外文翻譯---寬帶接入技術(shù)

1、<p>　　附錄1 外文文獻(xiàn)譯文</p><p><b>　　寬帶接入技術(shù)</b></p><p>　　寬帶接入技術(shù)是在電信和視頻網(wǎng)絡(luò)行業(yè)的催生影響下產(chǎn)生出來的，其應(yīng)用主要體現(xiàn)在物理層。相反，數(shù)據(jù)網(wǎng)絡(luò)行業(yè)的關(guān)注點(diǎn)主要集中在網(wǎng)絡(luò)層和運(yùn)輸層上（例如IP電話）。形成這種應(yīng)用方法上的區(qū)別的理由是，一旦傳輸?shù)膬?nèi)容被數(shù)字化，就能作為數(shù)據(jù)進(jìn)行處理。換句話說，從網(wǎng)絡(luò)的觀點(diǎn)

2、來看，網(wǎng)絡(luò)只是以數(shù)字的形式來傳輸數(shù)據(jù)。在應(yīng)用層上，這些數(shù)據(jù)位不論實(shí)際代表話音、數(shù)據(jù)，還是代表視頻流都沒有區(qū)別。因此，數(shù)據(jù)網(wǎng)絡(luò)行業(yè)在讓數(shù)據(jù)流適應(yīng)跨網(wǎng)傳輸這方面并沒有做什么。另外一方面，電信和視頻網(wǎng)絡(luò)行業(yè)必須解決利用現(xiàn)有接入基礎(chǔ)設(shè)施將內(nèi)容以分組數(shù)據(jù)傳輸?shù)姆椒ㄟM(jìn)行傳輸。這成為在開發(fā)下述幾類寬帶接入技術(shù)中的重要催化劑。</p><p>　　銅回路接入技術(shù)，又稱為數(shù)字用戶線（DSL）技術(shù)。DSL技術(shù)又統(tǒng)稱XDSL。XDSL

3、是由電信行業(yè)開發(fā)的，利用了世界范圍內(nèi)的幾百萬英里長(zhǎng)的現(xiàn)有銅回路通信設(shè)施。</p><p>　　光纖/同軸電纜的電纜接入技術(shù)。電纜接入是由視頻網(wǎng)絡(luò)行業(yè)中的有線電視部分開發(fā)的，利用了已經(jīng)連接全世界幾百萬個(gè)（大多數(shù)是常駐的）用戶的視頻信道電纜基礎(chǔ)設(shè)施。</p><p>　　在線衛(wèi)星接入技術(shù)。衛(wèi)星接入是由視頻網(wǎng)絡(luò)行業(yè)中的無線部門開發(fā)的，通過使用衛(wèi)星基礎(chǔ)設(shè)施，經(jīng)視頻信道接到有線電視中心終端局并傳送到

4、世界范圍內(nèi)的幾百用戶（大多數(shù)是常駐的）。</p><p>　　非對(duì)稱數(shù)字用戶線（ADSL）</p><p>　　20世紀(jì)80年代，ADSL作為電信行業(yè)為應(yīng)對(duì)電纜行業(yè)支持視頻點(diǎn)播需求而提供的一種解決方案，首先被開發(fā)了出來。然而直到20世紀(jì)90年代中期才認(rèn)識(shí)到，它也可以作為能訪問像因特網(wǎng)這樣的高速業(yè)務(wù)的技術(shù)。ADSL提供非對(duì)稱傳輸，典型的下傳速率可達(dá)到9Mb/s (從中心局到設(shè)備），上傳速率為

5、16Kb/s到640Kb/s （從設(shè)備到中心局）。像所有的銅線傳輸系統(tǒng)一樣，速率越高，傳輸范圍越短。</p><p>　　混合光纖同軸網(wǎng)技術(shù)（HFC）</p><p>　　混合光纖同軸網(wǎng)技術(shù)是基于現(xiàn)有的有線電視（有線TV或CATV）技術(shù)的。最初，有線電視是基于從多系統(tǒng)運(yùn)作員（MSO）到客戶設(shè)備的同軸電纜設(shè)施的，并采用樹形拓?fù)?。這些系統(tǒng)大多數(shù)已升級(jí)到HFC。在這種系統(tǒng)中，信號(hào)通過一對(duì)光纜被傳

