Find out everything you wanted to know about DSL but were afraid to ask.

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DSL (digital subscriber line) is one of the two most common ways of going broadband, accelerating your Internet access to speeds that can be many times faster than a conventional dialup modem. DSL refers to a group of technologies that deliver higher speeds over standard telephone lines. The most common forms of DSL available to consumers today are:

  • ADSL (asymmetric DSL)
    By far the most common consumer class service, ADSL is designed to deliver much higher downstream speeds at the expense of upstream speeds. Although downstream can be as high as 8 megabits per second, consumer-class service is commonly limited to 1.5 megabits per second; upstream speed is commonly limited to 128 kilobits per second.

    One advantage of ADSL is that it can share an existing phone line with voice (and even a conventional dialup modem), minimizing cost. There are two principal and incompatible forms of ADSL, CAP (carrierless amplitude and phase modulation) and DMT (discrete multitone transmission), although DMT now predominates.

  • SDSL (single-pair HDSL)
    Usually marketed as a business-class service, SDSL normally has the same speeds for upstream and downstream transmission up to a maximum of about 1.5 megabits per second in each direction. SDSL cannot share a phone line with voice, so a dedicated phone line is needed, which increases cost.

  • IDSL (DSL over ISDN)
    IDSL effectively turns ISDN into an "always on" data service. Speed is 140 kilobytes per second (sometimes 128 kilobytes per second) for both upstream and downstream. Although speed is much lower than other forms of DSL, a major advantage of IDSL is its range: roughly double that of ADSL or SDSL, on the order of about 27,000 feet as compared with about 14,000-17,000 feet. Repeaters can further extend the range of IDSL, but not ADSL or SDSL.


How does DSL work?

DSL is based on high-frequency transmission between a subscriber DSL modem (which looks much like a conventional dialup modem) and a DSLAM (digital subscriber line access multiplexor), a relatively large "headend" box that is usually located at the telephone company central office. The range of DSL can be extended if the DSLAM is instead placed in a remote terminal (or gateway) that is located between the central office and the subscriber, but such deployments are relatively new and not yet available in many areas.

DSL gets around the frequency and speed limitations of conventional dialup modems and the standard telephone network (commonly referred to as POTS, for plain old telephone service) by splitting off the high-frequency DSL signal at the DSLAM rather than passing through the telephone company switch in the central office and over the low-speed POTS network. In the case of ADSL, the telephone line can be shared between voice and DSL, because ADSL is confined to different frequencies that are higher than the first 4,000 hertz that is used for voice.

John Navas is the author of Navas Cable Modem/DSL Tuning Guide.

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