宽带接入—“最后一公里”技术.doc

BROADBAND ACCESS“LAST KILOMETRE”TECHNOLOGY Although broadband network techniques have been succeeded both in WAN and in LAN，a bottle neck of communication still exists between telecommunication offices and subscribersThis is so called“Last Kilometre”high-speed telecommunicationCause of the bottleneck is that the broadband networks almost use optical fiber as transmission medium while the users use copper wire and the later cannot transmit high-bandwidth signalsDiscussions about what to solve this problem tend to focus on two solutions1The straightforward onerunning a fiber from the end office into everyones house is called FTTH（fiber to the home）This solution fits in well with the current system but will not be economically feasible for decadesIt is simply too expensive The second solution that is much cheaper is FTTC ( fiher to the curb)In this model，the telephone company runs an optical fiber from each end office into each neighborhood（the curb）that is servesThe fiber is terminated in a junction box that all the local loops enterSince the local loops are now much shorter（perhaps 100 meters instead of 3 km），they can be run at higher speeds，probably around 1 Mbps，which is just enough for compressed videoThis design is shown Fig. 15-1In this manner，multiple videos（or other information channels）can pour down the fiber at high speed and be split over the twisted pairs at the end2By sharing a 1-Gbps fiber over 100 to 1000 customers，the cost per customer can be reduced，and considerably higher bandwidth can be provided than nowA typical application of this FTTC is ADSL（asymmetrical digital subscriber line）It is emerging as the technology for home-and small-office Internet connectivityIt provides either 15 Mb/s or 6 Mb/s from the network to the user and 64 Kb/s or 640 Kb/s in the reverse direction depending on the distance，12 000 or 18 000 feet 3The different speed for each direction gives it the asymmetric labelADSL is designed to take advantage of the fact that video-on demand，telecommuting，and Internet access traffic are inherently asymmetricalThe user sends a brief message up to the network and receives a ton of data coming back，either a movie or a piece of data downloadUnder such a scenario，low-speed traffic to the network is just fineADSL delivers high bandwidth where you need it and only uses a single copper pairThrough multiplexing，it also reserves 4 kHz of the bandwidth for POTSHowever，if you need symmetrical traffic，which you may need in a campus setting or with videoconferencing，ADSL wont be satisfactory4Besides ADSL there are another types of FTTC using the telephone networkThey are SDSL（symmetrical DSL），HDSL（high bit rate DSL），VDSL（very high DSL），and RADSL（rate adaptive DSL），and these DSL variations can be represented by xDSL。