6、送到一個(gè)光纜節(jié)點(diǎn)上，然后再經(jīng)過同軸電纜分發(fā)給客戶。在中心終端局上，各種源信號(hào)，如傳統(tǒng)的衛(wèi)星業(yè)務(wù)、使用廣域網(wǎng)的模擬和數(shù)字業(yè)務(wù)，以及使用專用主干網(wǎng)的因特網(wǎng)服務(wù)提供商（ISP）的業(yè)務(wù)被復(fù)用，并從電信號(hào)射頻轉(zhuǎn)換成為光信號(hào)。光纜上的通信是單向的：從中心終端局到光纜節(jié)點(diǎn)的每一對(duì)光纜，都以不同的方向承載單向業(yè)務(wù)。光信號(hào)在光纜節(jié)點(diǎn)處轉(zhuǎn)換成射頻信號(hào)并以雙工方式沿同軸電纜傳送。從中心終端局到客戶設(shè)備的信號(hào)稱為下傳信號(hào)或者前向通路信號(hào)。從客戶設(shè)備到中心終端局

7、的信號(hào)稱為上傳信號(hào)或者反向通路信號(hào)。</p><p><b>　　光纖通信介紹</b></p><p>　　80年代一項(xiàng)最最重要的技術(shù)發(fā)展是光纖通信成為一個(gè)主要的國(guó)際性產(chǎn)業(yè)。用光纖敷設(shè)總長(zhǎng)度可以表明其發(fā)展程序。據(jù)估計(jì)，到1987年底僅美國(guó)的光纖敷設(shè)總長(zhǎng)將達(dá)320萬公里，其中90% 以上是在1982-1987年間敷設(shè)并開通的，而長(zhǎng)度干線占主導(dǎo)地位，數(shù)量約為光纖總長(zhǎng)的95

8、% 。</p><p>　　雖然現(xiàn)在人們對(duì)于纖維光學(xué)的興趣主要在于通信，但早期發(fā)展纖維光學(xué)的目的并不在此。50年代初研究人員制造出第一根具有包層玻璃光纖時(shí)，并不想用于通信而是想用它們傳送內(nèi)窺鏡需要的成像光束。1966年Kao和Hockham發(fā)表了那篇著名的論文，建議將低損耗光纖用于通信，此時(shí)纖維光學(xué)已發(fā)展為一項(xiàng)很實(shí)用的技術(shù)了。</p><p>　　1970年10月，第一根低損耗（20dB/

9、km）石英光纖問世了。有時(shí)將這一日期作為光纖通信時(shí)代的開端。雖然這一成果當(dāng)時(shí)在研究領(lǐng)域確實(shí)引起了極大的關(guān)注，但這種光纖距離通信所要求的條件還相差甚遠(yuǎn)：每千米20dB的損耗對(duì)于長(zhǎng)途通信系統(tǒng)仍然是太大了；光纖易斷裂，必須尋找保護(hù)方法；沒有合適的光源。研究人員不知道光纜的終端和接頭是否會(huì)發(fā)展到實(shí)用階段，至于生產(chǎn)這些器件是否在經(jīng)濟(jì)上可行，從而使之在市場(chǎng)上占有重要地位，他們更是存有嚴(yán)重的疑慮。</p><p>　　雖然技術(shù)

10、障礙好像不可逾越，但經(jīng)濟(jì)潛力卻非常明顯。正因?yàn)槿绱?，?0年代早期研究和開發(fā)工作發(fā)展迅速，一些重要問題得以解決。</p><p>　　70年代中后期，由于發(fā)展重點(diǎn)由研究領(lǐng)域轉(zhuǎn)入工程實(shí)用，因而加速發(fā)展了適應(yīng)市場(chǎng)的產(chǎn)品。在實(shí)驗(yàn)室研制的光纖損減值接近瑞利極限值：0.8µm波長(zhǎng)處為2dB/km，1.3µm波長(zhǎng)處為0.3dB/km和1.55µm波長(zhǎng)處為0.15dB/km。通過改進(jìn)光纖外涂層方法

