外文翻譯---理論研究通用分組無(wú)線業(yè)務(wù)

1、<p><b>　　外文翻譯材料</b></p><p>　　電子工程 系 自動(dòng)化 </p><p>　　學(xué) 生 姓 名： </p><p>　　學(xué) 號(hào)： 01207111 　 　</p><p>　　設(shè) 計(jì) 地 點(diǎn)： 東南

2、大學(xué)成賢學(xué)院 </p><p>　　指 導(dǎo) 教 師： </p><p>　　Theoretical Study of the General Packet Radio Service</p><p>　　Abstract: As communications technology continues to mature a -n

3、d users’ quality have become increasingly demanding, new da -ta applications are emerging and are reaching the general public. Through the use of GPRS (General Packet Radio Service) techn -ology, the existing GSM network

4、s can be easily achieved with simple high-speed data packet access, enabling mobile operators to respond rapidly to market demands and gain competitive adva -ntage. This paper aims to provide a comprehensive yet simpl<

5、;/p><p>　　Keywords:GPRS networks;Voice/data integration; Multiple-channels; Packet radio; Telecommunications</p><p>　　1 Introduction</p><p>　　In the late 1800s, Marconi did the pioneer

6、 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 subscri

7、bers 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

8、 a century, with i</p><p>　　The systems were isolated from each other and only a few of the communicated with the PSTNs (Public Switched Telephone Networks). Today, the cellular systems consist of a cluster

9、of ba -se station with lowpower radio transmitters. Each base station s -erves a small cell within a large geographic area. The total num -ber of users served is increased because of channel reuse and al -so larger frequ

10、ency bandwidth. </p><p>　　Global system for mobile communications is the European standard for cellular communications developed by the ETSI (European Telecommunications Standards Institute).Throughout Europ

11、e 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 isvoice telephony. Voice is digitally encoded and carried by the GSM netwo

12、rk as a digital stream in a circuit-switched mode. </p><p>　　Data services have been gaining popularity in cellular networks since they were first introduced through the circuit-switched connection. As cellu

13、lar 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 increasi

14、ng the radio resource utilization.</p><p>　　2 GPRS-related technologies</p><p>　　GSM offers data services already but they have been constrained by the use of circuit-switched data channels over

15、 the air interface allowing a maximum bit rate of 14.4 kbit/s. For this reason, the GSM standard has continued its natural evolution to</p><p>　　accommodate the requirement for higher bit rates.The HSCSD (Hi

16、gh-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

17、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 c&l

18、t;/p><p>　　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 transmissi

19、ons. It is possible theoretically for GPRS subscribers to use several time slots (packet data channels) simultaneously reaching a bit rate of about 170 kbit/s. Volume-based charging is possible because channels are alloc

20、ated to users only when packets are to be sent or received. Bursty data applicati</p><p>　　2.1 Basic Principle</p><p>　　In a cellular network, an entire geographic area is divided into cells, wi

21、th 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 p

22、lanning 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 acco</p>&l

23、t;p>　　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 differ

24、ent 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 syste

25、ms. It uses a common shared channel to locate the users within the se</p><p>　　2.2 Multiple Access Technique</p><p>　　Multiple access is a technique to allow users to share a communication mediu

26、m 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

27、). 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</p><

28、p>　　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

29、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 ca

30、n also be applied.</p><p>　　2.3 Power Control</p><p>　　Power control is one of the most important design features in wireless communication including FDMA, TDMA, and CDMA systems. It ensures eac

31、h 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 interfe

32、rence management. This type of interference is caused by the frequency reuse in the limited available spectrum. Via a proper power level adju</p><p>　　Power control can be operated in a centralized form or a

33、 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 o

34、f 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.</p><p>　　3 GPRS architecture</p>

35、<p>　　GPRS 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 GP

36、RS deployment with minor impact on existing GSM PLMN components. Fig. 1 illustrates the logical architecture of a GSM network supporting GPRS.</p><p>　　Figure 1. Architecture of GPRS network</p><p

