【正文】 s information sent to the HLR normally the SS7 address of the new VLR , although it may be a routing number .The reason a routing number is not normally assigned,even though it would reduce signaling , is that there is only a limited number of routing numbers available in the new MSC/VLR and they are allocated on demand for ining calls. If the subscriber is entitled to service , the HLR sends a subset of the subscriber information, needed for call control, to the new MSC/VLR, and sends a message to the old MSC/VLR to cancel the old registration. For reliability reasons,GSM also has a periodic location updating procedure .If an HLR or MSC/VLR fails, to have each mobile register simultaneously to bring the data base up to date would cause overloading. Therefore, the database is updated as
。 cell boundary problem andreducesco channel interference, but it is quite plicated. Mobility management The Mobility Management layer (MM) is built on top of the RR layer , andhandles the functions that arise from the mobility of the subscriber , as well as the authentication and security management is concerned with the procedures that enable the system to know the current location of a poweredon mobile station so that ining call routing can be pleted. Location updating A powered on mobile is informed of an ining call by a paging message sent over the PAGCH channel of a cell. One extreme would be to page every cell in the work for each call, which is obviously a waste of radio bandwidth .The other extreme would be for the mobile to notify th e system, via location updating messages, of its current location at the individual cell level . This would require paging messages to be sent to exactly one cell, but would be very wasteful due to the large number of location updating messages . A promise solution used in GSM is to group cells into location areas .Updating messages are required when moving between location areas, and mobile stations are paged in the cells of their current location area. The location updating procedures , and subsequent call routing , use the MSC and two location registers: the Home Location Register (HLR) and the Visitor Location Register (VLR). When a mobile station is switched on in a new location area,or itmoves to a new location area or different operator39。 method uses handover to try to maintain or improve a certain level of signal quality at the same or lower power level. It thus gives precedence to hand over over power control. It avoids the 39。 cell boundaries when a mobile transmitting at peak power goes some distance beyond its original cell boundaries into another cell. The 39。 algorithm gives precedence to powercontrol over handover, so that when the signal degrades beyond a certain point , the power level of the mobile is increased . If further power increases do not improve the signal, then a handover is considered. This is the simpler and more mon method , but it creates 39。 The mobile station measures the signal strength or signal quality(basedon the Bit Error Ratio), and passes the information to the BaseStationController ,which ultimately decides if and when the power level should be changed. Power control should be handled carefully, since there is the possibility of instability .This arises from having mobiles in cochannel cells alternatingly increase their powerin response toincreased cochannel interference caused by the other mobile increasing its in unlikely to occur in practice but it is under study. . Frequency hopping The mobile station already has to be frequency agile,meaning it can move between a transmit, receive, and monitor time slot within one TDMA frame,which normally are on different frequencies. GSM makes use of this inherent frequency agility to implement slow frequency hopping , where the mobile and BTS transmit each TDMA frame on a different carrier frequency. The frequency hopping algorithm is broadcast on the Broadcast Control Channel. Since multipath fading idependent on carrier frequency, slow frequencyhoppinghelps alleviate the problem In addition,cochannel interference is in effect randomized. Multipath equalization At the 900 MHz range, radio waves bounce off everything – buildings ,hills, cars, airplanes, etc. Thus many reflected signals, each with a different phase , can reach an is used to extract the desired signal from the unwanted reflections. It works by finding out howa known transmitted signal is modified by multipath fading, and constructing an inverse filter to extract the rest of the desired signal. This known signal is the 26bit training sequence transmitted in the middle of every timeslot burst. The actual implementation of the equalizer is not specified in the GSM specifications. Discontinuous transmission Minimizing cochannel interference is a goal in any cellular system, since it allows better service for a given cell size,or the use of smaller cells, thus increasing the overall capacity of the system. Discontinuous transmission (DTX) is a method that takes advantage of the fact that a person speaks less that 40 percent of the time in normal conversation, by turning the transmitter off during silence periods . An added benefit of DTX is that power is conserved at the mobile unit. The most important ponent of DTX is, of course, Voice Activity Detection. It must distinguish between voice and noise inputs, a task that is not as trivial as it appears, considering background avoice signal is misinterpreted as noise ,the transmitter is turned off and a very annoying effect called clipping is heard at the receiving end. If, on the other hand , noise is misinterpreted as a voice signal too often , the efficiency of DTX is dramatically decreased . Another factor to consider is that when the transmitt