GSM 移動通信系統(tǒng)綜述 GSM 的歷史 在十九世紀(jì)八十年代，蜂窩電話系統(tǒng)在歐洲迅速發(fā)展起來，特別是在斯堪的納維亞和聯(lián)合國，還有法國和德國。每個國家發(fā)展自己的系統(tǒng)，在設(shè)備和運(yùn)營方面和別的其他國家不相同。這是一個不受歡迎的情況，因?yàn)橐苿釉O(shè)備不僅受國界的限制，（這在統(tǒng)一的歐洲變的越來越不重要），而且還受每種設(shè)備類型的市場限制， 以至于如此的經(jīng)濟(jì)規(guī)模和儲蓄不能被實(shí)現(xiàn)。 歐洲首先認(rèn)識到這種情況，在 1982 年歐洲郵電行政大會成立了一個歐洲移動特別小組，簡稱 GSM，形成這個小組為了研究和發(fā)展歐洲的移動陸地通信系統(tǒng)，所提出的這 個系統(tǒng)必須遵循以下幾個標(biāo)準(zhǔn)； 好的話音質(zhì)量。 低的終端服務(wù)成本。 支持國際漫游。 支持手持終端。 支持新的服務(wù)和設(shè)備。 高效的光譜。 ISDN 兼容性。 在 1989 年， GSM 的責(zé)任是被歐洲電訊學(xué)會標(biāo)準(zhǔn)所接受。 GSM 規(guī)范的第一階段于 1990 年被公布，商業(yè)服務(wù)在 1991 年被推行，到 1993 年 ，在 22 個國家有 36個 GSM 網(wǎng)絡(luò)。雖然標(biāo)準(zhǔn)定型在歐洲，但 GSM 不只是歐洲的標(biāo)準(zhǔn)，超過 200 個 GSM網(wǎng)絡(luò)（包括 DCS1800 和 PCS1900）在世界上 110 個國家運(yùn)營。在 1994 年初，世界上有 1.3 百萬個用戶，到 1997 年 10 月已經(jīng)超過了 55 百萬個用戶。 北美洲進(jìn)入 GSM 領(lǐng)域比較晚，而且隨之有一個 GSM 派生物叫 PCS1900， GSM 在每個大陸存在，而縮寫詞 GSM 代表了全球移動通信系統(tǒng)。 GSM 的發(fā)展選擇了一個（在時間上）被分割的數(shù)字系統(tǒng)，相反的是，像美洲的 AMPS 和聯(lián)合國 TACS 一樣標(biāo)準(zhǔn)的模擬的細(xì)胞系統(tǒng)。他們相信那個處于壓縮狀態(tài)的算法和數(shù)字信號處理器的進(jìn)展，允許實(shí)現(xiàn)原來的標(biāo)準(zhǔn)和在連續(xù)不斷改進(jìn)的系統(tǒng)方面的質(zhì)量和費(fèi)用。超過八千頁的 GSM 系統(tǒng)介紹盡量允許給中間供給者以靈活性和競爭性，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 。這個被通過為每個在系統(tǒng)中的定義的功能實(shí)體提供功能和交織描述。 GSM 所提供的服務(wù) 從開始， GSM 的計劃者想在提供的服務(wù)和信號使用的控制方面考慮 ISDN 的兼容性。然而，無線傳輸限制，帶寬方面和費(fèi)用，不允許 ISDNB 標(biāo)準(zhǔn)的通道點(diǎn)率64KPS 被實(shí)際達(dá)到。 使用 ITU-T 定義，電訊服務(wù)被劃分為送信人服務(wù)， 電話 服務(wù)和其它補(bǔ)充服務(wù)。最基本的電話是被 GSM 支持的電話。比如其它通信，語音被進(jìn)行數(shù)字化編碼，作為數(shù)字流通過 GSM 網(wǎng)絡(luò)被傳輸。有一種緊急服務(wù)，在最近的緊急事件發(fā)生處，服務(wù)提供者被通知撥打三位電話號碼（比如 911）。 各種各樣的數(shù)據(jù)服務(wù)被提供。 GSM 使用者可以發(fā)送和接收數(shù)據(jù)給使用者通過POPTS（普通的老的電話服務(wù)）、 ISDN、轉(zhuǎn)換公共數(shù)據(jù)的網(wǎng)絡(luò)包和轉(zhuǎn)換公共數(shù)據(jù)網(wǎng)絡(luò)的電路，使用多樣化的接口方法和協(xié)議，比如 X.25 或 X.32。速率可以達(dá)到9600bps，由于 GSM 是一個數(shù)字網(wǎng)絡(luò)，在用戶和 GSM 網(wǎng)之間是不需要調(diào)制解調(diào)器的，但是調(diào)制解調(diào)器要求在 GSM 網(wǎng)和 POTS 里交織工作。 其他數(shù)據(jù)服務(wù)包括組 3 傳真，作為在 ITU-T 協(xié)議 T.30 中被描述，這個被傳真編劇的使用者所支持， GSM 的唯一特性就是在較老的類 似系統(tǒng)中找不到，是短信息系統(tǒng)（ SMS）， SMS 是一種字母數(shù)字的雙向的信息服務(wù)。消息被傳輸采用一種存儲轉(zhuǎn)交的形式。對于點(diǎn)對點(diǎn)的 SMS，一條信息被發(fā)送到另一個用戶，收到的謝意被提供給發(fā)送人。 SMS 能被用于細(xì)胞 播送模式，用來發(fā)送諸如交通更新資料或消息更新資料。消息能被存儲在 SIM 卡里。 補(bǔ)充的服務(wù)被提供在電話服務(wù)或送信人服務(wù)之上。在第一階段的說明中，他們包括若干形式的呼叫轉(zhuǎn)接如呼叫轉(zhuǎn)接當(dāng)移動用戶不能到達(dá)網(wǎng)絡(luò)時），外出呼叫，或入局呼叫，比如在其它國家漫游時。在第二階段的規(guī)范中許多補(bǔ)充服務(wù)將被提供，比如呼叫者識別， 呼叫等待，多政黨談話。 信道控制 普通的信道模式能被分為兩類：專用呼叫信道模式和標(biāo)明空閑信道模式。普通的信道被空閑模式使用，從而改變要求變成專用信道的信號信息。專用信道方式監(jiān)控周圍的基站和其它信息，普通信道被定義在 51 幀，以及專用信道使用 26幀， TCH 結(jié)構(gòu)仍然能監(jiān)控信道，普通信道包括： 廣播控制信道（ BCCH） 連續(xù)不斷的發(fā)送，在下行鏈路上，消息包括基站識別，頻率分配和 跳頻序列 頻率校正信道（ FCCH）和同步信道（ SCH） 同步突發(fā)脈沖序列用于移動臺的時間同步。在 GSM 蜂窩網(wǎng)絡(luò)中，廣播確切的指一個頻 率校正信道和一個同步信道。 隨機(jī)接入信道（ RACH） 被 移動臺 使用的時隙提出入網(wǎng)。 尋呼信道 (PCH) 對下一個呼叫向移動臺發(fā)出報警。 傳輸基站準(zhǔn)許接入信道（ AGCH） 尋呼 移動臺 的信息。分配獨(dú)立專用信道（為了獲得一個獨(dú)立專用信道），對 RACH 作出應(yīng)答。 信道編碼和調(diào)制 由于自然的和人為的電磁干擾，語音編碼或數(shù)據(jù)信號通過無線電接口傳輸必是從差錯保護(hù)的。 