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DSL (Digital Subscriber Line) comes in a number of variants, several of which are described below. It is a networking technology that can transmit information at high speed (see Broadband) over existing copper and aluminium cables between the local telephone exchange and a subscribers residential or business premise.
Transmission speeds reduce as the distance from the local exchange increases, and other factors such the quality of the cable (copper is better than aluminium) and cable joints have an impact on maximum distance and transmission speed too. Improvements in DSL technology (e.g. ADSL2) are resulting in increased transmission speeds, but the maximum distance currently achievable even at reduced speeds is still less than the longest telephone lines.
As DSL operates over the same local cables used for the public telephone network, a small plug-in filter is required to separate out the telephone signal, if required.
ADSL (Asynchronous Digital Subscriber Line) transmits data at the different speeds in each direction. ADSL is the most common service for home and business broadband Internet access. High download speeds from the Internet to the customer are available (256kbps up to 2Mbps and for the lucky few up to 8Mbps), at the expense of reduced upload speeds which typically range from 128kbs up to around 800Kbps.
SDSL (Synchronous Digital Subscriber Line) is a less common form of DSL that transmits data at the same speed in both directions. The high download speed of ADSL is sacrificed to achieve an increase in upload speed. 2Mbps in both directions can frequently be achieved. SDSL is more appropriate for professionals or businesses that need to share or transfer (upload) large files.
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