Asymmetric Digital Subscriber Line (ADSL) is a communications technology that enables high-speed data to be sent simultaneously with voice over the existing telephone line. The data flow is asymmetric which means that the speed of the downstream link from the network to the user is substantially higher than the upstream link from the customer into the network. The downstream speed is maximum 8 Mbit/s. ADSL is the most predominant technology for high speed Internet access around the world.

ADSL is specified in ITU-T Recommendation G.992.1. ITU Recommendation G.992.3 specifies ADSL2 which provides slightly better performance than G.992.1. ITU Recommendation G.992.5 specifies ADSL2Plus, which provides twice the bandwidth so that bit rates as high as 20 Mb/s downstream can be achieved on relatively short lines.