Asymmetric Digital Subscriber Line is a form of DSL. Collectively, all types of DSL are reffered to as xDSL. xDSLs are all considered "last mile" technologies. Voice-band modems and ethernet both fall into this category, also. These technologies bring an internet connection from the high-speed backbone lines of major networks(for instance, fibre optic lines) that connect high traffic nodes (not nodes, nodes).

The primary limiting factor of ADSL (in fact, xDSL) is the distance from the remote end (that's your house) to the Central Office (CO) end (thats the phone company). If that distance is 18000 feet or less (a more practical number is 15000), then chances are the service will be available to you pending the phone company installs the correct equipment at the CO.

The line rates can be up to 8Mbps (8,000 kbps - think about it in relation to a 56kbps modem that you may have used) for the downstream and 800kbps upstream. Of course, any service provider would charge you a pretty penny for those speeds. The going rate is 256 upstream by 256 downstream for $50+/mo. "800 upstream and 8000 downstream", you say? "Sounds asymmetric", you say? Well, hence the name.

A brief description of how it all works follows. Telephones were invented. They run on one pair of copper wires, which are twisted to reduce interference with other phone lines. These are called twisted pairs. ADSL runs on your standard, everyday, run-of-the-mill twisted pair phone line. Every medium, and by medium I mean a material used to propagate a signal from point A to point B, has a certain range of frequencies that it will propagate. The copper in a twisted pair has a certain frequency range. Phones traditionally use a very small portion of that; when they were invented, someone decided to use the smallest frequency range they needed for a decent quality sound. This leaves a -ton- of extra frequencies on the copper wire just sitting there, hanging out.

Some smart guy a long time ago said, "Hey, we can send data over phone lines just like we send voice data when we talk on the phone. Voila, the first modem (MOdulator/DEModulator) was born. Then some smart guy (presumably not the same fellow who I just mentioned) about 10 years ago said, "Hey, why don't we use that extra space for something." Voila, xDSL is invented, utilizing the previously untouched frequencies of the copper wires that run practically everywhere these days, yet leaving POTS(Plain Old Telephone Service) untouched.