11、和成纜技術(shù)，克服了微彎損耗。生產(chǎn)了加強(qiáng)型光和多纖連接器用于室外作業(yè)。工作在0.8-0.85µm波長(zhǎng)區(qū)的商用鎵鋁砷激光器的室溫閾值電流減少到20-30mA范圍。據(jù)稱，激光器和發(fā)光管的設(shè)計(jì)壽命達(dá)10萬到100萬小時(shí)。開發(fā)了工作于1.3µm波長(zhǎng)附近的光源和改進(jìn)的光電檢測(cè)器，從而可以利用光纖在長(zhǎng)波長(zhǎng)區(qū)的低損耗和低色散特性。這一時(shí)期進(jìn)行的室外實(shí)驗(yàn)較重要的有AT&T于1976年在亞特蘭大的實(shí)驗(yàn)，1977年在芝加哥的實(shí)驗(yàn)和

12、1977年日本的用戶通路項(xiàng)目。</p><p>　　到了80年代，光纖器件在性能、價(jià)格和可靠性方面的改善使眾多電話公司受益匪淺。光纖很快成為長(zhǎng)途干線的首選傳輸媒質(zhì)。一些早期敷設(shè)的光纜線路采用0.8µm光源和漸變折射率多模光纖，但到1983年，城市間線路的設(shè)計(jì)者們就考慮使用1.3µm單模光纖系統(tǒng)了。單模光纖與1.3µm激光器相連，可以提供寬帶特性，增加了高速率系統(tǒng)的中繼距離。最近敷設(shè)的

13、光纖系統(tǒng)的數(shù)據(jù)速率已移至每秒幾比特范圍。這種系統(tǒng)采用光譜純的分布反饋激光器，將光纖色散效應(yīng)減至最小。在1.55µm波長(zhǎng)上設(shè)計(jì)的低色散光纖，相應(yīng)地具有低損耗特性，目前廣泛用于長(zhǎng)途通信。為進(jìn)一步增加光纖的信息容量，逐漸廣泛采用波分復(fù)用方法。</p><p>　　人們對(duì)于光纖在其他領(lǐng)域的潛力剛剛開始認(rèn)識(shí)。用于計(jì)算機(jī)系統(tǒng)和辦公室的光纖網(wǎng)絡(luò)逐漸變得更加重要。在電話系統(tǒng)中，光纖在主要城市地區(qū)中心交換局間互聯(lián)和低級(jí)交

14、換中的使用繼續(xù)迅速增加。入戶光纜已經(jīng)有了示范工程。許多觀察家相信，全國(guó)電話系統(tǒng)將使用光纖傳輸視頻寬帶信號(hào)而逐漸升級(jí)。這些寬帶用戶環(huán)路系統(tǒng)將為可視電話、視頻娛樂節(jié)目等業(yè)務(wù)提供通路。寬帶業(yè)務(wù)廣泛使用光纖將會(huì)變得經(jīng)濟(jì)可行。</p><p><b>　　異步轉(zhuǎn)移模式</b></p><p>　　ATM（異步轉(zhuǎn)移模式）既是復(fù)用技術(shù)又是交換技術(shù)。最初，人們是想用ATM來處理高比特

15、率的數(shù)字信號(hào)，事實(shí)卻證明它是一種通用技術(shù)，可以用來傳輸和交換任何類型并具有各種比特率的數(shù)字化信息。</p><p>　　無論傳輸?shù)男畔⑹鞘裁?，ATM都以稱作“ 信元”的短的分組采傳送信息。信元是由固定的48字節(jié)加上5字節(jié)的信頭組成。信元尋找路由是基于帶有雙重識(shí)別的邏輯信道。</p><p>　　ATM既與電路方式有關(guān)又與分組方式有關(guān)。由于使用簡(jiǎn)單的協(xié)議，信元至網(wǎng)絡(luò)節(jié)點(diǎn)的轉(zhuǎn)移可完全由硬件處理