GSM 使用編碼，并阻塞交錯達(dá)到這個保護(hù)，這個算法使用由于語音和不同的數(shù)據(jù)率而不同，用于語音的方法塊將被描述如下： 取消語音編碼每 20ms 產(chǎn)生一個 260 點(diǎn)塊的抽樣，從主觀測試來看，一些點(diǎn)塊感知的語音量比其他人感知的更重要。這些點(diǎn)被分為 3 類： Ia 類 50 比特 對點(diǎn)差錯最敏感 Ib 類 132 比特 對點(diǎn)差錯適中敏感 II 類 78 比特 對點(diǎn)差錯最不敏感 Ia 類為錯誤檢測出有 3 比特的循環(huán)冗余碼，如果一個錯誤被檢測出，偵被判定被損壞，而需要被丟棄。如果它稍微被削弱而被換以前正確接收到的偵。這53 個比特，和 Ib 類 132 比特以及 4 比特尾巴序列一起（總數(shù) 189 比特）被輸入1/2 長度為 4 的譯碼。每個輸入比特被兩個輸出比特譯碼，是基于以前 4 輸入比特的譯碼。譯碼器輸出 378 比特，在保持 II 類位的基礎(chǔ)上增加了 78 個比特這是不被保護(hù)的。每 20ms 語音采樣被作為 456 比特譯碼。 進(jìn)一步為了避免無線電接口帶來的突發(fā)誤差，每一個采樣被隔行掃描， 456比特輸出被譯碼劃分為 8 個點(diǎn)塊的，這些點(diǎn)塊被 8 個連續(xù)的時間傳輸，時隙脈沖。每一個時隙攜帶兩個 57 點(diǎn)塊，每個脈沖從兩個不同的語音采樣中開始。 話音編碼 GSM 是一個數(shù)字系統(tǒng)，所以語音被固定的模擬而且被數(shù)字化。這種方法被ISDN 和采用在高速中繼線和光纖上的多路復(fù)用技術(shù)的當(dāng)前的電話系統(tǒng)所使用，被稱作脈幅調(diào)制。從 PCM 流出的信息流是 64KBPS，太 高的速率通過無線電線路不是切實(shí)可行的。 GSM 小組在主觀語音質(zhì)量和復(fù)雜性上研究若干語音編碼。（這關(guān)系到費(fèi)用，延遲處理和消費(fèi)力量的執(zhí)行）基本上，來自以前的采樣信息不 會迅速的改變，而是被用來預(yù)測當(dāng)前采樣信息。以前的采樣信息的線性結(jié)合的系數(shù)，再加上剩余的編碼形式，以及預(yù)測和實(shí)際的采樣中的不同，來代表信號。語音被分為 50HZ 的采樣頻率間隔，每個間隔被分為 260 個比特，這就是所謂的呼叫充足率語音編碼，近來提高充足率語音編碼算法被一些美國 GSM1900 運(yùn)營商所執(zhí)行。這就是說通過使用現(xiàn)有的 13bps 的點(diǎn)率改進(jìn)語音質(zhì)量。 功 率控制 有五個不同的移動臺被定義，根據(jù)它們的高峰傳輸功率，額定功率分別為20， 8， 5， 2 瓦特。為了最小化次通道干擾和保持功率，移動臺和基站收發(fā)信臺以最小功率運(yùn)行，這將會保持一個可以接受的信號質(zhì)量，功率水平能被加強(qiáng)或減弱 2dB 從最高功率下降到最小功率為 13dBm. 移動臺測量信號強(qiáng)弱和信號質(zhì)量（主要通過比特差錯率），通過發(fā)送信息到基站臺控制器，這將最終決定功率水平是否被改變。功率控制應(yīng)該被細(xì)心處理，因?yàn)槟怯锌赡艿牟环€(wěn)定性，這是因?yàn)橐苿优_在次通道細(xì)胞中不斷增加它們的功率來回應(yīng)由其它移動臺在增加功率時造成的次通道 干擾。這在實(shí)際中不可能發(fā)生但它可以用于理論研究。 跳頻 移動臺已經(jīng)不得不采用跳頻，意味著 GSM 可以在一個 TDMA 幀內(nèi)接收，傳輸，監(jiān)控時隙，這樣在不同的頻率間改變從而使用頻率靈活性來執(zhí)行慢頻率跳變。這里移動臺和收發(fā)信臺在不同的載波頻率上傳輸 TDMA 幀。跳頻算法在廣播控制通道上進(jìn)行。 由于多徑衰落是依靠載頻，緩慢的頻率跳躍來提高抗干擾性能的，此外，次通道實(shí)際上是干擾隨機(jī)化。 多路均衡 在 900MHZ 頻段，無線電波受建筑物，汽車，飛機(jī)等等的影響。許多反射信號，都有不同的頻段，可以到達(dá)一個天線。平等被用來從不需要反 映的信號中提取想要得到的信號。它通過查明一個已知發(fā)射的信號怎樣通過多徑衰落被改進(jìn)，建造一個反轉(zhuǎn)的濾波器去提取想要得到的信號。這個已知的信號是 26 比特點(diǎn)序列在中間所有的時間突發(fā)。這個均衡器的實(shí)際執(zhí)行不是被指定在 GSM 中。 間斷傳輸 最大限度的減少通道干擾是蜂窩系統(tǒng)的目標(biāo)，由于它允許給一個特定的小區(qū)提供更好的服務(wù)，或者為更小的小區(qū)使用，從而增加系統(tǒng)的容量。不連續(xù)的傳輸（ DTX）是一種方法，利用一個人在正常交談中講話不少于 40%的時間，通過在無語音信號時關(guān)閉發(fā)射器。 DTX 的另一個增加的好處是功率在移動單元中被保存 。 DTX 最重要的組成部分是語音激活檢測。 它必須在聲音和噪音輸入時區(qū)分開，一個任務(wù)就是不和它出現(xiàn)時一樣考慮到背景噪音。 如果一個聲音信號被作為噪音信號曲解，立即關(guān)閉發(fā)射機(jī)。一個非常討厭的稱作剪輯的東西被在接收端聽到。如果，另一方面，噪音也會被做為一種聲音信號曲解，間斷傳輸?shù)墓πЬ蜁粶p小。要考慮的另一個因素是當(dāng)發(fā)射機(jī)關(guān)閉時，在接收端就完全無話音，這是由于數(shù)字 GSM 的性質(zhì)。為了保證接收者檢測出的是無話音，通過盡力與發(fā)射端背景噪聲相匹配在接受端建立舒適噪聲。 間斷接收 另一種在移動臺上保存功率的辦法是間斷接收。尋呼 信道通過一個入局的呼叫給基站一個信號，被構(gòu)造成次通道。每一個移動臺僅僅需要聽到自己的次信道。這時在連續(xù)的尋呼次信道中，當(dāng)沒有被使用的功率時，移動臺可以進(jìn)入休眠模式。 在 GSM中的信號協(xié)議結(jié)構(gòu) 在 GSM 中，信號協(xié)議被分為三層，依賴于接口，當(dāng)時引入了數(shù)字 3，第一層是物理層，它為物理層提供無線信道，第二層是數(shù)據(jù)鏈路層，通過 Um接口，數(shù)據(jù)鏈路層使用在 ISDN 中被 LAPD 協(xié)議，叫做 LAPDm，通過 A 接口， 7 號信令的信息傳輸層被使用， GSM 的第 3 層，被分為 3 個子層。 無線資源管理 控制建立，維持，終止無線電裝置和 固定信道，包括轉(zhuǎn)發(fā)器。 連接控制功能 處理呼叫控制 與 CCITT 協(xié)議的 Q.931 相似，管理補(bǔ)充服務(wù)和短信息服務(wù)。 