cells to be searched and the maximum time for search. The MS answers among others with cell ID of the cell it is registered with. After the MS is located that way and a route is built in the core network the MS is assigned a radio channel.