外文翻译--有关GSM的介绍

Introduction to GSM1.1. BACKGROUND AND REQUIREMENTSAt 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. 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 GSMDue 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 GSMThe 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 1710-1785 uplink 1805-1880 downlink2.1. OPEN INTERFACES OF GSMThe 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.Figure 3.1 The three Subsystems of GSMThe 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 CELLSA 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:Frequency ReuseNow 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.Figure 3.1 Frequency Planning exercise Figure 3.2 Frequency reuse pattern exampleAs 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 3.2The 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.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.Figure 3.SS7 stack for INThe 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. 3. 有关GSM的介绍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动系统的欧洲。在那时，电子通讯服务需求迅速的增长。正因为如此，欧洲邮电行政大会CEPT（Conference Europe of Post and Telecommunications）成立了欧洲移动特别小组，建立全欧洲统一的蜂窝移动通信系统。这一组织称为欧洲移动特别小组，这一系统只取首字母的缩写称为GSM。这种表述已经改变了好多次，但时至今日最常用的表达是全球移动通信系统。九十年代初，一个共有的移动通信系统的缺乏是如此的明显的，世界范围内的问题。这就是为什么GSM系统现在在东欧各国，非洲，亚洲以及澳大利亚如此广泛的传播。美利坚合众国，整个南美洲以及日本已经决定采用另一种移动通信系统标准，这种标准与GSM不相兼容。但是个人通信系统（PCS）使用同样的GSM技术，尽管这种技术有一点点差别，但同样在美国使用。在GSM 系统正在被统一的时代他就预见到国际电信将被废除。这种发展制定一些规则包括GSM系统标准，而这些规则在某种意义上形成标准。*在各个国家应当有很多网络运营商。这必将引起竞争和服务提供者竞争。这无意当中形成一种保证GSM系统快速发展的最佳方式；这种设备的价格将会下跌而用户能够负担打电话的费用。*这种系统应该是一种公开的系统，意思是它应当包含不同的系统之间比较好比较合理的接口规范。它使得不同生产商生产的设备之间很好的符合，以使得能够从运营商的角度改善系统有效运行的费用。*GSM网络必须建立在对已经存在的公用电话交换网（PSTN）没有产生很大主要变化的基础上。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：系统必须是泛欧标准。系统必须保持好的语音通话质量。系统必须尽可能有效地利用频率资源。系统必须有一个比较高的，合理的容量。系统必须与现有的ISDN（综合业务数字网）相兼容。系统必须与其他数字通信标准相匹配。系统必须具有好的安全性，不论是在用户还是信息传输都要具有安全性。3.2. 1.2 GSM的优势基于以上提到的要求所建立起来的系统，可以看出GSM产生很多明显的优势。总体上说有以下这些优势：GSM更有效地利用了现有的频谱资源，基于数字传播路径，这种系统减少了小区之间的干扰。平均语音通话质量比现有的模拟系统完成得更好。整个系统支持数据传输。保证语音通话质量清晰和用户个人信息的安全。由于与综合业务数字网相兼容，和模拟系统一样提供新的服务。在所包含的各个国家中尽可能地实现国际漫游。巨大的市场空间缓解了竞争并降低了投资者和用户的费用。3.3. 1.3 GSM的发展过程以下列出的是GSM发展的简短历史中一些重要年份的重大历史性事件1982年，欧洲邮电行政大会CEPT产生一种新的系统GSM。1985年，欧洲邮电行政大会CEPT决定在适当的时间采取行动。