信號在網(wǎng)絡(luò)的固定部分的不同實(shí)體之間，比如歸屬位置寄存器和訪問位置寄存器，它們通過移動應(yīng)用部分協(xié)議完成。移動應(yīng)用協(xié)議是被建立在 7 號信令部分的最高層。 MAP 的說明非常復(fù)雜，超過了 500 頁，是 GSM 協(xié)議中最長的文件。 無線資源管理 無線資源管理層在移動臺和移動交換中心之間的一層接口協(xié)議。主要的功能組成部分是移動臺，和基站控制器，還有移動交換中心。無線資源管理層與無線資源管理 -會議的管理有關(guān)，這里 移動臺有一個專門的功能，無線通信信道的結(jié)構(gòu)包括無線信道的分配。一個無線管理層協(xié)議總是移動臺通過接口協(xié)議開始，或者是一次向外的呼叫，或者是調(diào)度信息的接口，這個接口和調(diào)度程序的細(xì)節(jié)，如同當(dāng)一個專用信道被分配在移動臺上和調(diào)度次信道結(jié)構(gòu)被在無線資源管理層被處理。另外，它處理無線電特性包括功率控制，間斷傳輸和接收。 轉(zhuǎn)發(fā)器 在一個蜂窩網(wǎng)中，無線電和固定聯(lián)系不是永久的被分配在依次呼叫間。轉(zhuǎn)發(fā)器是一個對向外呼叫給不同的信道和蜂窩的開關(guān)，轉(zhuǎn)發(fā)器所需要的執(zhí)行器和測量方法是無線資源管理層最基本的功能之一。 在 GSM 系統(tǒng)中有四個 不同類型的轉(zhuǎn)發(fā)器，包括轉(zhuǎn)移呼叫： 在一個蜂窩網(wǎng)中的信道（時隙）。 在同一基站控制器下的基站移動臺。 在不同基站控制器控制下的細(xì)胞，但是屬于在相同的移動交換中心 在不同的移動交換中心。 前兩種類型的轉(zhuǎn)發(fā)器，叫內(nèi)部轉(zhuǎn)發(fā)器，只包括基站控制器，為了節(jié)省信號帶寬，它們利用基站控制器不包括移動交換中心，除此之外在完成轉(zhuǎn)發(fā)時通知它。最后兩種轉(zhuǎn)發(fā)器稱做外部轉(zhuǎn)發(fā)器，被移動交換中心所處理。 GSM 一個重要的方面是原始移動交換中心，仍然對其余的呼叫相關(guān)功能起作用。除了后來的內(nèi)部基站控制器外，轉(zhuǎn)發(fā)器在新的移動交換中心控制下，被稱作中 繼器。 轉(zhuǎn)發(fā)器能被其它移動臺或移動交換中心所啟動。在一個空閑時隙里，移動臺掃描相鄰的 16 個蜂窩中的廣播控制信道，對于可能的轉(zhuǎn)發(fā)器形成一張六個最好的侯選列表，基于收到的信號的強(qiáng)度。這個信息通過基站控制器和移動交換中心運(yùn)用轉(zhuǎn)發(fā)算法被使用，至少每秒一次， 算法適用在當(dāng)轉(zhuǎn)發(fā)器決定應(yīng)該被用在 GSM 協(xié)議中時。有兩個基本的運(yùn)算法則被使用，都密切被束縛在功率控制中，這是因?yàn)榛究刂破魍ǔ２恢廊跣盘柺欠褚驗(yàn)槎鄰剿ヂ浠蛞苿优_移動到另一個蜂窩網(wǎng)中了。這種情況在郊區(qū)蜂窩網(wǎng)中確實(shí)是存在。 最小的可以接受的性能算法為轉(zhuǎn)發(fā)器優(yōu)先，以致 于在一定的點(diǎn)上，當(dāng)信號減弱時移動臺的功率在增強(qiáng)。如果功率一直增加而不改變信號的話，這個轉(zhuǎn)發(fā)器是可以考慮使用的。這是一個既簡單又普通的方法，但是當(dāng)一個移動臺以很高的功率從一段距離的原始的蜂窩邊界進(jìn)入另一個蜂窩時，它抹去了一些蜂窩邊界。功率預(yù)算法使用轉(zhuǎn)發(fā)器在相同或更小的功率水平上去維持或改進(jìn)信號質(zhì)量。它優(yōu)先于功率控制法。避免了抹去細(xì)胞邊界的問題，減少了次信道的干擾，但是非常復(fù)雜。 移動管理 移動管理層是建立在無線資源管理的上一層，和處理用戶移動和安全，鑒權(quán)方面的問題。位置管理與程序有關(guān)，這程序是使系統(tǒng)能 夠知道當(dāng)前加強(qiáng)移動臺功率的程序，這樣的話，入局呼叫路由選擇能被完成。 位置更新 信號最強(qiáng)的基站被入局呼叫通過尋呼信號通知。一個極端就是在網(wǎng)絡(luò)中的一個尋呼對應(yīng)一次呼叫，這顯然是一種資源的浪費(fèi)。另一個極端就是為了移動用戶通知系統(tǒng)，經(jīng)當(dāng)前位置更新信息使它的當(dāng)前位置在一個單獨(dú)的細(xì)胞中。這將需要尋呼信息被確切地發(fā)送到一個細(xì)胞中，但是這將會對于大量更新的位置信息造成浪費(fèi)。一個折衷的被使用在 GSM 系統(tǒng)中的辦法就是在位置區(qū)域組成蜂窩網(wǎng)。當(dāng)在位置區(qū)域之間移動時，要求更新信息。移動臺在它們當(dāng)前位置的區(qū)域中被尋呼。 位置更新程序 和后來的呼叫路由選擇，使用移動交換中心和兩個位置寄存器：歸屬位置寄存器，訪問位置寄存器。當(dāng)移動臺在一個新的位置區(qū)域被轉(zhuǎn)換時或者它移動到一個新的位置區(qū)域時或不同運(yùn)營商的公共陸地移動網(wǎng)，它必須向網(wǎng)絡(luò)登記表明它當(dāng)前的位置。在正常情況下，一個位置更新信息被發(fā)送到新的移動交換中心 /訪問位置寄存器，這時記錄當(dāng)前位置然后發(fā)送當(dāng)前位置信息給用戶歸屬位置寄存器。被送到歸屬位置寄存器的信息正常是新的訪問位置寄存器的 SS7地址，雖然它只可能是一個路由選擇數(shù)字。路由選擇數(shù)字的原因不是正常的被指定，甚至它將減少信號。是在新的移動交換 中心 /訪問位置寄存器里可以得到的路由選擇數(shù)字里的有限的數(shù)字，它們按入局呼叫的要求分配。如果用戶有權(quán)享受服務(wù)，歸屬位置寄存器發(fā)送用戶信息的一個子集，為了呼叫控制需要新的移動交換中心 /歸屬位置寄存器。然后發(fā)送信息到舊的移動交換中心 /訪問位置寄存器去取消舊的登記信息。由于可靠性的原因， GSM 也有一種位置更新程序，如果一個歸屬位置寄存器或移動交換中心不能工作了，同時有每一次移動臺登記更新新的數(shù)據(jù)將造成超載的情況，然而當(dāng)更新位置信息發(fā)生時，數(shù)據(jù)庫也被更新，且能夠周期性的更新，且被運(yùn)營商所控制，信號傳輸和恢復(fù)的速度之間 是一種貿(mào)易。如果一個移動用戶不注冊，在更新的周期后，它將被撤消。 一個關(guān)系到位置更新的過程是 IMSI 的附加且派遣。派遣讓網(wǎng)絡(luò)知道移動臺是不可能到達(dá)的，并避免不必要的分配信道和發(fā)送尋呼信息，附加類似于位置更新，通知系統(tǒng)移動用戶是可以到達(dá)的。 