16、完成，這就縮短了轉(zhuǎn)送時(shí)間，提高了傳輸路徑的速率，使比特速率甚至可以達(dá)到每秒幾百兆比特。另一方面，ATM保留了分組方式所有的靈活性：只傳送所需要的信息，提供簡(jiǎn)單、獨(dú)特的復(fù)用方法而不管不同信息流的比特率，并且允許比特流的變化。</p><p>　　ATM開始于80年代初，那時(shí)人們?cè)噲D找到一種更適合的技術(shù)用于交換超過100Mbit/s的高比特率的信道。1988年，ITU批準(zhǔn)了I .121建議，該建議選擇ATM作為用于各

17、種類型信息寬帶網(wǎng)絡(luò)的目標(biāo)傳送模式，其中包括如話音的低比特率的信息。1991年一些運(yùn)營(yíng)公司和廠商建立了ATM論壇以加速ATM標(biāo)準(zhǔn)化工作。現(xiàn)在ATM論壇的成員已超過600個(gè)，它對(duì)ATM 的標(biāo)準(zhǔn)化和規(guī)范化有著重要影響。第一批ATM產(chǎn)品1992年面世，主要用于局域網(wǎng)，其設(shè)計(jì)主要是用來解決計(jì)算機(jī)終端間隨計(jì)算機(jī)數(shù)量和功能不斷增加所帶來的共享同一承載電路的問題。</p><p>　　ATM網(wǎng)絡(luò)可以近似在看作是由三個(gè)覆蓋功能層組

18、成：業(yè)務(wù)和應(yīng)用層、ATM網(wǎng)絡(luò)層和傳輸層。應(yīng)用層提供端到端的業(yè)務(wù)。應(yīng)用層使用ATM網(wǎng)絡(luò)層的邏輯連接，當(dāng)信元通過由邏輯連接（稱作虛連接）共享的傳輸鏈路時(shí)，ATM網(wǎng)絡(luò)層依次對(duì)信息流復(fù)用并尋找信息流的邏輯路由。傳輸層提供物理鏈路并處理信元的實(shí)際物理傳輸。</p><p>　　ATM網(wǎng)絡(luò)能夠傳輸和交換話音、數(shù)據(jù)和視頻業(yè)務(wù)，從接入的角度看這些業(yè)務(wù)使用傳統(tǒng)的數(shù)字接口并具有同</p><p>　　樣的服務(wù)

19、質(zhì)量。這就意味著任何兩個(gè)終端間的物理連接都可由等效的邏輯連接代替，邏輯連接可在共用的傳輸。</p><p>　　鏈路中與其他的邏輯連接復(fù)用。資源可在所有連接中動(dòng)態(tài)共享</p><p>　　與同步時(shí)分復(fù)用技術(shù)相比，同步復(fù)用技術(shù)僵硬地將業(yè)務(wù)與傳輸資源相連，而異步技術(shù)的優(yōu)勢(shì)是根據(jù)其確切的需要來占用傳輸鏈路。</p><p>　　ATM技術(shù)將網(wǎng)絡(luò)傳輸?shù)膽?yīng)用和業(yè)務(wù)與所使用的傳

20、輸資源完全分開。構(gòu)成虛網(wǎng)絡(luò)的能力意味著物理網(wǎng)絡(luò)可以由許多用戶動(dòng)態(tài)實(shí)時(shí)地共享，因而使網(wǎng)絡(luò)結(jié)構(gòu)得到低價(jià)高效的使用，對(duì)高比特率業(yè)務(wù)也一樣。對(duì)所有網(wǎng)絡(luò)層的投資都是適應(yīng)未來需要的，因?yàn)椴煌膽?yīng)用在出現(xiàn)新的需求時(shí)可及時(shí)在同一網(wǎng)絡(luò)結(jié)構(gòu)中進(jìn)行重新分配。ATM提供一種獨(dú)特的方式將傳輸不同業(yè)務(wù)的網(wǎng)絡(luò)協(xié)調(diào)成單一的物理網(wǎng)絡(luò)。</p><p>　　以上所描述的ATM優(yōu)點(diǎn)說明了為什么人們對(duì)ATM充滿熱情。</p><p&

