1.8 Mobility Management and Call Control
As all components of a GSM mobile network have now been introduced, the following section gives an overview of the three processes that allow a subscriber to roam throughout the network.
1.8.1 Call Reselection and Location Area Update
As the network needs to be able to forward an incoming call, the subscriber's location must be known. After the mobile device is switched on, its first action is to register with the network. Therefore, the network becomes aware of the current location of the user, which can change at any time because of the mobility of the user. If the user roams into the area of a new cell, it may need to inform the network of this change. To reduce the signaling load in the radio network, several cells are grouped into a location area. The network informs the mobile device via the BCCH of a cell not only of the cell ID but also of the LAC that the new cell belongs to. The mobile device thus has to report only its new location if the new cell belongs to a new location area. Grouping several cells into location areas not only reduces the signaling load in the network but also the power consumption of the mobile. A disadvantage of this method is that the network operator is only aware of the current location area of the subscriber but not of the exact cell. Therefore, the network has to search for the mobile device in all cells of a location area for an incoming call or SMS. This procedure is called paging. The size of a location ...