37、>　　3.1 Mobile Station</p><p>　　GPRS 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

38、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 p

39、robably will not be available when GPRS is initially deployed due to its complexity and high cost. An MS of</p><p>　　exclusive manner. </p><p>　　The GPRS MS has two components: a MT (Mobile Term

40、inal) 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 MSs will also come as one unit

41、 combining the functionalities of an MT and a TE.</p><p>　　3.2 Base Station Subsystem</p><p>　　GPRS has minor impact on the existing GSM BSS making it easy to reuse existing component and links

42、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.</p><p>　　

43、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 voic

44、e 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 t

45、o adapt such connections.</p><p>　　3.3 Network Switching Subsystem</p><p>　　The GPRS NSS can be viewed as an overlay network ensuring the link between mobile users and data networks. GPRS introd

46、uces 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

47、 data services. The network is generally divided into several service areas controlled by separate SGSNs. Only one SGSN serves an MS at a given time provided it i</p><p>　　Several interfaces have been introd

48、uced in GPRS to define entity-to-entity interactions. For instance, the Gb interface is required between the BSC and the SGSN. Two GSNs communicate through a Gn interface, and the SGSN sends queries and receives subscrib

49、er information to/from the HLR through the Gr interface. The Gs interface between the SGSN and the MSC/VLR was left optional while the Gi interface which connects a GGSN to a PDN was not specified in the standard to allo

50、w implementation preferen</p><p>　　As mentioned, GPRS standard activities focused mainly on PTP connections to IP PDNs at the Gi interface. An example of such IP PDN can be a corporate Intranet where access

51、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 a

52、nd related services.</p><p>　　4 Conclusion</p><p>　　GPRS acting as GSM network bear service represents a major development toward the next</p><p>　　generation wireless mobile networ

53、k 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 da

54、ta 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.</p><p>　　理

55、論研究通用分組無(wú)線業(yè)務(wù)</p><p>　　摘要：隨著通信技術(shù)的不斷成熟和用戶(hù)的質(zhì)量要求越來(lái)越高，新的數(shù)據(jù)應(yīng)用不斷涌現(xiàn)，并被廣泛運(yùn)用。通過(guò)GPRS技術(shù)的使用，現(xiàn)有的GSM網(wǎng)絡(luò)可以簡(jiǎn)單方便地實(shí)現(xiàn)高速數(shù)據(jù)分組接入，使移動(dòng)運(yùn)營(yíng)商迅速響應(yīng)市場(chǎng)需求和獲得競(jìng)爭(zhēng)優(yōu)勢(shì)。本文旨在從用戶(hù)和架構(gòu)的角度提供一個(gè)全面而又簡(jiǎn)單的GPRS系統(tǒng)概述。</p><p><b>　　1 介紹</b>&l

56、t;/p><p>　　在19世紀(jì)后期，馬可尼做了一項(xiàng)具有開(kāi)創(chuàng)性意義的工作，就是成功連接了一個(gè)路基站和一艘船之間的無(wú)線鏈路。從那時(shí)開(kāi)始，無(wú)線通訊系統(tǒng)就在以非常驚人的速度朝前發(fā)展著。移動(dòng)用戶(hù)在過(guò)去的幾十年里以巨大的速度在增長(zhǎng)著。在早期階段，無(wú)線通訊系統(tǒng)等技術(shù)主要用于軍事方面，根據(jù)需要和要求對(duì)軍事進(jìn)行各種支持。在過(guò)去的半個(gè)世紀(jì)世紀(jì)，隨著越來(lái)越多的移動(dòng)用戶(hù)使用移動(dòng)服務(wù)，商用無(wú)線通信系統(tǒng)已經(jīng)處于領(lǐng)頭地位。</p>