21、gt;　　隨著數(shù)字化和圖像編碼技術(shù)的進(jìn)步，交互視頻業(yè)務(wù)和更通常的多媒體業(yè)務(wù)開始出現(xiàn)。這些業(yè)務(wù)將會(huì)對(duì)網(wǎng)絡(luò)產(chǎn)生很大的影響。今天，ATM是唯一能夠提供這些業(yè)務(wù)所需的高比特率和靈活性的傳輸?shù)募夹g(shù)。</p><p>　　ATM，遠(yuǎn)比任何其他電信技術(shù)更能滿足運(yùn)營(yíng)公司和用戶對(duì)當(dāng)前和未來業(yè)務(wù)的需求。與其他有可能在某些應(yīng)用領(lǐng)域與ATM競(jìng)爭(zhēng)的技術(shù)相比，ATM（主要是由于其通用性，無論是比特率還是傳輸?shù)男畔㈩愋停┒季哂刑厥獾膬?yōu)點(diǎn)。AT

22、M對(duì)所有比特率的信號(hào)都可提供交換功能，這一點(diǎn)特別適合于高比特率和可變化比特率信號(hào)。</p><p>　　ATM的獨(dú)特性將使它成為卓越的多媒體業(yè)務(wù)的自然載體，特別是對(duì)于可變比特率的視頻，并且使它成為能夠提供如視頻點(diǎn)播新業(yè)務(wù)的未來信息高速公路必不可少的一部分。在很短的時(shí)間內(nèi)，運(yùn)營(yíng)者對(duì)ATM產(chǎn)生了很大的興趣，主要是由于將連接的概念與</p><p>　　實(shí)際資源分開所引入網(wǎng)絡(luò)的靈活性和虛擬性。這

23、就簡(jiǎn)化了網(wǎng)絡(luò)的管理功能</p><p>　　并能最佳地使用網(wǎng)絡(luò)資源，特別是通過統(tǒng)計(jì)復(fù)用和建立虛擬專用網(wǎng)絡(luò)。</p><p>　　當(dāng)然，在ATM技術(shù)普遍使用之前仍會(huì)有很長(zhǎng)的路要走，但是這場(chǎng)正在進(jìn)行的技術(shù)革命將會(huì)深刻在影響數(shù)據(jù)處理和視頻處理，影響電信世界。這一革命所產(chǎn)生的影響無疑會(huì)比在模擬網(wǎng)絡(luò)中出現(xiàn)數(shù)字技術(shù)的影響要大得多。</p><p>　　附錄2 外文文獻(xiàn)原文&l

24、t;/p><p>　　BROADBAND ACCESS TECHNOLOGIES</p><p>　　Broadband access technologies have been spawned by the efforts of the telecommunications and video networking industries to move toward convergence

25、 at the physical layer. By contrast, the efforts of the data networking industry toward convergence have generally focused on the networking and transport layers (e.g., IP telephony). The reason for this difference in ap

26、proach is that, once content has been digitized, it is treated as data. In other words，from a networking perspective, the network mer</p><p>　　Copper-loop access technologies, also known as digital subscribe

27、r line (DSL) technologies. DSL technologies are collectively referred to as XDSL.XDSL was developed by the telecommunications industry to make use of the several million miles of existing copper loop infrastructure aroun

28、d the world.</p><p>　　Cable access technologies over fiber/coaxial cable. Cable access was developed by the cable TV portion of the video networking industry to take advantage of the cable infrastructure tha

29、t feeds video channels to several million (mostly residential) subscribers around the world.</p><p>　　Satellite access technologies over wireless medium. Satellite access was developed by the wireless portio

30、n of the video networking industry to make use of the satellite infrastructure that feeds video channels to cable TV head end offices and to several hundred(mostly residential) subscribers around the world.</p>&l