IMSI 附加 /派遣激活到一個單獨(dú)的細(xì)胞基礎(chǔ)上運(yùn)行。 鑒權(quán)與安全 由于無線媒介能被任何人訪問，使用者的鑒定為了證明他們是誰，這是一個移動網(wǎng)絡(luò)中非常重要的部分，鑒定包括兩個功能實(shí)體， SIM 卡和認(rèn)證中心。每一個用戶被一把秘密的鑰匙，一部分被儲存在 SIM 卡里，其它的儲存在認(rèn)證中心，在認(rèn)證期間，認(rèn)證中心產(chǎn)生一個隨機(jī)號碼把它發(fā)送到移動用戶，然后認(rèn)證中心和移動終端使用這個隨機(jī)號碼。一種秘密算法稱作 A3 算法，產(chǎn)生一個回應(yīng)信號（ SRES）被返回到認(rèn)證中心。如果被移動用戶發(fā)送的數(shù)字和被認(rèn)證的一樣，那么用戶被鑒別。同樣的初始隨機(jī)碼和用戶鑰匙被使用一種的秘密鑰匙叫 A8，這個秘密鑰匙與 TDMA 偵一道，使用 A5 算法去創(chuàng)建一個 114 突發(fā)點(diǎn)序列，譯成密碼是相當(dāng)大膽的選擇，由于信號已經(jīng)被以一種 TDMA 偵方式編碼，掃描和傳輸。這樣以免受到所有的保護(hù)，而采用專用竊聽器。 另一個水平的安全是在移動設(shè)備本身執(zhí)行，與移 動用戶相反。正如以前提到的，每一個移動終端被唯一的國際移動設(shè)備識別號碼辨別，網(wǎng)絡(luò)中的一張 IMEIS 在識別登記設(shè)備中被存儲，狀態(tài)回應(yīng)到 EIR 的 IMEI 是下列之一： 白色單：合法的移動設(shè)備識別號。 灰色單：是否允許由運(yùn)營商所決定。 黑色單：終端既沒有被報告偷竊，也沒有被類型認(rèn)可，禁止使用的移動設(shè)備識別號。 Overview of the Global System for Mobile Communications History of GSM During the early 1980s,analog cellular telephone systems were experiencing rapid growth in Europe, particularly in Scandinavia and the United Kingdom, but also in France and Germany. Each country developed its own system , which was income paible with everyone elses in equipment and operation .This was an undesir- able sitution, because not only was the mobile equipment limited to operation within national boundaries, which in a unified Europe were increasingly unimportant , but there was also a very limited market for each type of equipment , so economies of scale and the subsequent savings could not be realized. The Europeans realized this early on , and in 1982 the Conference of European Posts and Telegraphs (CEPT) formed a study group called the Groupe Spcial Mobile (GSM) to study and develop a pan-European public land mobile system .The proposed system had to meet certain criteria: Good subjective speech quality Low terminal and service cost Support for international roaming Ability to support handheld terminals Support for range of new services and facilities Spectral efficiency ISDN compatibility In 1989, GSM responsibility was transferred to the EuropeaTelecommunication Standards Institute (ETSI), and phase I of the GSM specifications were published in 1990. Commercial service was started in mid-1991 , and by 1993 there were 36 GSM networks in 22 countries.Although standardized in Europe, GSM is not only a European standard. Over 200 GSM networks ( including DCS1800 andPCS1900) are operational in 110 countries around the world. In the beginning of 1994, there were 1.3 million subscribers worldwide which had grown to more than 55 million by October 1997. With North America making a delayed entry into the GSM field with derivative of GSM called PCS1900 , GSM systems exist on every continent, and the acronym GSM now aptly stands for Global System for Mobilecommunications. The developers of GSM chose an unproven (at the time)digital system,as opposed to the then-standard analog cellular systems like AMPS in the United States