7 Network Functions

The function location update provides data about the actual location of the MS to the databases HLR and VLR about the actual location of the MS. This procedure is started by the MS, if an update of its location becomes necessary, that is if the MS moves to the location of another MSC/LA or if it is powered up or down. There also exists the possibility of periodic location updates forced by the network. The MS sends data to the VLR, which transfers it to the HLR. The procedure is task of the MS’s mobility management, which sends a request (identified by IMSI) to the VLR. Only if the MS’s LA is known, a call can be setup.

Location update starts after the setup of the RRC (radio resource control) connection. The MS sends a LOCATION UPDATE REQUEST containing old TMSI and old LAI (location area identifier) to the new MSC/VLR. This connects to the old MSC/VLR, which it has derived from the data sent by the MS. With the TMSI it asks for IMSI and authentication data. From that moment encrypted communication with the MS becomes possible. The new TMSI and LAI are transferred via this secure connection as soon as they are assigned. The HLR is informed and updates the location data with the new VLR. It informs the old VLR that it can delete the MS’s data. The new VLR gets information about the services the MS is allowed to access. The MS is provided with TMSI and LAI. After that the RRC connection is terminated.
The data flow for circuit and packet switched mode is the same; it only differs in time, because of the different sizes of the LA/RA.