31、t;p><b>　　ADSL</b></p><p>　　ADSL was first developed in the 1980s as the telecommunications industry's answer to the cable industry' request to support video on demand. In the mid 1990s

32、, however, it was quickly recognized as a viable technology to enable access to high-speed services such as the Internet. ADSL delivers asymmetric transmission rates typically up to 9 Mbps downstream (from the CO to the

33、premises) and 16 Kbps to 640 Kbps upstream (from the premises to the CO). Like all copper transmission systems, the higher </p><p>　　HFC Technology</p><p>　　HFC technology is based on existing c

34、able television (cable TV or CATV) technology. Originally, cable TV systems were based on coaxial cable facilities from an MSO to a customer premises and used a tree topology. Most of these systems have been upgraded to

35、HFC, by which the signal is brought to a fiber node via a pair of optical fibers and then distributed via coaxial cable to customers. At the head end, signals from various sources, such traditional satellite services, an

36、alog and digital servic</p><p>　　Introduce to Optical Fiber Communication </p><p>　　One of the most important technological developments during the 1980s has been the emergence of optical fiber

37、communication as a major international industry. One indication of the extent of this development is the total length of installed fiber, which was estimated to be 3.2 million kilometers in the U.S. alone by the end of 1

38、987. Over 90% of this fiber was placed in service during the time period of 1982~1987. Long-haul trunk installations have been dominated, accounting for about 95% of the </p><p>　　Although telecommunication

39、is the rationale for most of the current interest in fiber optics, this was not the case during the early days of the technology. The researchers who produced the first clad glass optical fibers in the early 1950s were n

40、ot thinking of using them for communications; they wanted to make imaging bundles for endoscopy. Fiber optics was already a well-established commercial technology when the famous paper by Kao and Hockham, suggesting the

41、use of low-loss optical fibers fo</p><p>　　The first low-loss (20dB/km) silica fiber was described in a publication which appeared in October of 1970. The date of this publication is sometimes cited as the b

42、eginning of the era of fiber communication. Although this development did receive considerable attention in the research community at the time, it was from inevitable that a major industry would evolve. The 20dB/km loss

43、figure was still too high for long-hall telecommunication systems. The fibers were fragile, and a way to protect the</p><p>　　Although the technological barriers appeared formidable, the economic potential w

44、as very significant. As a consequence, research and development activity expanded rapidly, and a number of important issues were resolved during the early 1970s.</p><p>　　During the middle and late 1970s, th

45、e rate of progress toward marketable products accelerated as the emphasis shifted from research to engineering. Fibers with losses approaching the Rayleigh limit of 2dB/km at a wavelength of 0.8um, 0.3dB/km at 1.3um, and

46、 0.15dB/km at 1.55um, were produced in the laboratory .Microbend loss problems were overcome through the use of improved fiber coatings and cabling techniques . Rugged cables and multiform connectors were produced for fi

47、eld installation. Rom te</p><p>　　Improvements in component performance, cost, and reliability by 1980 led to major commitments on the part of telephone companies. Fiber soon became the preferred transmissio

48、n medium for long-haul trunks. Some early installations used 1.8um light sources and graded-index multimode fiber, but by 1983, designers of intercity links were thinking in terms of 1.3um, single-mode systems. The singl

49、e-mode fiber, used in conjunction with a 1.3um laser, provides a bandwidth advantage which translates into </p><p>　　Data rates for installed fiber optic systems have recently moved into gigabit per second r

50、ange. Such systems use the spectrally pure distributed-feedback lasers to minimize fiber dispersion effects. Fibers designed for low dispersion at 1.55um wavelength, which corresponds to minimum fiber lass, are now commo

51、nly used in long distance transmission. The use of wavelength multiplexing to further increase the fiber information capacity is becoming more widespread.</p><p>　　The potential of fiber optics in other area

52、s is only beginning to be realized. Fiber optic networks for computer systems offices are becoming more prominent. In the telephone system, the use of fiber optics for interconnecting central offices within a metropolita

53、n area and for lower levels in the switching hierarchy is still increasing rapidly. Fiber links to the home have been used in demonstration projects. Many observers believe that national telephone systems will eventually

