How does the network find the mobile phone?
When a mobile is switched on it sends a registration message to a local cell (usually the one with the best quality signal). Data about where that mobile is can then be sent to (& stored in) the nearest Visitor Location Register (VLR). In addition, the user’s Home Location Register (HLR) record is updated to point to the VLR that they are currently visiting. These databases can be interrogated by any switch on the network that is trying to call that mobile. When mobiles move from one location area to another, they will update their VLR and HLR records accordingly. At regular intervals (typically around 15 minutes) the network will interrogate the phone to check that it is still present. This ensures that the HLR is kept up-to-date. When a call is made to a mobile, calling signals will be sent out to the location area (not just the individual cell) in which the mobile is currently registered.