57、<p>　　早期的無(wú)線信息系統(tǒng)包括了一個(gè)一個(gè)基地站與一個(gè)高功率的發(fā)射器和一個(gè)大的地理區(qū)域。每一個(gè)基地站只能支持一小部分用戶(hù)的服務(wù)，并且費(fèi)用十分昂貴。這些系統(tǒng)是互相隔離開(kāi)的，只有一小部分與公眾溝通交換電話網(wǎng)絡(luò)保持連接。如今，移動(dòng)系統(tǒng)包括了一個(gè)低功率的基站簇?zé)o線電發(fā)射機(jī)。每個(gè)基站在一個(gè)大的地理區(qū)域內(nèi)提供一個(gè)小的細(xì)胞服務(wù)。該服務(wù)通道的重復(fù)使用和頻率帶寬變大會(huì)使得用戶(hù)總數(shù)隨之增加。</p><p>　　全球移

58、動(dòng)通信系統(tǒng)是由歐洲電信標(biāo)準(zhǔn)協(xié)會(huì)開(kāi)發(fā)的歐洲標(biāo)準(zhǔn)的移動(dòng)通信。在整個(gè)歐洲和世界上的其他地區(qū)，GSM系統(tǒng)已經(jīng)被廣泛使用。它已經(jīng)在100多個(gè)國(guó)家被實(shí)施使用。在GSM系統(tǒng)中，最重要的服務(wù)莫過(guò)于語(yǔ)音電話。語(yǔ)音信息在GSM網(wǎng)絡(luò)中作為數(shù)字流以一種電路交換模式進(jìn)行數(shù)字編碼。</p><p>　　由于數(shù)據(jù)服務(wù)首先通過(guò)電路交換鏈接介紹了移動(dòng)網(wǎng)絡(luò)，它們已經(jīng)越來(lái)越受到了大眾的認(rèn)可。隨著運(yùn)營(yíng)商開(kāi)始部署像GPRS這樣的分組數(shù)據(jù)服務(wù)，他們可以以更

59、高效帶寬的方式提供移動(dòng)網(wǎng)絡(luò)服務(wù)，從而提高無(wú)線資源的利用率。</p><p>　　2 GPRS的相關(guān)技術(shù)</p><p>　　雖然GSM已經(jīng)提供了數(shù)據(jù)服務(wù)，不過(guò)由于使用電路交換數(shù)據(jù)通道接口，它們?nèi)允艿娇罩性试S最大比特率14.4千比特/秒的限制。出于這樣的原因，GSM繼續(xù)其自然發(fā)展演變以滿足更高的比特率要求。為了使每個(gè)用戶(hù)更好的分配時(shí)隙從而提高利用率，高速電路交換數(shù)據(jù)成為了一個(gè)解決途徑。但是它

60、仍然不足以應(yīng)對(duì)像Web瀏覽這樣的突發(fā)數(shù)據(jù)應(yīng)用。除此之外，HSCSD依靠電路交換技術(shù)吸引了一部分的用戶(hù)，他們希望收取的費(fèi)用以數(shù)據(jù)流量為準(zhǔn)而非實(shí)際持續(xù)連接的時(shí)間。而另一方面，服務(wù)提供商則需要更有效的手段在越來(lái)越多的用戶(hù)之間分享越來(lái)越少的無(wú)線資源。在一個(gè)電路交換模式下，一個(gè)通道只被分配用于記錄一個(gè)用戶(hù)的連接時(shí)間。這種對(duì)無(wú)線電資源的一對(duì)一訪問(wèn)在數(shù)據(jù)包交換技術(shù)中并非是必需的。</p><p>　　GPRS以一個(gè)非常重大的發(fā)

