外文翻譯--有關GSM的介紹

1 (12)錯誤 !未指定書簽。 Introduction to GSM 1.1. BACKGROUND AND REQUIREMENTS At the beginning of the 80s it was realised that the European countries were using many different, incompatible mobile systems. At the same time, the needs for telecommunication services were remarkably increased. Due to this, CEPT (Confrence Europenne des Postes et Tlcommunications) founded a group to specify a common mobile system for Western Europe. This group was named Groupe Speciale Mobile and the system name GSM arose. The abbreviation has since been interpreted in other ways, but the most common expression nowadays is Global System for Mobile communications. At the beginning of the 90s, the lack of a common mobile system was seen to be a general, world-wide problem. This is why the GSM system has now spread also to the Eastern European countries, Africa, Asia and Australia. The USA, South America in general and Japan had made a decision to adapt to another kind of common mobile system specification which is not compatible with GSM. However Personal Communication System (PCS) which uses the same GSM technology with few variations has also been adopted in the USA. During the time the GSM system was being specified it was foreseen that national telecommunication monopolies would be disbanded. This development set some requirements concerning the GSM system specifications and these requirements are in a way built into the specifications: There should be several network operators in each country. This should lead to the tariff and service provisioning competition. This was presumed to be the best way to ensure the rapid expansion of the GSM system； the prices of the equipment would fall and the users would be able to afford to call. The system must be an open system, meaning that it should contain well defined interfaces between different system parts. This enables the equipment from several manufacturers to coexist and hence improves the cost efficiency of the system from the operators point of view. GSM networks must be built without causing any major changes to the already existing Public Switched Telephone Networks (PSTN). In addition to the above-mentioned commercial demands, some other main objectives were defined: The system must be Pan European. The system must maintain a good speech quality. 2 (12)錯誤 !未指定書簽。 The system must use radio frequencies as efficiently as possible. The system must have high / adequate capacity. The system must be compatible with ISDN (Integrated Services Digital Network). The system must be compatible with other data communication specifications. The system must maintain good security concerning both subscriber and transmitted information. 1.2. ADVANTAGES OF GSM Due to the above-mentioned requirements set for the system, many remarkable advantages will be achieved. Roughly, these advantages are: GSM uses radio frequencies efficiently, and, due to the digital radio path, the system tolerates more intercell disturbances. The average quality of speech achieved is better than in existing analogue systems. Data transmission is supported throughout the system. Speech is encrypted and subscriber information security is guaranteed. Due to the ISDN compatibility, new services are offered compared to the analogue systems. International roaming is technically possible within all the countries concerned. The large market toughens the competition and lowers the prices both for investments and usage. 