nsive Web data need lots of downstream bandwidth, but user requests and responses are small and require little upstream bandwidth. Using ADSL, up to 6.1 megabits per second of data can be sent downstream and up to 640 Kbps upstream. The high downstream bandwidth means that your telephone line will be able to bring motion video, audio, and 3-D images to your computer or hooked-in TV set. In addition, a small portion of the downstream bandwidth can be devoted to voice rather data, and you can hold phone conversations without requiring a separate line. The original intent of x.DSL was to compete with CATV companies in delivering video on demand content to their subscribers. Although ADSL can realistically deliver near-broadcast quality at 300Kbps data rates and best viewed at a 1/3 of the screen, it is still not comparable to CATV. Applications such as instructional videos will work well via ADSL because of low motion involved. Marginal full screen video, for video on demand purposes is much more desirable. It is currently in trial via RADSL (rate-adaptive DSL) with Pacific Bell and US west, requiring at least 750 Kbps for an MPEG-4 video stream. The more definite answer to directly compete with CATV is VDSL (Very High data rate DSL). VDSL promises between 51 and 55 Mbps over lines up to 1000 feet in length. Downstream rates range from 13Mbs to 30Mbps. It is based on fiber-to-the-curb architecture (FTTC/N) and is more expensive to deploy. Currently in limited trial deployment by US West and NEXT Level in Phoenix,AZ provides True broadcast quality video. The Phoenix system, featuring NEXT Level supplied servers, switches, and CPE, serves clusters of 16 to 32 users whose homes are fed by fiber from local switches. The system easily supports 265 channels of video per home. Thousands of channels of programming could easily be delivered over a VDSL network, allowing any school, church, community group or commercial programmer to distribu...