61、展而在GSM標(biāo)準(zhǔn)中凸顯出其地位來(lái)，這就體現(xiàn)在分組交換技術(shù)上，為更多的移動(dòng)用戶(hù)提供突發(fā)數(shù)據(jù)需要傳輸?shù)母弑忍芈省＠碚撋蟻?lái)說(shuō)，GPRS用戶(hù)可以同時(shí)使用幾個(gè)時(shí)段，并且可以達(dá)到約170千比特每秒的比特率。由于發(fā)送或接受數(shù)據(jù)包時(shí)渠道被分配給相應(yīng)用戶(hù)，所以基于卷的收費(fèi)是很有可能的。因?yàn)楣?yīng)商可以使用其他用戶(hù)活動(dòng)傳輸?shù)牟罹?，所以突發(fā)數(shù)據(jù)的應(yīng)用使人們有可能更有效地平衡用戶(hù)與用戶(hù)之間的網(wǎng)絡(luò)資源。</p><p><b>　

62、　2.1 基本原則</b></p><p>　　在一個(gè)移動(dòng)網(wǎng)絡(luò)中，整個(gè)地理區(qū)域被分為很多部分，每個(gè)部分由一個(gè)基站提供服務(wù)。由于基站的發(fā)射功率較低，所以在重復(fù)使用同樣的渠道時(shí)，另一個(gè)單元并不會(huì)受到太多的干擾。對(duì)小單元的配置和選擇可以盡量減少對(duì)另一個(gè)單元的干擾，從而可以達(dá)到最大的容量。一個(gè)單元通常被描繪成一個(gè)六邊形，但在現(xiàn)實(shí)中的實(shí)際形狀則根據(jù)不同的地理環(huán)境和無(wú)線電傳播而稍有不同。我們通?；谟脩?hù)的密度來(lái)進(jìn)

63、行信道的分配。如果一個(gè)單元要服務(wù)很多的用戶(hù)，通常就會(huì)有更多的頻道分配。這些渠道就重復(fù)使用相鄰的單元或者單元群。在具有相同廣播頻道但被空間分離的單元中，較低的發(fā)射功率和天線的方向使同頻道干擾保持在一個(gè)可接受的水平內(nèi)。</p><p>　　可移動(dòng)性是無(wú)線通信系統(tǒng)最關(guān)鍵的特征之一。有時(shí)我們需要追蹤用戶(hù)的一些應(yīng)用，如在不同單元間轉(zhuǎn)換以及改變無(wú)線電通道。一個(gè)移動(dòng)在不同單元內(nèi)轉(zhuǎn)換通道被稱(chēng)作切換。運(yùn)用信息和呼叫處理的程序來(lái)支持

64、用戶(hù)移動(dòng)性和切換使得移動(dòng)電話能夠被順利完成。分頁(yè)是移動(dòng)系統(tǒng)中另外一個(gè)非常重要的特征。通常我們?cè)诜?wù)范圍內(nèi)使用一個(gè)共享信道查找用戶(hù)并且廣播一些信號(hào)消息。</p><p>　　2.2 多址接入技術(shù)</p><p>　　多址接入是一種允許用戶(hù)共享通信媒介的技術(shù)，這樣一來(lái)系統(tǒng)整體的能力就可以被增強(qiáng)。一般有三種常用的多址接入方案：FDMA（頻多分址），TDMA（時(shí)多分址），CDMA（碼多分址）。在F

65、DMA中，每個(gè)呼叫都為它自己的通話時(shí)間分配了其相應(yīng)的頻率。整個(gè)頻帶被分成了很多小的個(gè)人渠道提供給用戶(hù)訪問(wèn)。在TDMA中，用戶(hù)共享同一頻段的頻率。每個(gè)呼叫被分配一個(gè)不同的傳輸時(shí)隙。在CDMA中，用戶(hù)共享同一頻段的頻率和時(shí)間。每個(gè)呼叫被分配一個(gè)唯一的代碼，它可以傳播到整個(gè)頻譜的頻段。這些擴(kuò)頻電話被同時(shí)發(fā)送出去，并且有一個(gè)獨(dú)特的逆運(yùn)算碼接收機(jī)分離開(kāi)。由三個(gè)多址接入方案合成的方法也可以被運(yùn)用。</p><p><b