1.3. EVOLUTION OF GSM The following list highlights some important years in the short history of GSM. 1982 CEPT initiated a new system, GSM 1985 CEPT made decision on time schedule and action plan 1986 CEPT tested eight experimental systems in Paris 1987 Memorandum of Understanding (MoU), allocation of the frequencies 890-915 uplink (from mobile to base station) 935-960 downlink (from base station to mobile) 1988 European Telecommunications Standard Institute (ETSI) was created includes members from administrations, industry and user groups 1989 Final recommendations and specifications 1.7.1991 First official call in the world with GSM 1992 Australian operators were first non-European signatories of the GSM MoU 1992 New frequency allocation: GSM 1800 3 (12)錯誤 !未指定書簽。 1710-1785 uplink 1805-1880 downlink 2.1. OPEN INTERFACES OF GSM The main idea behind the GSM specifications is to define several open interfaces which then are limiting certain parts of the GSM system. Because of this interface openness, the operator maintaining the network may obtain different parts of the network from different GSM network suppliers. Also, when an interface is open it defines strictly what is happening through the interface and this in turn strictly defines what kind of actions/procedures/functions must be implemented between the interfaces. Nowadays, GSM specifications define two truly open interfaces: The first one is between the mobile Station and the Base station. This open air interface is appropriately termed as Air interface. The second one is between the Mobile Services Switching Centre (which is the switching exchange in GSM) and the Base Station Controller. This interface is called the A interface. The two network elements just mentioned will be discussed in greater detail in later chapters. The system includes more than two interfaces defined but they are not totally open because the system specification had not been completed when the commercial systems were launched. Network Subsystem (NSS) Base Station Subsystem (BSS) Network Management Subsystem (NMS) The actual network needed for call establishing is composed of the NSS and the BSS. The BSS is a network part responsible for radio path control. Every call is connected through the BSS. The NSS is a network part taking care of call control functions. Every call is always connected by and through the NSS. The NMS is the operation and maintenance related part of the network. It is also needed for the whole network control. The network operator observes and maintains network quality and service offered through the NMS. All these three subsystems are surrounded by the above-mentioned interfaces. 4 (12)錯誤 !未指定書簽。 A i r AO & MFigure 3.1 The three Subsystems of GSM The MS (Mobile Station) is a combination of terminal equipment and a subscriber. The terminal equipment as such is called ME (Mobile Equipment) and the subscribers data is stored to a separate module called SIM (Subscriber Identity Module). Hence, ME + SIM = MS. 1.4. DIMENSIONING CELLS A cell is the basic construction block of a GSM network. One cell is the geographical area covered by one BTS. The actual size of a cell depends on several factors: the environment, number of users, etc. Cells are grouped under Base Station Controllers (BSC). Dimensioning a cell means finding answers to two fundamental questions: How many traffic channels (TCH) does the cell need to handle and how