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DSL (Digital Subscriber Line) Modems

What is DSL?

DSL is a high-speed data service that works over copper telephone lines. As we all know, existing telephone lines offer significant advantages as a medium for providing high-speed network access to residential and small business users. These users currently rely on voiceband modems that use the telephone line as a relatively inexpensive way to gain network access. Although the rate at which voiceband modems can transmit data has evolved over the past two decades from 2.4 Kbps to 56 Kbps, current speeds are still too slow for some existing data applications and voiceband modems will be unable to provide the bandwidth for many anticipated applications.

Advances in semiconductor integration and digital signal processing have led to the development of a broadband access technology, known as Digital Subscriber Line or DSL, technology, which can transmit data over copper telephone lines significantly faster than voiceband modems.

This digital service can be used to deliver bandwidth-intensive applications like streaming audio/video, online games, application programs, telephone calling, video conferencing and other high-bandwidth services.

DSL delivers "always on" availability so eliminates the tedious dial-up process associated with voiceband modems. It is a point-to-point technology that connects the end user to a telecommunication service provider's central office or to an intermediate hub. DSL modem is connected at each end of the copper wire and the transmission speed depends on the length and condition of the existing wire and the distance to the central office...


There are some versions and flavors of DSL, which has led to the common designation of "xDSL" when referring to this type of technology in general.

The first DSL technology, known as Asymmetric Digital Subscriber Line or full-rate ADSL, was created in the late 1980s and enables data to be transmitted at speeds more than 100 times faster than 56 Kbps voiceband modems. Full-rate ADSL permits voice and multi-megabit data traffic to be transmitted simultaneously on the same line. So, it is possible to use the telephone line for network access and voice communication at the same time. However, this approach usually requires that a technician install a filter, known as a voice-data "splitter", at the user's site, which increases deployment time and cost.

In June 1999, a new global "splitterless" ADSL standard called "G.lite" is approved. G.lite services are capable of providing data transmission speeds between ten and thirty times faster than those of voiceband modems, while permitting voice and data traffic to be transmitted simultaneously without the need to install splitters. As a result, the combination of G.lite modems can provide high-speed network access to residences and small offices at a lower cost than full-rate ADSL.

Some of the other variations include HDSL (High-bit-rate DSL), SDSL (Symmetric DSL) and VDSL (Very-high-bit-rate DSL).

  • HDSL uses 4 copper wires (2 pairs) and offers a wider coverage area than previous methods.
  • SDSL is an enhanced version of HDSL that allows it to work with only one pair of wires. HDSL and SDSL have the same bandwidth capability, 2 Mbps, in both directions.
  • When we come to VDSL (Very-high-speed DSL), it is targeted at high-access demanding companies and can support speeds of 52Mbps downstream and 13Mbps upstream.

At METU Campus, pair of SDSL modems is connecting the new Sport Center to the campus backbone. After the installation on July 2001, the data transmission rate is measured to be 2 Mbps that is really satisfactory compared to the data transmission rate of the voiceband modem, which was 33.6 Kbps. The new connection is serving the local area network of the center, which currently consists of a few personal computers.

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2001 METU CC
Design: CC - INFO