GPRS外文翻译--理论研究通用分组无线业务.doc

山 东 科 技 大 学外 文 翻 译 学 院 名 称 电气信息系 专 业 班 级 电气工程及其自动化10-4 班 学 生 姓 名 梅玉金 学 号 201003201414 指 导 教 师 姚 福 强 翟 绪 梅 Theoretical Study of the General Packet Radio ServiceAbstract: As communications technology continues to mature and users quality have become increasingly demanding, new data applications are emerging and are reaching the general public. Through the use of GPRS (General Packet Radio Service) technology, the existing GSM networks can be easily achieved with simple high-speed data packet access, enabling mobile operators to respond rapidly to market demands and gain competitive advantage. This paper aims to provide a comprehensive yet simple overview of the GPRS system from the users and from the architectural perspectives.Keywords: GPRS networks; Voice/data integration; Multiple-channels; Packet radio; Telecommunications1 IntroductionIn the late 1800s, Marconi did the pioneer work establishing the first successful radio link between a land-based station and a tugboat. Since then, wireless communication systems have been developing and evolving with a furious pace. The number of mobile subscribers has been growing tremendously in the past decades. In the early stages, wireless communication systems were dominated by military usage and supported according to military needs and requirements. During the last half a century, with increasing civil applications of mobile services, commercial wireless communication systems have been taking the lead. The early wireless systems consisted of a base station with a high-power transmitter and served a large geographic area. Each base station could serve only a small number of users and was costly as well.The systems were isolated from each other and only a few of the communicated with the PSTN (Public Switched Telephone Networks). Today, the cellular systems consist of a cluster of ba -se station with low power radio transmitters. Each base station serves a small cell within a large geographic area. The total number of users served is increased because of channel reuse and al -so larger frequency bandwidth. Global system for mobile communications is the European standard for cellular communications developed by the ETSI (European Telecommunications Standards Institute).Throughout Europe and the rest of the world, GSM has been widely adopted. It has already been implemented in over 100 countries. The most important service in GSM is voice telephony. Voice is digitally encoded and carried by the GSM network as a digital stream in a circuit-switched mode. Data services have been gaining popularity in cellular networks since they were first introduced through the circuit-switched connection. As cellular carriers, however, start deploying the packet data service such as GPRS (General Packet Radio Service), carriers can provide data services in a more bandwidth-efficient way over the cellular network, thereby increasing the radio resource utilization.2 GPRS-related technologiesGSM offers data services already but they have been constrained by the use of circuit-switched data channels over the air interface allowing a maximum bit rate of 14.4 kb/s. For this reason, the GSM standard has continued its natural evolution to accommodate the requirement for higher bitrates. The HSCSD (High-speed Circuit-switched Data) are one solution that address this requirement by allocating more time slots per subscriber and thus better rates. It remains however insufficient for bursty data applications such as Web browsing. Moreover, HSCSD rely on circuit-switching techniques making it unattractive for subscribers who want to be charged based on the volume of the data traffic they actually use rather than on the duration of the connection. In turn, service providers need effective means to share the scarce radio resources between more subscribers. In a circuit-switched mode, a channel is allocated to a single user for the duration of the connection. This exclusive access to radio resources is not necessary for data applications with the use of packet switched techniques. GPRS