TACS in the United Kingdom. They had faith that advancements in compressionalgorithm and digital signal processors would allow the fulfillment of the original criteria and the continual improvement of the system interms of quality and cost . The over 8000 pages of GSM recommendations try to allow flexibility and cometitive innovation among suppliers but provide enough standardization to guarantee proper interwork between the components of the system.This is done by providing functionaandinter-face descriptions for each of the functional entities defined in the system. Services provided by GSM From the beginning, the planners of GSM wanted ISDN compatibility in terms of the services offered and the control signalling used . However, radio transmission limitations, in terms of bandwidth and cost, do not allow the standard ISDN B-channel bit rate of 64 kbps to be practically achieved. Using the ITU-T definitions , telecommunication services be divided into bearer services, teleservices , and supplementary services . The basic teleservice supported by GSM is telephony . As with anothercommunications , speech is digitally encoded and transmitted through the GSM network as a digital stream . There is emergency service , where the narest emergency- service provider is notified by dialing three digits (similar to 911). A variety of data services is offered.GSM users can send and receive data,at rates up to 9600 bps , tousers on POTS ISDN, Packet Switched Public Data Networks , and Circuit Switched Public Data Networks using a variety of access methods and protocols, such as X.25 or X.32.Since GSM is a digital network,a modem is not required between the user and GSM network,although an audio modem is required inside theGSMnetwork to interwork with POTS. Other data services include Group 3 facsimile , as described in ITU-T recommend- dation T.30, which is supported by use of an appropriate fax adaptorA unique feature of GSM, not found in older analog systems, is the Short Message Service (SMS). SMS is a bidirectional service for short alphanumeric messages . Messages are transported in a store-and-forward fashion.For point -to-point SMS, a message can be sent to another sub scriber to the serviceand anacknowledgement of receipt is provided to the sender . SMS can also be used in a cell-broadcast mode , for sending messages such as traffic up-dates or news updates. Messages can also be stored in the SIM card for later retrieval . Supple mentary services are provided on top of teleservices or bearer services.Inthe current (Phase I) specifications, they include several forms of call forward ( such as call forwarding when the mobile subscriber is unreachable by the network ) , and call barring of outgoing or incoming calls , for example , when roaming in another country . Many additional supplementary services will be provided in the Phase 2 specifications , such as caller identification, call waiting, multi-party