ADSL splits high speed data off from voice data at the telephone company switch and requires updating equipment at the Telco end of the link. User equipment must by upgraded to use the new bandwidth, but older equipment can be used without modification. Conventional phones used on ADSL do not require facility power. ADSL does not require the monumental task of rewiring telephone company local loops.
There are two (incompatible) competing ADSL modulation standards -- the ANSI standard Discrete Multitone Technology (DMT) and the cheaper Carrierless Amplitude and Phase Modulation (CAP).
ADSL was field tested in 1997. Deployment by some local carriers in the US started in 1998.
The available references don't talk much about limitations, but it is likely that the same constraints that affect 56K modems -- line quality; presence of PBXs; or Subscriber Line Concentrators -- will also affect ADSL links. It is at least possible that basic "lifeline" voice service powered by batteries at the telephone company office will by available on ADSL enabled lines even in the event of a general power failure.
ADSL info: http://web.archive.org/web/20071020220331/www.dslforum.org/
For information on availability:
Last Update 990131.
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