stands out as one major development in the GSM standard that benefits from packet switched techniques to provide mobile subscribers with the much needed high bit rates for bursty data transmissions. It is possible theoretically for GPRS subscribers to use several time slots (packet data channels) simultaneously reaching a bit rate of about 170 kb/s. Volume-based charging is possible because channels are allocated to users only when packets are to be sent or received. Bursty data applications make it possible to balance more efficiently the network resources between users because the provider can use transmission gaps for other subscriber activities. 2.1 Basic PrincipleIn a cellular network, an entire geographic area is divided into cells, with each cell being served by a base station. Because of the low transmission power at the base station, the same channels can be reused again in another cell without causing too much interference. The configuration and planning of the cell is chosen to minimize the interference from another cell and thus maximum capacity can be achieved. The cell is usually depicted as a hexagon, but in reality the actual shape varies according to the geographic environment and radio propagation. Channel allocation is chosen based on the density of the users. If a cell has many users to serve, usually more channels are allocated. The channels are then reused in adjacent cells or cluster of cells. The spatial separation of the cells with the same radio channels, in conjunction with the low transmission power and antenna orientation, keeps the co-channel interference at an acceptable level. Mobility is one of the key features in wireless communication systems. There is a need to track the users moving into different cells and changing radio channels. A mobile switched to another channel in a different cell is called handoff. A signaling and call processing procedure is needed to support user mobility and handoff such that a mobile phone can be completed successfully. Paging is another key feature in cellular systems. It uses a common shared channel to locate the users within the service area and to broadcast some signaling messages. 2.2 Multiple Access TechniqueMultiple access is a technique to allow users to share a communication medium so that the overall capacity can be increased. There are three commonly used multiple access schemes: FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), CDMA (Code Division Multiple Access). In FDMA, each call is assigned its own band of frequency for the duration of the call. The entire frequency band is divided into many small individual channels for users to access. In TDMA, users share the same band of frequencies.Each call is assigned a different time slot for its transmission. In CDMA, users share the same band of frequencies and time slots. Each call is assigned a unique code, which can spread the spectrum to the entire frequency band. The spectrum spread calls are sent on top of each other simultaneously, and are separated at the receiver by an inverse operation of the unique codes. A combination of the three multiple access schemes can also be applied.2.3 Power ControlPower control is one of the most important design features in wireless communication including FDMA, TDMA, and CDMA systems. It ensures each user transmits and receives at a proper energy level to convey information successfully while reducing the interference to other users. Power control is needed in FDMA and TDMA systems because of the co-channel interference management. This type of interference is caused by the frequency reuse in the limited available spectrum. Via a proper power level adjustment, the co-channel interference can be reduced. This allows a higher frequency reuse factor and thus increases the system capacity. Owner control is the most essential requirement in CDMA systems. Without power control, all the mobiles transmit to the base station with the same power not taking into account path loss and fading effect. Mobiles close to the base station will cause