many traffic channels are necessary? To solve these problems, i.e. to determine the traffic capacity, we have to calculate the number of Erlangs. Erlang is the measuring unit of network traffic. One Erlang equals the continuous use of a mobile device for one hour. The traffic is calculated using a simple formula:x c a l l s p e r hour a v e r a g e c o n v e r s a t i on t i m eE r l a n g s S e c o n d s ( ) ( )3600Frequency Reuse Now we have to resolve another problem. There is a limited number of frequencies available to each Base Station Subsystem and they must be distributed between the cells to ensure a balanced coverage throughout the BSS. Lets take an exercise to illustrate the situation. You are the network planner and the number of frequencies assigned to this project is 9. Your task is to distribute the frequencies in the network that is shown in the following figure with one frequency per cell. 5 (12)錯誤 !未指定書簽。 1 234 57 896 1 234 57 896 1 234 57 896 1 234 57 896 1 234 57 896 1 234 57 896 Figure 3.1 Frequency Planning exercise Figure 3.2 Frequency reuse pattern example As you can see, the frequencies have to be reused. If you do not distribute the frequencies properly throughout the network the result will be a high level of interference caused by overlapping frequencies. To avoid this, the GSM network includes a specification of the Frequency reuse patterns, one of which is presented in figure 3.2 The next step involves the dimensioning of the Location Areas. This is carried out according to the traffic characteristics of each area. The final phase is the dimensioning of the Fixed Network on the basis of the traffic requirements and dimensioning of the entire radio network. 6 (12)錯誤 !未指定書簽。 2. INTELLIGENT NETWORK The Intelligent Network has been made possible through developments in many fields including: signalling system no.7, stored program control exchanges, advanced software languages and powerful computers. The purpose of this section is to focus in on those parts of the network directly required for the Intelligent Network. Signalling is required for exchanges, network data bases and other intelligent nodes in the network to exchange messages related to call set up, call supervision, call connection control information needed for distributed application processing and network management information. M T P L a y e r 1M T P L a y e r 2M T P L a y e r 3S C C PI S U P T U PT C A PC o r e I N A P M A P C a l l C o n t r o l A p p l i c a t i o n sFigure 3. SS7 stack for IN The SS7 is a signalling system recommended by ITU-T and specifically designed for telephone networks. The SS7 is the generic name for a suite of protocols. These protocols are layered and closely match the Open System Interconnection (OSI) seven layer model. The lowest three layers are called the Message Transfer Part (MTP). Its function is to route a message efficiently and reliably between two signalling points in the network. Above MTP is the signalling connection control part (SCCP). The MTP together with SCCP provide most of the layer services specified in layers 1 through 3 of the OSI model. The SS7 application protocols sit immediately above MTP and SCCP. The Telephony User Part (TUP) is an application protocol for setting up and clearing (non-ISDN) telephone calls. The ISDN User part (ISUP) protocol is an application protocol designed to support ISDN calling. The Transaction Capabilities and Application part (TCAP) is an application protocol for invoking applications on a remote network element. TCAP is used for signalling between the SSP and the SCP of the Intelligent Network. It is essential therefore distinguish between functions and products in the Intelligent Network concepts. 