66、>　　2.3 功率控制</b></p><p>　　功率控制是無(wú)限通信中最重要的設(shè)計(jì)功能之一，包括了FDMA，TDMA，和CDMA。它確保了用戶(hù)以適當(dāng)?shù)哪芰克絹?lái)發(fā)送和接受數(shù)據(jù)從而成功傳達(dá)信息，同時(shí)減少對(duì)其他用戶(hù)的干擾。</p><p>　　由于同頻道干擾管理的存在，功率控制在FDMA和TDMA系統(tǒng)中是不可或缺的。造成這種類(lèi)型的干擾的原因是對(duì)有限的可用頻譜的頻率復(fù)用。通過(guò)

67、適當(dāng)?shù)墓β孰娖降恼{(diào)整，同頻干擾可以相應(yīng)減少。這就產(chǎn)生了一個(gè)更好的頻率復(fù)用因子，從而增加了系統(tǒng)的容量。功率控制是CDMA系統(tǒng)中最重要的需求。如果沒(méi)有功率控制，所有具有相同功率的手機(jī)都會(huì)不考慮路徑損耗和衰減效應(yīng)而被傳送到基站。越靠近基站的手機(jī)就會(huì)對(duì)基站產(chǎn)生更大的信息干擾。這種效果就是所謂的近/遠(yuǎn)影響。因此，一個(gè)設(shè)計(jì)良好的功率控制算法對(duì)于一個(gè)CDMA系統(tǒng)的正常運(yùn)作起了至關(guān)重要的作用。在功率控制的情況下，系統(tǒng)的容量相比其他系統(tǒng)是非常低的。<

68、;/p><p>　　功率控制的另外一個(gè)優(yōu)點(diǎn)是它可以延長(zhǎng)使用最低功率傳輸?shù)碾姵貕勖７聪蜴溌饭β士刂圃诮?遠(yuǎn)效果的影響下比前向鏈路更加嚴(yán)格。在一個(gè)前向鏈路中，為了減少小單元之間的干擾，功率控制仍是必要的。</p><p>　　功率控制可以以集中或者分布的形式工作。中央控制器接收所有渠道的連接信息，并控制其傳輸功率水平。集中式的方法可以?xún)?yōu)化整個(gè)網(wǎng)絡(luò)或網(wǎng)絡(luò)的一部分，因此用電量是非常高效的。這需要在網(wǎng)

69、絡(luò)中廣泛地控制信號(hào)，然而在實(shí)際中則難以應(yīng)用。</p><p><b>　　3 GPRS的架構(gòu)</b></p><p>　　GPRS被視為GSM的傳送功能。它是由ETSI設(shè)計(jì)的，在不干擾現(xiàn)有服務(wù)的情況下實(shí)施了現(xiàn)有的GSM的基礎(chǔ)設(shè)施。這樣做的目的是在對(duì)現(xiàn)有的GSM PLMN影響最小的情況下加快GPRS的部署。圖1說(shuō)明了支持GPRS的GSM網(wǎng)絡(luò)的邏輯結(jié)構(gòu)。</p>

70、;<p>　　圖1 GPRS網(wǎng)絡(luò)的體系結(jié)構(gòu)</p><p><b>　　3.1 移動(dòng)站</b></p><p>　　GPRS和GSM系統(tǒng)在用戶(hù)之間提供了跨工作和動(dòng)態(tài)共享資源的服務(wù)。出于這樣的原因，三個(gè)類(lèi)型的終端已經(jīng)被確定：A類(lèi)的MS可以攜帶一種電路交換和一種分組交換連接，在不中斷數(shù)據(jù)傳輸?shù)耐瑫r(shí)發(fā)起或接受語(yǔ)音呼叫。這種類(lèi)型的終端由于其性能復(fù)雜成本高，

