Asymmetrical Digital Subscriber Line, working on the copper ‘local loop’ originally installed to connect phones to the exchange for communication by voice, ADSL adds a broadband downstream channel (to the user) of maximum 8 Mb/s and a narrower band upstream channel (from the user) of maximum 128-1024 kb/s, according to class. As the upstream and downstream speeds are different the service is called ‘asymmetrical’ (a widely used technique to keep the customers’ equipment technically simpler, and lower cost, with larger, more complex equipment at the supplier, e.g., telephone and TV services, MPEG compression, etc). Exactly how fast an ADSL circuit can run ultimately depends on the performance of the line (including the customer’s own wiring), and is often dictated by the distance from the telephone exchange where the DSLAM terminates the line. The highest speeds are usually only available within 1.5 km of the DSLAM. The service is normally always-on, no need to dial up. Its uses include high-speed Internet connections and streaming video.
ADSL-2 can run up to 12 Mb/s over up to 2.5 km, and ADSL-2+ can deliver 24 Mb/s over up to 1.5 km. ADSL-2/2+ effectively doubles this rate by putting two services together (all distances are approximate). These are sufficient to carry live SD or HD provided that the service is continuous, or can be recorded before viewing.