7 (12)錯誤 !未指定書簽。 3. 有關 GSM的介紹 3.1. 1.1背景 在 80年代初它起源于使用很多不同的不相兼容的移動系統(tǒng)的歐洲。在那時，電子通訊服務需求迅速的增長。正因為如此，歐洲郵電行政大會 CEPT（ Conference Europe of Post and Telecommunications）成立了歐洲移動特別小組，建立全歐洲統(tǒng)一的蜂窩移動通信系統(tǒng)。這一組織稱為歐洲移動特別小組，這一系統(tǒng)只取首字母的縮寫稱為 GSM。 這種表述已經(jīng)改變了好多次，但時至今日最常用的表達是全球移動通信系統(tǒng)。 九十年代 初，一個共有的移動通信系統(tǒng)的缺乏是如此的明顯的，世界范圍內(nèi)的問題。這就是為什么 GSM系統(tǒng)現(xiàn)在在東歐各國，非洲，亞洲以及澳大利亞如此廣泛的傳播。美利堅合眾國，整個南美洲以及日本已經(jīng)決定采用另一種移動通信系統(tǒng)標準，這種標準與 GSM不相兼容。但是個人通信系統(tǒng)（ PCS）使用同樣的 GSM技術，盡管這種技術有一點點差別，但同樣在美國使用。 在 GSM 系統(tǒng)正在被統(tǒng)一的時代他就預見到國際電信將被廢除。這種發(fā)展制定一些規(guī)則包括 GSM系統(tǒng)標準，而這些規(guī)則在某種意義上形成標準。 *在各個國家應當有很多網(wǎng)絡運營商。這必將引起競爭和 服務提供者競爭。這無意當中形成一種保證 GSM系統(tǒng)快速發(fā)展的最佳方式；這種設備的價格將會下跌而用戶能夠負擔打電話的費用。 *這種系統(tǒng)應該是一種公開的系統(tǒng)，意思是它應當包含不同的系統(tǒng)之間比較好比較合理的接口規(guī)范。它使得不同生產(chǎn)商生產(chǎn)的設備之間很好的符合，以使得能夠從運營商的角度改善系統(tǒng)有效運行的費用。 *GSM網(wǎng)絡必須建立在對已經(jīng)存在的公用電話交換網(wǎng)（ PSTN）沒有產(chǎn)生很大主要變化的基礎上。 除了上面提到的一些基本要求之外，一些其他的主要要求也被提出： 系統(tǒng)必須是泛歐標準。 系統(tǒng)必須保持好的語音通話質(zhì)量。 8 (12)錯誤 !未指定書簽。 系統(tǒng) 必須盡可能有效地利用頻率資源。 系統(tǒng)必須有一個比較高的，合理的容量。 系統(tǒng)必須與現(xiàn)有的 ISDN（綜合業(yè)務數(shù)字網(wǎng)）相兼容。 系統(tǒng)必須與其他數(shù)字通信標準相匹配。 系統(tǒng)必須具有好的安全性，不論是在用戶還是信息傳輸都要具有安全性。 3.2. 1.2 GSM的優(yōu)勢 基于以上提到的要求所建立起來的系統(tǒng)，可以看出 GSM產(chǎn)生很多明顯的優(yōu)勢。總體上說有以下這些優(yōu)勢： GSM更有效地利用了現(xiàn)有的頻譜資源，基于數(shù)字傳播路徑，這種系統(tǒng)減少了小區(qū)之間的干擾。 平均語音通話質(zhì)量比現(xiàn)有的模擬系統(tǒng)完成得更好。 整個系統(tǒng)支持數(shù)據(jù)傳輸。 保證語音通話質(zhì) 量清晰和用戶個人信息的安全。 由于與綜合業(yè)務數(shù)字網(wǎng)相兼容，和模擬系統(tǒng)一樣提供新的服務。 在所包含的各個國家中盡可能地實現(xiàn)國際漫游。 巨大的市場空間緩解了競爭并降低了投資者和用戶的費用。 3.3. 1.3 GSM的發(fā)展過程 以下列出的是 GSM發(fā)展的簡短歷史中一些重要年份的重大歷史性事件 1982年，歐洲郵電行政大會 CEPT產(chǎn)生一種新的系統(tǒng) GSM。 1985年，歐洲郵電行政大會 CEPT決定在適當?shù)臅r間采取行動。 1986年，歐洲郵電行政大會 CEPT在巴黎測試八個試驗系統(tǒng)。 1987年， GSM諒解備忘錄 (MOU)，申請以下 頻段 9 (12)錯誤 !未指定書簽。 上行鏈路： 890MHz-915MHz （從移動臺到基站） 下行鏈路： 935MHz-960MHz （從基站到移動臺） 1988年，歐洲電信標準協(xié)會（ ETSI）成立包括倡導者，工業(yè)和用戶群在內(nèi)的成員。 1989年，最后的建立和標準 1991年 1月 7日，世界上第一例利用 GSM的官方呼叫。 1992年，澳大利亞運營商成為第一個非歐洲的 GSM諒解備忘錄 (MOU). 成員。 1992年，新的頻段申請： GSM 1800MHz 上行鏈路： 1710MHz-1785MHz （從移動臺到基站） 下行鏈路： 1805MHz-1880MHz （從基站到移動臺） 3.4. 2 GSM接口 GSM背后的主要想法是定義一些能夠限制 GSM系統(tǒng)的特定部分的公開接口。由于接口是公開的，運營商保證網(wǎng)絡能夠獲得從不同的 GSM網(wǎng)絡提供者那提供的網(wǎng)絡的不同部分。同樣，當接口公開，那意味著通過這些接口所發(fā)生的被嚴格的限制，相反的什么樣的行為，產(chǎn)品以及功能實體應當在接口之間被提及也應該被嚴格的限制。 當今時代， GSM標準定義了兩種真正的公開接口：第一種是移動臺和基站臺之間的接口。這種公開的空中接口確切的應該定義為空中接口。第二種是移動服務交換中心（在 GSM中 用于使交換改變的）和基站控制臺之間的接口。這種接口稱為A接口。這兩種網(wǎng)絡實體剛剛提出的將在遲一些的例會中對更多的細節(jié)進行討論。這一系統(tǒng)包括多于兩個接口被定義但是它們并不總是公開的，因為直到原來的系統(tǒng)被廢除新的系統(tǒng)標準才完整。 10 (12)錯誤 !未指定書簽。 網(wǎng)絡子系統(tǒng)（ NSS） 基站子系統(tǒng)（ BSS） 網(wǎng)絡控制子系統(tǒng)（ NMS） 建立呼叫的平常網(wǎng)絡所需要的主要由網(wǎng)絡子系統(tǒng)（ NSS）和基站子系統(tǒng)（ BSS）組成?；咀酉到y(tǒng)（ BSS）是負責無線路徑控制的網(wǎng)絡的一部分。每一個呼叫的建立都是通過基站子系統(tǒng) BSS的連接。網(wǎng)絡子系統(tǒng)（ NSS）是負責呼叫控制 功能的網(wǎng)絡的一部分。每一次呼叫總是通過網(wǎng)絡子系統(tǒng) NSS建立連接的。網(wǎng)絡控制子系統(tǒng)（ NMS）是網(wǎng)絡部分相關的執(zhí)行和保證。它同樣也需要整個網(wǎng)絡的控制。網(wǎng)絡規(guī)劃者需要和保持網(wǎng)絡的質(zhì)量和整個網(wǎng)絡控制子系統(tǒng)（ NMS）提供服務。所有的這三個子系統(tǒng)由上面所提到的接口所連接。 A i r AO & M圖 3-1 GSM的三大組成部分 移動臺 MS是由終端設備和用戶組成。像這樣的終端設備叫著移動設備，而用戶數(shù)據(jù)儲存在一個獨立的 SIM卡（用戶識別卡）中。因此， ME+SIM=MS. 3.5. 2.1 小區(qū)分裂 一個小區(qū)是 GSM網(wǎng)絡的基本組成模塊。一個小區(qū)由一個 BTS所覆蓋的地理范圍組成。一個小區(qū)的大小取決于許多因素：環(huán)境