conversations . Control channels Common channels can be accessed both by idle mode and dedicated mode mobiles. The common channels are used by idle mode mobiles to exchange the signallinginforma tion required to change to dedicated mode . Mobiles already in dedicated mode monitor the surrounding base stations for handover and other information .The common channels are defined within a 51frame multiframe , so that dedicatedmobile using the 26-frame multiframe TCH structure canstill monitor control channels .The common channels include: Broadcast Control Channel (BCCH) Continually broadcasts, on the downlink, information including base station identity, frequency allocations, and frequency-hopping sequences. Frequency Correction Channel (FCCH) and Synchronisation Channel ( SCH ) Used to synchronise the mobile to the time slot structure of a cell by defining the boundaries of burst periods , and the time slot numbering . Every cell in a GSM network broadcasts exactly one FCCH andone SCH , which are by definition on time slot number 0 (within a TDMA frame). Random Access Channel (RACH) Slotted Aloha channel used by the mobile to request access to the network. Paging Channel (PCH) Used to alert the mobile station of an incoming call. Access Grant Channel (AGCH) Used to allocate an SDCCH to a mobile for signalling (in order to obtain adedicated channel), following a request on the RACH. Channel coding and modulation Because of natural and man-made electromagnetic interference,the encoded speech or data signal transmitted over the radio interface must be protected from errors . GSM uses convolutional encoding and block interleaving to achieve this protection . The exact algorithms used differ for speech and for different data rates .The method used forspeech blocks will be described below. Recall that the speech codec produces a 260 bit block for every 20ms speechsample. From subjective testing , it was found that some bits of this block were more important for perceived speech quality than others . The bits are divided into three classes: Class Ia 50 bits - most sensitive to bit errors Class Ib 132 bits - moderately sensitive to bit errors Class II 78 bits - least sensitive to bit errors Class Ia bits have a 3 bit Cyclic Redundancy Code added for error detection. If an error is detected,the frame is judged too damaged to be comprehensible and itisdiscarded. It is replaced by a slightly attenuated version of the previous correctly received frame . These 53 bits, together with the 132 ClassIb bits and a 4 bit tail sequence (a total of 189 bits), are input into a 1/2 rate convolutional encoder of constraint length 4 . Each input bit is encoded as two output bits, based on a combination of the previous 4 input bits.The convolutional encoder thus outputs 378 bits,to which are added the 78remaining Class II bits, which are unprotected. Thus every 20 ms speech sample is encoded as 456 bits. To