(xDSL) A whole class of digital telecommunication technologies that can offer BROADBAND data rates, up to 50 megabits per second (Mbps), over the existing copper wires of the analogue public telephone system. By employing advanced modulation schemes, xDSL technologies support Internet or other data access simultaneously with voice telephone calls. In the meta-acronym xDSL, the x stands for the first letter of any of the individual acronyms used for these technologies: ADSL, HDSL, SDSL and VDSL.
The xDSL technologies vary in the relative speeds of their incoming and outgoing signals, and in the distance from the exchange over which they can operate, each making a different trade-off in its exploitation of the available BANDWIDTH. All are, in telecom jargon, 'last mile' technologies, designed only to carry signals over the last mile into the home or office, rather than between cities or countries.
The best-known variant ADSL (Asymmetric Digital Subscriber Line) supports a downstream data rate into the subscriber's home of 1.5 Mbps, sufficient for an MPEG compressed video signal, at up to 18,000 feet from the exchange. However the upstream rate back to the exchange is only 16-640 kilobits per second, hence the name, 'Asymmetric'. ADSL is currently being deployed commercially in several areas of the USA and in the UK.
HDSL (High-data-rate Digital Subscriber Line) employs more advanced modulation techniques to deliver 2 Mbps in both directions over two wires, at up to 12,000 feet, while SDSL (Single-line Digital Subscriber Line) delivers similar rates over a single wire.
VDSL (Very-high-data-rate Digital Subscriber Line) is asymmetric like ADSL but with a higher downstream data rate of 13Mbps at 4500 feet and up to 50 Mbps at 1000 feet (sufficient for a high-definition television signal): its upstream rate is around 2 Mbps.
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