he terms and detailed operation of a particular network technology. 5.3.1 MS in Idle Mode The MS is preprogrammed to know the frequencies of the control channels. When it is switched on, the mobile scans these frequencies and selects the BS with the strongest common control channel. Then the MS transmits its unique identification code, which may be its telephone number (or other identification code depending on the system), over the control channel in order to inform the VLR. The VLR, with the help of the identification of the MS, determines the address of the subscriber's home country and the home network. Then the MSC/VLR transmits the signaling message toward the Mobile Communications home network. The message is then routed to the HLR, which is then informed that this specific subscriber is now located in the area of a certain VLR. The HLR stores this information. Now the HLR is able to route the calls to the right MSC/VLR, which routes it further to the mobile subscriber. The MS then continuously listens to the common control channel and, if necessary, transfers to the control channel of another cell (Figure 5.4). Each network is divided into small location areas that contain a group of cells. All BSs inside a certain location area send the same global code dedi- cated for that location area on the common control channel. If the MS moves, changes the channel and the location information sent by the net- work changes; the MS notices it and informs the network, which then updates the location information stored in the VLR and HLR (if needed). 5.3.2 Outgoing Call The number that a user wants to call is entered into the memory of the mobile telephone through its keypad. When the user presses the Call button, the mobile telephone sends a set of signaling messages to the BS via the com- mon control channel, as shown in Figure 5.4. These messages contain the dialed digits, which the BS passes to the MSC for routing. In idle mode the mobile station listens to the common Dialed digits control channel, CCCH. Channel a