en a call is not connected. The MS surveys the radio environment constantly and, when it detects that it could be served best in a new LA, performs a normal location update procedure to change the location information in its present VLR and in the HLR (if needed). We say that the mobile station has roamed to another LA. In dedicated mode, during a call, the procedure called handover, which we will discuss later, may be required. If the LA is changed during a call, the location update takes place after the call is cleared. Location update may take place inside one network when the LA is changed or between different networks that may be located in different countries. The latter case requires a roaming agreement between network operators to allow a subscriber to use the other network in addition to her home network. Figure 5.11 illustrates the location update procedure that Introduction to Telecommunications Network Engineering Network X Network Y AuC/EIR VLR (new) PSTN or ISDN VLR (old) Mobile station Signaling transfer HLR = Home location register AuC = Authentication center VLR = Visitor location register EIR = Equipment identity register Figure 5.11 Location update in GSM network. occurs when a mobile station is switched on in another network Y in another country. This example assumes that the mobile station has been switched off in the home network, network X, and that the network operators of net- works Y and X have a roaming agreement that allows cellular subscribers to use the services of another network. For location update the following main operations are carried out (see Figure 5.11): 1. When the MS has roamed to another LA, it scans the common control channels. When it finds a common control channel, it detects the LA code, which contains country and network identifi- cations. If the MS cannot find the same LA code it has stored previ- ously, it requests a location update from the network. 2. The MSC/VLR requests the global identity code of the mobile [international mobile identity subscriber (IMSI) st