Asymmetric Digital Subscriber Lines (ADSL) are used to deliver high-rate digital data over existing ordinary phone-lines. A new modulation technology called Discrete Multitone (DMT) allows the transmission of high speed data.
ADSL facilitates the simultaneous use of normal telephone services, ISDN, and high speed data transmission, eg., video.
DMT-based ADSL can be seen as the transition from existing copper-lines to the future fiber-cables. This makes ADSL economically interesting for the local telephone companies. They can offer customers high speed data services even before switching to fiber-optics.