7 Network Functions

Call setup with mobile equipment is more complex than for fixed line communication networks. Mobility data has to be managed by the databases HLR and VLR to enable a call setup. To activate the transmission in the corresponding cell, the position of the mobile equipment needs to be known, especially when a mobile is called. Then the HLR is involved to determine its actual location or the responsible VLR.

Therefore, in hierarchical networks, administrative regions are built. They have at least one administrative region managed by one switching center. Each administrative region - it is called LA (location area) in GSM and RA (routing area) in UMTS - consists of at minimum one location area that contains more than one cell. The signaling effort to find a user in the network can be reduced a lot, if the network is subdivided in more location areas. But with the number of location areas the necessity of location updates, and thus additional signaling, increases.

The VLR is responsible for a number of location areas (LA) and the user data for users moving in these LAs. According to its data base a fast association from user to LA is possible. The VLR can assign Temporary Mobile Subscriber Identity (TMSI) to protect the identity of the user