宽带接入—“最后一公里”技术(中英对照)[1]

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 subscribers．This is so called“Last Kilometre”high-speed telecommunication．Cause 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 signals．
　　Discussions about what to solve this problem tend to focus on two solutions[1]．The straightforward one—running a fiber from the end office into everyone’s house is called FTTH（fiber to the home）．This solution fits in well with the current system but will not be economically feasible for decades．It 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 serves．The fiber is terminated in a junction box that all the local loops enter．Since 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 video．This design is shown Fig. 15-1．来源：www.examda.com
　　In 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 end[2]．By 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 now．

　　A typical application of this FTTC is ADSL（asymmetrical digital subscriber line）．It is emerging as the technology for home-and small-office Internet connectivity．It provides either 1．5 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 [3]．The different speed for each direction gives it the asymmetric label．ADSL is designed to take advantage of the fact that video-on demand，telecommuting，and Internet access traffic are inherently asymmetrical．The 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 download．Under such a scenario，low-speed traffic to the network is just fine．
　　ADSL delivers high bandwidth where you need it and only uses a single copper pair．Through multiplexing，it also reserves 4 kHz of the bandwidth for POTS．However，if you need symmetrical traffic，which you may need in a campus setting or with videoconferencing，ADSL won’t be satisfactory[4]．
　　Besides ADSL there are another types of FTTC using the telephone network．They 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 system．It is likely that both Fig. 15-1 And Fig．15-2 will coexist in the future，as telephone companies and cable TV operators become direct competitors for voice，data， and possibly even television service[5]．