54、 be upgraded to h</p><p><b>　　ATM</b></p><p>　　ATM (Asynchronous Transfer Mode) is both a multiplexing and switching technique. It was initially intended to handle high bit rates, bu

55、t it has in fact proved to be a universal technique for transporting and switching any type of digitized information at a wide variety of bit rates.</p><p>　　ATM transfers information in short packets called

56、 “cells” with a fixed length of 48 bytes plus five header bytes, irrespective of the underlying type of transmission. Cell routing is based on the principle of logical channels with dual identification: the cell header c

57、ontains the identifier of the basic connection to which the cell belongs-called a virtual circuit (VC) and the identifier of the group of VCs to which the connection belongs-called a virtual path (VP) .</p><p&

58、gt;　　ATM is related to both circuit and packet modes. Because of the simplicity of the protocol used, the transfer of cells to the network nodes can be handled entirely by hardware, which leads to very short transit time

59、 and high usage of transmission paths, even at bit rates of several hundred megabits a second. On the other hand, ATM retains all the flexibility of the packet mode, enabling only required information to be conveyed, off

60、ering a simple, unique multiplexing method irrespective of the bi</p><p>　　ATM dated from the beginning of the 1980s: at the time , people were trying to find the most suitable technique for switching high b

61、it rate channel at more than 1000 Mbit/s . In 1988, the ITU approved recommendation 1.121 which ratified the choice of ATM as the target transfer mode for broadband networks for all types of information, including low bi

62、t rate information such as voice. In 1991 several operators and manufactures founded the ATM Forum to expedite standardization. The ATM Forum now ha</p><p>　　An ATM network can be considered, in a first appr

63、oximation, as being three overlaid functional levels: a service and applications level, an ATM network level and a transmission level. The applications provide an end-to-end service. They use the logical connections of A

64、TM network level which in turn multiplexes and logically routes the information flow as ATM cells go through the transmission links shared by logical connections called virtual connections. </p><p>　　The tra

65、nsmission level provides these physical links and handles the actual physical transport of the cells.</p><p>　　An ATM network can transport and switch voice, data and video which, seen from the access, use t

66、raditional digital interfaces with the same quality of service. This means that a physical connection between any two terminals can be replaced with an equivalent logical connection which is multiplexed with others in a

67、common transmission link. The resource is shared dynamically between all the connections.</p><p>　　Compared with the synchronous time division multiplexing techniques which rigidly link service to resource,

68、the asynchronous technique has the advantage of occupying the transmission link only in proportion to the exact requirement.</p><p>　　The ATM technique completely separates the applications and services tran

69、sported over a network from the transmission resources used. The ability to construct virtual networks means that the physical network can be shared by many users dynamically and in real time , thereby achieving cost-eff

70、ective use of infrastructure, for high bit rate services too. Investments at all levels are also future-proofed, because of the different applications can be reallocated in time over the same network infrast</p>&

71、lt;p>　　As digitization and image encoding progress, interactive video services, and more generally multimedia services, are starting to emerge. Their impact on the network will be considerable. Today, ATM is the only

72、transfer technique to offer the high bit rates and flexibility required by these services.</p><p>　　ATM, much more than any other telecommunications technique, is able to meet the current and the future requ

73、irements of both operators and users. Compared with other techniques that may compete in certain applications, ATM is special mainly due to its universal nature, both in terms of bit rate and type of information transfer

74、red. ATM offers a switching function for all bit rates and this is particularly suitable for high and variable bit rates.</p><p>　　ATM’s specific features will make it the preeminent nature vehicle for multi

75、media services, and especially for varying bit rate video, and will make it one of the essential components of future information superhighways offering new services such as video on demand. In the short term, ATM is als

76、o proving of great interests to the operators, because of the flexibility and virtuality that it can introduce into networks, by separating the concept of connection from that of physical resources. This </p><

77、p>　　Of course, there is still a long way to go before the ATM techniques is in general use , but a revolution is underway which will deeply affect the worlds of telecommunications, data processing and video. The impac