71、可能無(wú)法使用GPRS的最初部署。B類(lèi)的MS是能夠在同一時(shí)間連接到GSM和GPRS，但一接到語(yǔ)音電話時(shí)，GPRS數(shù)據(jù)交換可以為語(yǔ)音呼叫而暫停。GPRS數(shù)據(jù)交換可在語(yǔ)音通話結(jié)束后恢復(fù)使用。最后，一個(gè)C類(lèi)的MS允許用戶(hù)在一個(gè)特定的時(shí)間以一種特定方式訪問(wèn)。</p><p>　　GPRS的質(zhì)譜有兩個(gè)組成部分：一是MT（移動(dòng)終端），它就是通常作為無(wú)線調(diào)制解調(diào)器用于訪問(wèn)無(wú)限接口的手機(jī)；二是TE（終端設(shè)備），通常是一個(gè)筆記本電腦

72、或一個(gè)PDA（個(gè)人數(shù)字助理）。GPRS的MS也可作為將MT和TE相結(jié)合的單元。</p><p><b>　　3.2 基站子系統(tǒng)</b></p><p>　　GPRS對(duì)于現(xiàn)有的GSM BSS只有很小的影響，這樣不用進(jìn)行大型的修改的時(shí)候就可以重用現(xiàn)有的組件和鏈接。這是由可行性的，因?yàn)镚PRS就和GSM一樣采用同樣的頻段，同樣的TDMA幀結(jié)構(gòu)和突發(fā)結(jié)構(gòu)。</p>

73、<p>　　一個(gè)叫PCU（分組控制單元）的新的功能組件被添加到GPRS標(biāo)準(zhǔn)中的BSS里去支持?jǐn)?shù)據(jù)包的處理。PCU被邏輯地放置在BSS和GPRS NSS中。與語(yǔ)音電路連接不同的是，只有當(dāng)數(shù)據(jù)需要通過(guò)空中接口傳輸?shù)臅r(shí)候，GPRS連接必須建立和釋放在BSS和MS之間。因此，歐洲電信標(biāo)準(zhǔn)協(xié)會(huì)定義了新的程序去適應(yīng)這種連接方式。</p><p>　　3.3 網(wǎng)絡(luò)交換子系統(tǒng)</p><p>

74、;　　GPRS NSS可以被看做是確保移動(dòng)用戶(hù)和數(shù)據(jù)網(wǎng)絡(luò)之間連接的覆蓋網(wǎng)絡(luò)。GPRS在GSM基礎(chǔ)設(shè)施中加入了一個(gè)新的功能元素：GSN（GPRS支持節(jié)點(diǎn)），它可以是一個(gè)SGSN或者是一個(gè)GGSN。這在GSM網(wǎng)絡(luò)中用于支持分組數(shù)據(jù)服務(wù)是非常必要的。網(wǎng)絡(luò)一般被分為幾個(gè)由獨(dú)立SGSN控制的服務(wù)地區(qū)。在某一特定時(shí)間內(nèi)，在其服務(wù)范圍內(nèi)只有一個(gè)SGSN服務(wù)一個(gè)MS。SGSN的主要職責(zé)就是追蹤它所服務(wù)的MS，并且負(fù)責(zé)數(shù)據(jù)訪問(wèn)控制服務(wù)。在另一方面，GGS

75、N提供連接到外部PDN的接口。SGSN通過(guò)幀與BSS連接，并且通過(guò)GPRS骨干網(wǎng)與幾個(gè)GGSN連接。HLR數(shù)據(jù)更新用于覆蓋GPRS的用戶(hù)信息。改變現(xiàn)有的MSC/VLR并不是必須的，但GPRS標(biāo)準(zhǔn)提出一些SGSN和MSC之間的協(xié)調(diào)改進(jìn)建議和兩個(gè)接口之間的選擇則是可以被支持的。</p><p>　　在GPRS中引入了幾個(gè)接口用于定義實(shí)體之間的相互作用。例如，在BSC和SGSN之間需要Gb接口。</p>