further protect against burst errors commontothe radio interface each sample is interleaved . The 456 bits output by the convolutional encoder are divided into 8 blocks of 57 bits, and these blocks are transmitted in eight consecutive time-slot bursts . Since each time-slot burst can carry two 57 bit blocks , each burst carries traffic from two different speech samples. Speech coding GSM is a digital system, so speech which is inherently analog, has to be digitized. The method employed by ISDN, and by current telephone systems for multiplex- ing voice lines over high speed trunks and optical fiber lines, is PulseCoded Modulation (PCM) . The output stream from PCM is 64 kbps , too high a rate tobe feasible over a radio link. The 64 kbps signal,although simple to implement contains much redundancy. The GSM group studied several speech coding algorithms on the basis of subjective speech quality and complexity ( which is related to cost, processing delay, and power consumption once implemented)before arriving at the choice of a Regular Pulse Excited - Linear Predictive Coder (RPELPC ) with a Long Term Predictor loop.Basically, information from previous samples , which does not change very quickly , is used to predict the current sample. The coefficients of the linear combination of the previous samples , plus an encoded form of the residual, the difference between the predicted and actual sample, represent the signal. Speech is divided into 20 millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps . This is the so- called Full-Rate speech coding. Recently , an Enhanced Full-Rate (EFR) speech coding algorithm has been implemented 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 0.8 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 。 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 co-channel cells alternatingly increase their powerin response toincreased cochannel interference caused by the other mobile increasing its power.This 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,co-channel 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 antenna.Equalization 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 time-slot burst. The actual implementation of the equalizer is not specified in the GSM specifications. Discontinuous transmission Minimizing co-channel 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 component 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 noise.If 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 transmitter is turned off, there is total silence heard at the receiving end, due to the digital nature of GSM. To assure the receiver that the connection is not dead, comfort noise is created at the receiving end by trying to match the characteristics of the transmitting ends 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 incoming call ,is structured into sub-channels. Each mobile station needs to listen only to its own sub- channel. In