1986年，欧洲邮电行政大会CEPT在巴黎测试八个试验系统。1987年，GSM谅解备忘录(MOU)，申请以下频段上行链路：890MHz-915MHz （从移动台到基站）下行链路：935MHz-960MHz （从基站到移动台）1988年，欧洲电信标准协会（ETSI）成立包括倡导者，工业和用户群在内的成员。1989年，最后的建立和标准1991年1月7日，世界上第一例利用GSM的官方呼叫。1992年，澳大利亚运营商成为第一个非欧洲的GSM谅解备忘录(MOU).成员。1992年，新的频段申请：GSM 1800MHz 上行链路：1710MHz-1785MHz （从移动台到基站）下行链路：1805MHz-1880MHz （从基站到移动台）3.4. 2 GSM接口GSM背后的主要想法是定义一些能够限制GSM系统的特定部分的公开接口。由于接口是公开的，运营商保证网络能够获得从不同的GSM网络提供者那提供的网络的不同部分。同样，当接口公开，那意味着通过这些接口所发生的被严格的限制，相反的什么样的行为，产品以及功能实体应当在接口之间被提及也应该被严格的限制。当今时代，GSM标准定义了两种真正的公开接口：第一种是移动台和基站台之间的接口。这种公开的空中接口确切的应该定义为空中接口。第二种是移动服务交换中心（在GSM中用于使交换改变的）和基站控制台之间的接口。这种接口称为A接口。这两种网络实体刚刚提出的将在迟一些的例会中对更多的细节进行讨论。这一系统包括多于两个接口被定义但是它们并不总是公开的，因为直到原来的系统被废除新的系统标准才完整。网络子系统（NSS）基站子系统（BSS）网络控制子系统（NMS）建立呼叫的平常网络所需要的主要由网络子系统（NSS）和基站子系统（BSS）组成。基站子系统（BSS）是负责无线路径控制的网络的一部分。每一个呼叫的建立都是通过基站子系统BSS的连接。网络子系统（NSS）是负责呼叫控制功能的网络的一部分。每一次呼叫总是通过网络子系统NSS建立连接的。网络控制子系统（NMS）是网络部分相关的执行和保证。它同样也需要整个网络的控制。网络规划者需要和保持网络的质量和整个网络控制子系统（NMS）提供服务。所有的这三个子系统由上面所提到的接口所连接。图3-1 GSM的三大组成部分移动台MS是由终端设备和用户组成。像这样的终端设备叫着移动设备，而用户数据储存在一个独立的SIM卡（用户识别卡）中。因此，ME+SIM=MS. 3.5. 2.1 小区分裂一个小区是GSM网络的基本组成模块。一个小区由一个BTS所覆盖的地理范围组成。一个小区的大小取决于许多因素：环境，使用者的数目等等。小区是基站控制器下的成员。小区分裂意味着发现两个基本问题的答案：在小区中有多少话务量信道需要处理和有多少话务量信道是必需的？为了解决这些问题，比如说决策话务量容量，我们必须计算出爱尔兰数量。爱尔兰是网络话务量的计量单位。一爱尔兰等于每小时移动装置连续使用的话务量。话务量率的计算是用一个简单的公式：X爱尔兰=（每小时呼叫*平均通话时间）/3600秒频率复用现在我们必须解决另一个问题。将有限的频率资源更有效的用于每个基站子系统，我们必须分配给整个基站子系统（BSS）中的小区之间以确保平衡覆盖。 让我们通过一个试验表明这一问题。你是网络规划者，分配给这个项目的频点数目是9。你的任务是给整个网络分配频点，如下图所示每个小区分配一个频点。图3-1 频率规划方案图 3-2 频率复用方式例子正如你所看到的，频率应该得到复用。如果你没有在整个网络中合理的划分频谱，结果将会是互相重叠的频率之间产生很严重的相互干扰。为了避免该种干扰，GSM网络包括一频率复用体制的标准 ，其中一种如图3.2所描述。下一步是包括当地区域的划分。这由每一个地区的话务量特征不同采取相应的措施。基于整个无线网络的划分和话务量需求，最后阶段时固定网络的划分。3.6. 3 智能网在很多领域发展的整个过程智能网使以下成为可能：七号信令系统，存储的程序控制改变，高级的软件语言和高级的计算机。这一部分的目的是以智能网直接要求的网络的那些部分为目标。信令要求改变时，网络数据基础和网络中其他智能符号改变关于呼叫建立的信息，呼叫监督,呼叫连接控制需要区分应用过程的信息和网络控制的信息。 图3 七号信令呼入分层七号信令系统是信令系统中由国际电信联盟ITU-T制订的和专门为电话网络设计的。七号信令系统是一系列协议的总体名称。这些协议和分层和和公开互联网系统OSI七层分层方式非常符合。底下三层叫做信息传输部分MTP。它的功能是在网络中两个信令之间有效及时地传送信息。在信息传输部分MTP上面是信令连接控制部分SCCP。信息传输部分MTP和信令连接控制部分SCCP共同提供大部分层与层之间的服务对应于公开互联网系统OSI的底下三层。七号信令系统采用的协议信息传输部分MTP和信令连接控制部分SCCP之上迅速地建立。电话用户部分TUP是建立和清除（非综合业务数字网ISDN）的电话呼叫所采用的协议。综合业务数字网ISDN用户部分（ISUP）协议是设计用来支持综合业务数字网ISDN用户所设计采用的协议。事务处理能力应用部分（TCAP）是提供在遥远网络实体中与电路无关的信令应用之间交换信息的能力采用的协议。事务处理能力应用部分TCAP 用于SSP 和智能网中的SCP之间的信令。区分智能网络实体中功能和产物之间将非常重要。12 (12)