Wikipedia: ADSL

Asymmetric Digital Subscriber Line (ADSL), a form of DSL, is a high speed data communications technology enabling use of existing analog telephone lines for access to the Internet and other services.

Using bandwidth higher that that of voice signals on the telephone line, it is always on, requiring no dialing to initiate a connection. To optimize performance in typical applications, it is asymmetric: the received or downstream rate is higher than the transmitted or upstream rate. Data and analog voice communications occur simultaneously on the same telephone line.

Upstream rates are typically 256 Kbps but can go as high as 768 Kbps Downstream rates can be as high as 8 Mbps over short ranges, although 2 Mbps is more common.

Because of the relatively low data rate (compared to optical backbone networks) ATM is an appropriate technology for multiplexing time-critical data such as digital voice with less critical data such as Web traffic; ATM is widely employed over ADSL technology to ensure that this remains a possibility.