the time between successive paging sub-channels, 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 LAPDm.Across 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 Recommendation Q.931 , and manages Supplementary Services and the Short Message Service. Signalling between the different entities in the fixed part of the network , such as between the HLR and VLR, is accomplished 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 complex, and at over 500 pages , it is one of the longest documents in the GSM recommendations . Radio resources management The radio resources management (RR ) layer oversees the establishment of a link, both radio and fixed , between the mobile stationand the MSC.The main functional components 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 RR-session 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 RR-session is always initiated by a mobile station through the access procedure, either for an outgoing call , or in response to a paging message.The details of the access and paging procedures , such as when a dedicated channel is actually assigned to the mobile,and the paging sub-channel 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 network,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 on-going 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 completion of the handover.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 call-related 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.The algorithm for when a handover decision should be taken is not specified in theGSM recommendations.There are two basic algorithms use ,both closely tied in with power control.This 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 performance 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 common method , but it creates smeared cell boundaries when a mobile transmitting at peak power goes some distance beyond its original cell boundaries into another cell. The power budget 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 smeared cell boundary problem andreducesco- -channel interference, but it is quite complicated. 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 aspects.Location management is concerned with the procedures that enable the system to know the current location of a powered-on mobile station so that incoming call routing can be completed. Location updating A powered - on mobile is informed of an incoming call by a paging message sent over the PAGCH channel of a cell. One extreme would be to page every cell in the network 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 compromise 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 operators PLMN, it must register with the network 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 subscribers HLR.The 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 incoming calls. If the subscriber is entitled to service , the HLR sends a subset of the subscribe