significant interference to mobiles that are farther away from the base station. This effect is the so-called near/far effect. Therefore, a well-designed power control algorithm is crucial for proper operation of a CDMA system. In the absence of power control, the system capacity is very low compared to other systems. Another advantage of power control is that it can prolong battery life by using a minimum required transmission power. Power control on a reverse link is more stringent than on a forward link because of the near/far effect. On a forward link, power control is still necessary to reduce the inter-cell interference.Power control can be operated in a centralized form or a distributed form. A centralized controller obtains the information of all the established connections and channel gains, and controls the transmission power level. The centralized approach can optimize the power usage of the entire or part of the network and thus is very efficient. It requires extensive control signaling in the network, however, and is difficult to apply in practice.3 GPRS architectureGPRS is considered as a service or feature of GSM. It was designed by ETSI to be implemented over the existing infrastructure of GSM without interfering with the already existing services. The aim is quick GPRS deployment with minor impact on existing GSM PLMN components. Fig. 1 illustrates the logical architecture of a GSM network supporting GPRS.Figure 1. Architecture of GPRS network3.1 Mobile StationGPRS and GSM systems provide inter-working and sharing of resources dynamically between users. For this reason, three types of terminals have been defined: a class-A MS can carry a circuit-switched and a packet switched connection simultaneously enabling the subscriber to initiate or receive a voice call without interrupting a data transmission or reception activity. This type of terminal probably will not be available when GPRS is initially deployed due to its complexity and high cost. An MS of class-B is able to connect to both GSM and GPRS at the same time but an incoming voice call requires GPRS data transactions in progress to be suspended for the duration of the call. GPRS data transactions can then resume at the end of the voice call. Finally, a class-C MS allows subscribers to access one service type only at a given time in an exclusive manner. The GPRS MS has two components: a MT (Mobile Terminal) which is typically a handset used to access the radio interface as a radio modem, and a TE (Terminal Equipment) which is typically a laptop or a PDA (Personal Digital Assistant). GPRS MS will also come as one unit combining the functionalities of an MT and a TE.3.2 Base Station SubsystemGPRS has minor impact on the existing GSM BSS making it easy to reuse existing component and links without major modifications. This is possible because GPRS uses the same frequency bands and hopping techniques, the same TDMA frame structure, the same radio modulation and burst structure as GSM.A new functional component called PCU (packet control unit) was added to the BSS in the GPRS standard to support the handling of data packets. The PCU is placed logically between the BSS and the GPRS NSS. Unlike the voice circuit connections however, connections in GPRS have to be established and released between the BSS and the MS only when data need to be transported over the air interface. Therefore, ETSI has defined new procedures to adapt such connections.3.3 Network Switching SubsystemThe GPRS NSS can be viewed as an overlay network ensuring the link between mobile users and data networks. GPRS introduces a new functional element to the GSM infrastructure: GSN (GPRS Support Node) which can be either a SGSN (Serving-GSN) or a GGSN (Gateway-GSN). This addition is necessary for the GSM network in order to support packet data services. The network is generally divided into several service areas controlled by separate SGSN. Only one SGSN serves an MS at a given time provided it is located in its service area. The SGSN is primarily responsible for keeping track of the