Another kind of FTTC that is designed to use existing cable TV infrastructure is shown in Fig. 15-2 Here a multidrop cable is used instead of the point-to-point system characteristic of the telephone systemIt is likely that both Fig. 15-1 And Fig15-2 will coexist in the future，as telephone companies and cable TV operators become direct competitors for voice，data， and possibly even television service5We usually call this access technology HFC（hybrid fiber coaxial）It has been developed by the cable TV industry to provide two-way，high-speed data access to the home using a combination of fiber optics and traditional coaxial cableNOTES1 由about引出的介词短语作定语，修饰discussions。2 并列谓语，主语multiple videos；最后的end是指junction box。3 12 000 or 18 000 feet为同位语，说明作用的距离不同，提供的传输速率不同。4 if引出的是条件状语从句，并由which引出的非限定性定语从句修饰；campus setting为校园环境，可译为校园网。5 It为先行代词，that引出的为主语从句。KEYWORDSbroadband access宽带接入broadband network宽带网络WAN（wide area network）广域网LAN（local area network）局域网bottle neck瓶颈optical fiber光纤bandwidth带宽end office终端局，末级局FTTH（fiber to the home）光缆到家庭local loop本地回路Junction box分线盒compressed video压缩视频ADSL（asymmetrical digital subscriber line）非对称数字用户线路multiplexing多路复用POTS（plain old telephone services）普通老式电话业务SDSL（Symmetrical DSL）对称数字用户线路HDSL（high bit rate DSL）高比特率数字用户线路VDSL（very high DSL）甚高数据传输率数字用户线路RADSL（rate adaptive DSL）速率自适应数字用户线路HFC（hybrid fiber coaxial）混合同轴光缆 翻译:宽带接入“最后一公里”技术 虽然宽带网络技术在广域网和局域网中已经很成功，但在电信局和用户之间始终存在着通信瓶颈所谓的“最后一公里”高速通信问题。瓶颈的产生是因为宽带网大多使用光纤作为通信介质，而用户则使用铜线，后者不能传输大带宽信号。 解决这一问题有两种倾向性方案。直接的方法是将光缆从端局延伸到每家每户，这叫做光缆到家庭（FTTH）。这种方案能很好地满足当前系统的要求，但在近几十年内都是不经济的，它太昂贵。 另外一种比较便宜的方案是光缆到路边（FTTC）在这种方式中，电话公司将其光缆从每一端局延伸到要服务的小区（路边）。光缆在分线盒处终止，从分线盒接入所有本地环路。由于本地环路比较短（大约l00m而不是3krn），因而传输速率较高，大约1Mb/s，这对压缩了的视频信号刚好够用。这种方案如图15-1所示。 在这种方法中，多路视频或其他信息信道可以高速传向光缆并在分线盒处分配给多对双绞线。由于100-1000个用户共享lGb/s光缆信号，故每一用户的费用就能降下来，并且今后能提供更大的带宽。FTTC的一种典型应用是非对称数字用户线路（ADSL）。 ADSL正在作为家庭和小型办公室的Internet连接技术出现。依据距离的不同，对于12 000英尺或18 000英尺，它提供了从网络到用户的1. 5Mb/s或6Mb/s和从用户到网络的64Kb/s或640Kb/s的高速连接。在两个方向有不同速度使它得到了不对称的称谓。ADSL的设计利用了这样一个事实：视频点播、在家工作和Internet信息访问等应用本来就是不对称的。用户向网络发送一个简短的报文，而收到大量的数据：或是一部电影或是数据下载。在这样的情况下，向网络发低速信息流是没问题的。 ADSL在用户需要之处给出了大带宽，并且只用一对铜线。通过多路复用，它还能为打普通电话保留4kHz的带宽。但如果需要对称的信息流（在校园网或电视会议应用中需要这种对称信息流）ADSL就不能令人满意。 除了ADSL以外，使用电话网的FTTC还有其他一些类型。它们是对称SDSL、高比特率HDSL、甚高数据传输率VDSL和速率自适应RADSL，这些DSL的变种都可用xDSL表示。 光缆到路边（FTTC）的另一种设计方案是使用现有的有线电视基础设施，如图15-2所示。这里使用的是多点电缆，而不是电话系统中的点对点系统。如图15-1与图15-2所示的这两种系统今后有可能共同存在，因为电话公司与有线电视将成为在语音、数据，甚至是电视业务上的直接竞争者。 通常把这种接入技术成为混合同轴光缆技术，它是由有线电视行业开发的，将光缆和传统的同轴电缆相结合，同家庭提供双向、高速数据接入。 EXERCISES1Fill in the blanks with appropriate words or phrases（1）still exists between the telecommunication offices and Internet users（2）Broadband networks usually useas transmission medium（3）To solve so called“Last Kilometer”problem we have two solutions，one is FTTH， another is （4）A typical application of the FTTC that a telephone subscriber uses is（5）will coexist in the future（6）A variety of DSL can be represented by（7）HFC networks use technology of（8）HDSL is abbreviated from（9）The access traffic in ADSL system are inherently（10）There is abetween fiber and copper cable TV wire aADSL and HFC bhigh bit rate DSL cA bottleneck djunction box eFTTC fasymmetrical gADSL hxDSL ioptical fiber jcable TV network2True / False（1）Broadband network techniques have been