【導(dǎo)讀】納維亞和聯(lián)合國(guó)，還有法國(guó)和德國(guó)。每個(gè)國(guó)家發(fā)展自己的系統(tǒng)，在設(shè)備和運(yùn)營(yíng)方。面和別的其他國(guó)家不相同。這是一個(gè)不受歡迎的情況，因?yàn)橐苿?dòng)設(shè)備不僅受國(guó)界。限制，以至于如此的經(jīng)濟(jì)規(guī)模和儲(chǔ)蓄不能被實(shí)現(xiàn)。動(dòng)特別小組，簡(jiǎn)稱GSM，形成這個(gè)小組為了研究和發(fā)展歐洲的移動(dòng)陸地通信系統(tǒng)，所提出的這個(gè)系統(tǒng)必須遵循以下幾個(gè)標(biāo)準(zhǔn)；低的終端服務(wù)成本。在1989年，GSM的責(zé)任是被歐洲電訊學(xué)會(huì)標(biāo)準(zhǔn)所接受。GSM規(guī)范的第一階段。存在，而縮寫(xiě)詞GSM代表了全球移動(dòng)通信系統(tǒng)。的AMPS和聯(lián)合國(guó)TACS一樣標(biāo)準(zhǔn)的模擬的細(xì)胞系統(tǒng)。超過(guò)八千頁(yè)的GSM系統(tǒng)介紹盡量允許給中間供給者以靈活。性和競(jìng)爭(zhēng)性，但是足夠的標(biāo)準(zhǔn)化保證在系統(tǒng)組成部分之間互相交織。最基本的電話是被GSM支持的電話。有一種緊急服務(wù)，在最近的緊急事件發(fā)生處，服。GSM使用者可以發(fā)送和接收數(shù)據(jù)給使用者通過(guò)。GSM小組在主觀語(yǔ)音質(zhì)量和復(fù)雜性上研究若干語(yǔ)音編碼。這就是說(shuō)通過(guò)使用現(xiàn)有的13bps的點(diǎn)率改進(jìn)語(yǔ)音質(zhì)量。

　　

【正文】 he 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。 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。 smeared 39。 cell boundaries when a mobile transmitting at peak power goes some distance beyond its original cell boundaries into another cell. The 39。power budget39。 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。smeared39。 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。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 subscriber39。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