MS it serves, and for access control to data services. The GGSN on the other hand provides the interface to external PDN (Packet Data Networks). The SGSN is connected to the BSS by frame relay and to possibly several GGSN via a GPRS backbone network. The HLR database is updated to contain GPRS subscriber information. Adaptations to an existing MSC/VLR are not required but the GPRS standard suggests some enhancements to coordinate between the SGSN and the MSC/VLR if the optional interface between the two is to be supported.Several interfaces have been introduced in GPRS to define entity-to-entity interactions. For instance, the interface is required between the BSC and the SGSN. Two GSN communicate through a interface, and the SGSN sends queries and receives subscriber information to/from the HLR through the interface. The interface between the SGSN and the MSC/VLR was left optional while the interface which connects a GGSN to a PDN was not specified in the standard to allow implementation preferences.As mentioned, GPRS standard activities focused mainly on PTP connections to IP PDN at the GI interface. An example of such IP PDN can be a corporate Intranet where access is restricted to authenticated corporate employees allowing them to access for instance the corporate web and mail servers. Another example is connectivity to an ISP (Internet service provider) offering Internet access and related services.4 ConclusionGPRS acting as GSM network bear service represents a major development toward the next generation wireless mobile network such as UMTS (Universal Mobile Telecommunication System). In the GPRS architecture, a Base Station Controller and the multiple Base Stations constitute the Base Station System. At the output of the BSC, the voice or data traffic is separated. Specifically, voice is transported to the MSC while data is sent to the SGSN. The GGSN is used as a gateway between GPRS and the external IP networks such as the Internet.理论研究通用分组无线业务摘要：随着通信技术的不断成熟和用户的质量要求越来越高，新的数据应用不断涌现，并被广泛运用。通过GPRS技术的使用，现有的GSM网络可以简单方便地实现高速数据分组接入，使移动运营商迅速响应市场需求和获得竞争优势。本文旨在从用户和架构的角度提供一个全面而又简单的GPRS系统概述。1 介绍在19世纪后期，马可尼做了一项具有开创性意义的工作，就是成功连接了一个路基站和一艘船之间的无线链路。从那时开始，无线通讯系统就在以非常惊人的速度朝前发展着。移动用户在过去的几十年里以巨大的速度在增长着。在早期阶段，无线通讯系统等技术主要用于军事方面，根据需要和要求对军事进行各种支持。在过去的半个世纪世纪，随着越来越多的移动用户使用移动服务，商用无线通信系统已经处于领头地位。早期的无线信息系统包括了一个一个基地站与一个高功率的发射器和一个大的地理区域。每一个基地站只能支持一小部分用户的服务，并且费用十分昂贵。这些系统是互相隔离开的，只有一小部分与公众沟通交换电话网络保持连接。如今，移动系统包括了一个低功率的基站簇无线电发射机。每个基站在一个大的地理区域内提供一个小的细胞服务。该服务通道的重复使用和频率带宽变大会使得用户总数随之增加。全球移动通信系统是由欧洲电信标准协会开发的欧洲标准的移动通信。在整个欧洲和世界上的其他地区，GSM系统已经被广泛使用。它已经在100多个国家被实施使用。在GSM系统中，最重要的服务莫过于语音电话。语音信息在GSM网络中作为数字流以一种电路交换模式进行数字编码。由于数据服务首先通过电路交换链接介绍了移动网络，它们已经越来越受到了大众的认可。随着运营商开始部署像GPRS这样的分组数据服务，他们可以以更高效带宽的方式提供移动网络服务，从而提高无线资源的利用率。2 GPRS的相关技术虽然GSM已经提供了数据服务，不过由于使用电路交换数据通道接口，它们仍受到空中允许最大比特率14.4千比特/秒的限制。出于这样的原因，GSM继续其自然发展演变以满足更高的比特率要求。为了使每个用户更好的分配时隙从而提高利用率，高速电路交换数据成为了一个解决途径。但是它仍然不足以应对像Web浏览这样的突发数据应用。除此之外，HSCSD依靠电路交换技术吸引了一部分的用户，他们希望收取的费用以数据流量为准而非实际持续连接的时间。而另一方面，服务提供商则需要更有效的手段在越来越多的用户之间分享越来越少的无线资源。在一个电路交换模式下，一个通道只被分配用于记录一个用户的连接时间。这种对无线电资源的一对一访问在数据包交换技术中并非是必需的。GPRS以一个非常重大的发展而在GSM标准中凸显出其地位来，这就体现在分组交换技术上，为更多的移动用户提供突发数据需要传输的高比特率。理论上来说，GPRS用户可以同时使用几个时段，并且可以达到约170千比特每秒的比特率。由于发送或接受数据包时渠道被分配给相应用户，所以基于卷的收费是很有可能的。因为供应商可以使用其他用户活动传输的差距，所以突发数据的应用使人们有可能更有效地平衡用户与用户之间的网络资源。2.1 基本原则在一个移动网络中，整个地理区域被分为很多部分，每个部分由一个基站提供服务。由于基站的发射功率较低，所以在重复使用同样的渠道时，另一个单元并不会受到太多的干扰。对小单元的配置和选择可以尽量减少对另一个单元的干扰，从而可以达到最大的容量。一个单元通常被描绘成一个六边形，但在现实中的实际形状则根据不同的地理环境和无线电传播而稍有不同。我们通常基于用户的密度来进行信道的分配。如果一个单元要服务很多的用户，通常就会有更多的频道分配。这些渠道就重复使用相邻的单元或者单元群。在具有相同广播频道但被空间分离的单元中，较低的发射功率和天线的方向使同频道干扰保持在一个可接受的水平内。可移动性是无线通信系统最关键的特征之一。有时我们需要追踪用户的一些应用，如在不同单元间转换以及改变无线电通道。一个移动在不同单元内转换通道被称作切换。运用信息和呼叫处理的程序来支持用户移动性和切换使得移动电话能够被顺利完成。分页是移动系统中另外一个非常重要的特征。通常我们在服务范围内使用一个共享信道查找用户并且广播一些信号消息。2.2 多址接入技术多址接入是一种允许用户共享通信媒介的技术，这样一来系统整体的能力就可以被增强。一般有三种常用的多址接入方案：FDMA（频多分址），TDMA（时多分址），CDMA（码多分址）。在FDMA中，每个呼叫都为它自己的通话时间分配了其相应的频率。整个频带被分成了很多小的个人渠道提供给用户访问。在TDMA中，用户共享同一频段的频率。每个呼叫被分配一个不同的传输时隙。在CDMA中，用户共享同一频段的频率和时间。每个呼叫被分配一个唯一的代码，它可以传播到整个频谱的频段。这些扩频电话被同时发送出去，并且有一个独特的逆运算码接收机分离开。由三个多址接入方案合成的方法也可以被运用。2.3 功率控制功率控制是无限通信中最重要的设计功能之一，包括了FDMA，TDMA，和CDMA。它确保了用户以适当的能量水平来发送和接受数据从而成功传达信