Asymmetric digital subscriber line (ADSL) is a type of digital subscriber line (DSL) technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. ADSL differs from the less common symmetric digital subscriber line (SDSL). In ADSL, bandwidth and bit rate are said to be asymmetric, meaning greater toward the customer premises (downstream) than the reverse (upstream). Providers usually market ADSL as an Internet access service primarily for downloading content from the Internet, but not for serving content accessed by others.
ADSL works by using spectrum above the band used by voice telephone calls. With a DSL filter, often called a splitter, the frequency bands are isolated, permitting a single telephone line to be used for both ADSL service and telephone calls at the same time. ADSL is generally only installed for short distances from the telephone exchange (the last mile), typically less than 4 kilometres, but has been known to exceed 8 kilometres (5 mi) if the originally laid wire gauge allows for further distribution.
At the telephone exchange, the line generally terminates at a digital subscriber line access multiplexer (DSLAM) where another frequency splitter separates the voice band signal for the conventional phone network. Data carried by the ADSL are typically routed over the telephone company‘s data network and eventually reach a conventional Internet Protocol network.
There are both technical and marketing reasons why ADSL is in many places the most common type offered to home users. On the technical side, there is likely to be more crosstalk from other circuits at the DSLAM end (where the wires from many local loops are close to each other) than at the customer premises. Thus the upload signal is weakest at the noisiest part of the local loop, while the download signal is strongest at the noisiest part of the local loop. It, therefore, makes technical sense to have the DSLAM transmit at a higher bit rate than does the modem on the customer end. Since the typical home user does prefer a higher download speed, the telephone companies chose to make a virtue out of necessity, hence ADSL.
The marketing reasons for an asymmetric connection are that, firstly, most users of internet traffic will require fewer data to be uploaded than downloaded. For example, in normal web browsing, a user will visit several websites and will need to download the data that comprises the web pages from the site, images, text, sound files etc. but they will only upload a small amount of data, as the only uploaded data is that used to verify the receipt of the downloaded data (in very common TCP connections) or any data inputted by the user into forms etc. This justifies for internet service providers to offer a more expensive service aimed at commercial users who host websites, and who therefore need a service that allows for as much data to be uploaded as downloaded. File sharing applications are an obvious exception to this situation. Secondly, internet service providers, seeking to avoid overloading of their backbone connections, have traditionally tried to limit uses such as file sharing which generate a lot of uploads.