ontrol Channels There are several logical common channels in each cell. These altogether typically occupy one fixed time slot (typically TSO) at a fixed frequency. The common control channel in the downlink direction transmits, for example, the following information from the network to the MSs: Synchronization information of frequency and time slots; Information about common channels that is used by neighboring cells; Location area and network identification; Paging messages for incoming calls and channel assignment for a new call. Mobile Communications In the uplink direction, from the MS to the network, the common con- trol channel is used, for example, for call-request messages from the MSs. 5.6 Operation of the GSM Network In this section we introduce the operating principles of a cellular network. To do this, we illustrate the GSM network with a few simplified examples. They show how location update is performed, how a mobile call is estab- lished, how handover is performed, and what the security functions of the GSM network are. Each GSM subscriber is registered into one HLR of his or her home network. This HLR is the central point that provides subscriber information regardless of where he or she is presently located. 5.6.1 Location Update The cellular mobile network has to be aware of the location of its subscribers at all times to be able to route incoming calls to them. The location update procedure takes place every time a MS moves to another location area or when a user switches her telephone on in a different location than where she was located previously. The geographical position of a GSM mobile is known at the accuracy of a location area (LA), which typically consists of a number of cells. The BTSs of those cells need not be connected to the same BSC. When an incoming call to a mobile subscriber arrives, it is paged through all the cells belonging to the LA where this specific subscriber is known to be. The MS is responsible for location updates and performs this updating in idle mode, that is, wh