ADSL (Asymmetric Digital Subscriber Line) is a type of digital subscriber line (DSL) technology for transmitting digital information at a high bandwidth on existing phone lines to homes and businesses. Compared to traditional modem lines, ADSL allows more data to be sent over existing copper telephone lines (POTS). Using with a splitter, or DSL filter, it allows a single telephone connection to be used for both ADSL service and voice calls at the same time. In general, ADSL is offered at downstream data rates from 512 Kbps to about 6 Mbps. In addition, ADSL can only be distributed over short distances from the telephone exchange (the last mile), typically less than 4 kilometres, but sometimes it has been exceeded up to 8 kilometres if the originally laid wire gauge allows for further distribution. Data carried by the ADSL are typically routed over the telephone company’s data network and eventually reach a conventional Internet Protocol network.
ADSL defines three “Transmission protocol-specific transmission convergence (TPS-TC)” layers:
In home installation, the prevalent transport protocol is ATM. On top of ATM, there are multiple possibilities of additional layers of protocols (two of them are abbreviated in a simplified manner as “PPPoA” or “PPPoE”), with the all-important TCP/IP at layers 4 and 3 respectively of the OSI model providing the connection to the Internet.