【正文】 mplemented by some North American GSM1900 operators. This is said to provide improved speech quality using the existing 13 kbps bit rate. Power control There are fiveclasses of mobile stations defined,accordin to their peak transmitter power, rated at 20 , 8, 5, 2, and watts . To minimize cochannel interference and to conserve power , both the mobiles and the Base Transceiver Stations operate at the lowest power level that will maintain an acceptable signal quality .Power levels can be stepped up or down in steps of 2 dB from the peak power for the class down to a minimum of 13 dBm 。s back ground noise. Discontinuous reception Another method used to conserve power at the mobile station is discontinuous reception . The paging channel, used by the base station to signal an ining call ,is structured into subchannels. Each mobile station needs to listen only to its own sub channel. In the time between successive paging subchannels, the mobile can go to sleep mode, when almost no power is used. Signalling protocol structure in GSM The signalling protocol in GSM is structured into three general layers, depending on the interface, as shown in Figure 3. Layer 1 is the physical layer, which uses the channel structures discussed above over the air interface. .Layer 2 is the data link layer. Across the Um interface,the data link layer is a modified version of the LAPD protocol used in ISDN,called the A interface,the Message Transfer Part layer 2 of Signalling System Number 7 is used. Layer 3of the GSM signalling protocol is itself divided into 3 sublayers. ? Radio Resources Management Controls the setup , maintenance,and termination of radio and fixed channels, including handovers. ? Mobility Management Manages the location updating and registration procedures,as well as security and authentication. ? Connection Management Handles general call control , similar to CCITT Remendation , and manages Supplementary Services and the Short Message Service. Signalling between the different entities in the fixed part of the work , such as between the HLR and VLR, is acplished throught the Mobile Application ( MAP Part).MAP is built on top of the Transaction Capabilities Application Part ( TCAP), the top layer of Signalling System Number 7. The specification of the MAP is quite plex, and at over 500 pages , it is one of the longest documents in the GSM remendations . Radio resources management The radio resources management (RR ) layer oversees the establishment of a link, both radio and fixed , between the mobile stationand the main functional ponents involved are the mobile station, and the Base Station Subsystem, as well as the MSC. The RR layer is concerned with the management of an RRsession which is the time that a mobile is in dedicated mode , as well as the configuration of radio channels including the allocation of dedicated channels. An RRsession is always initiated by a mobile station through the access procedure, either for an outgoing call , or in response to a paging details of the access and paging procedures , such as when a dedicated channel is actually assigned to the mobile,and the paging subchannel structure, are handled in the RR layer. In addition, it handles the management of radio features such as power control,discontinuous transmiss ion and reception, and timing advance. Handover In a cellular work,the radio and fixed links requiredare not permanentlyallocated for the duration of a call. Handover, or handoff as it is called in North America, is the switching of an ongoing call to a different channel or cell . The execution and measure ments required for handover form one of basic functions of the RR layer. There are four different types of handover in the GSM system, which involve transf erring a call between: ? Channels (time slots) in the same cell ? Cells (Base Transceiver Stations) under the control of the same Base Station Controller (BSC). ? Cells under the control of different BSCs, but belonging to the same Mobile services Switching Center (MSC). ? Cells under the control of different MSCs. The first two types of handover, called internal handovers,involve only one Base Station Controller (BSC). To save signaling bandwidth, they are managed by the BSC without involving the Mobile services ,Switching Center (MSC), except to notify it at the pletion of the last two types of handover, called external hand overs , are handled by the MSCs involved. An important aspect of GSM is that the original MSC, the anchor MSC , remains responsible for most callrelated functions, with the exception of subsequent inter BSC handovers under the control of the new MSC, called the relay MSC. Handovers can be initiated by either the mobile or the MSC (as a means of traffic load balancing ) . During its idle time slots , the mobile scans the Broadcast Control Channel of up to 16 neighboring cells , and forms a list of the six best candidates for possible handover, based on the received signal strength. This information is passed to the BSC and MSC, at least once per second,and is used by the handover algorithm for when a handover decision should be taken is not specified in theGSM are two basic algorithms use ,both closely tied in with power is because the BSC usually does not know whether the poor signal quality is due to multipath fading or to the mobile having moved to another cell ， This is especially true in small urban cells. The minimum acceptable performance39。 smeared 39。power budget39。smeared39。s PLMN, it must register with the work to indicate its current location. In the normal case, a location update message is sent to the new MSC/VLR,which records the location area information,and then sends the location information to the subscrib