p-to- date subscriber information such as where (in the area of which VLR) they Mobile Communications are located presently, what services they have the right to use, and a number where she has transferred calls. The HLR is the global central point where their information is available wherever they are located. When a call is routed to them, the dialed subscriber's telephone number tells the network where their HLR can be found. VLR stores information about every subscriber in its area. The VLR informs the HLR when a new subscriber arrives in its area. It also contains more accurate information of where (to which cell or group of cells) to con- nect incoming calls directed to a certain subscriber. The VLR is usually inte- grated into a mobile telephone exchange but the HLR is usually a physically separate efficient database system. 5.2.3 Radio Channels Each BS provides two main types of channels, as shown in Figure 5.3: the common control channel and the dedicated channels. In the downlink or forward direction (from network to mobile stations) information such as net- work identification, location information, designated power level, and pag- ing for incoming calls is sent on the common control channel of each cell. When MSs are in idle mode (no ongoing call) they are continuously listening to the common control channel of one cell. In the uplink or reverse direction HLR, home location register, stores subscriber information and updated location information (VLR address). Each subscriber is registered in one fixed HLR. VLR, visitors location register, stores subscriber information of each MS located in its area. Common control channel that all Microwave link mobiles listen to when they are in idle to mobile mode. Call request messages are sent on switching center this channel in the case of an outgoing call and paging messages are sent in the case of an incoming call. Lines to Dedicated channels that are used mobile switching for calls (speech or traffic channels) station when a mobile is in dedicated mode. cente