xDSL -- A family of technologies for providing high speed digital phone lines from Telephone Company Central Offices to users over copper wires.

HDSL - High Bit Rate DSL. Delivers 768K Bits Per Second over distances of up to 3700 meters without repeaters. Paired xDSL lines are often used to provide T1 Line Service. Costs and power needs are lower than those of ADSL.

ADSL - Asymmetric DSL Provides download speeds 9 MegaBytes per Second and Upload Speeds of 640K Bits per Second over distances of 2600 meters. At 3700 meters, speeds are down to 6.3 MegaBytes/Sec, At 4600 meters, 2 Megabytes/sec and at 5200 meters, 1.5MegaBytes/Sec (64KBits/Sec Uplink). Most US users (80%) are within 5.2km of a Central Office. ADSL is more expensive and requires more power than HDSL. Nonetheless, the telephone companies are touting it as the competition to Cable modems and are expected to sell it very cheaply in order to compete with the cable companies.

RADSL -- Rate Adaptable DSL is ADSL with modems that adjust their transmission speeds to line conditions. Its primary utility seems to be to ensure maximum possible rates over ADSL lines. RADSL can be used to provide ADSL links -- albeit slow ADSL links -- beyond 5.2 Kilometers.

SDSL -- Single Line DSL is a DSL variant that allows 1.5MegaBits/Sec in both directions for distances up to 2500 meters.

VDSL -- Very High Rate DSL. An emerging technology that will probably deliver 25 MegaBytes/Sec downlink and 3MegaBytes/Sec Uplink over lines up to 300 meters.

xDSL references: http://www.telechoice.com/xdslnewz/ not evaluated

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