DSL (Digital Subscriber Line) (Linktionary term)

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DSL (Digital Subscriber Line)

Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

DSL is a technology that provides high-speed data transmissions over the so-called "last-mile" of "local-loop" of the telephone network, i.e., the twisted copper wire that connects home and small office users to the telephone company central offices (COs). Demand for high-speed access methods is increasing with growing Internet access, electronic commerce, IP telephony, and videoconferencing. A number of methods for providing this bandwidth are available, including DSL technologies, cable (CATV) networks, and wireless and satellite technologies. All of these fit into the category of "residential broadband services."

This section provides a short briefing on DSL technologies. One of the best places to find DSL information is Telechoice's xDSL.com Web site at http://www.xdsl.com. There you will find the latest breaking DSL news, buyers guides, white papers, and company listings.

DSL technologies can enhance copper wire infrastructure to be the most effective way of delivering broadband services to the greatest number of people. In some cases, data rates up to 52 Mbits/sec can be achieved. DSL makes the local loop a multiservice access network that can support not only Internet access, but video and telephony services. No wiring upgrade is necessary for DSL. Only the equipment at the user end and at the telephone company end of the cable must be upgraded to new equipment.

While DSL transmission can share the same wire used to transmit traditional analog voice calls, it can also support multiple lines of digital telephony within the frequency range that it operates. Figure 1 illustrates how ADSL modems use frequency division multiplexing to support three separate channels: a traditional analog voice channel, the low-speed upstream channel, and the high-speed downstream channel.

Figure 1

DSL connections are point-to-point dedicated circuits, meaning that they are always connected. There is no dial-up. There is also no switching, which means that the line is a direct connection into the carrier's system. DSL modems are required at the customer site and the carrier site. Because there are different modulation techniques, users must ensure compatibility between their equipment and the carrier's equipment. The carrier will usually recommend suitable equipment.

There are actually seven types of DSL service, ranging in speeds from 16 Kbits/sec to 52 Mbits/sec. The services are either symmetric (traffic flows at the same speed in both directions) or asymmetric (the downstream capacity is higher than the upstream capacity). Asymmetric services are good for Internet users because more information is usually downloaded than uploaded. For example, a simple button click to download a Web page may produce 1K of upstream traffic, and thousands or millions of bytes of downstream traffic.

With all DSL technologies, there is a trade-off between the data rate and cable distance. As distance between the home and CO increases, the data rate drops. For example, the highest speed DSL service requires that customers be within 1,000 feet of the central office. Not too many homes qualify for that service, but businesses, apartment buildings, and condominium structures in downtown areas might.

Following is a description of the different versions of DSL. Note that these versions are often collectively referred to as xDSL.

HDSL (High-Speed Digital Subscriber Line) HDSL is the most common and mature of the DSL services. It delivers data symmetrically at T1 data rates of 1.544 Mbits/sec over lines that are up to 3.6 kilometers (12,000 feet) in length. Generally, HDSL is a T1 service that requires no repeaters but does use two lines. Voice telephone services cannot operate on the same lines. It is not intended for home users, but instead is intended for the telephone company's own feeder lines, interexchange connections, Internet servers, and private data networks.

SDSL (Symmetric Digital Subscriber Line) SDSL is a symmetric bidirectional DSL service that is basically the same as HDSL, but operates on one twisted-pair wire. It can provide data rates up to the T1 rate of 1.544 Mbits/sec.

ADSL (Asymmetric Digital Subscriber Line) ADSL is an asymmetric technology, meaning that the downstream data rate is much higher than the upstream data rate. As mentioned, this works well for a typical Internet session in which more information is downloaded from Web servers than is uploaded. ADSL operates in a frequency range that is above the frequency range of voice services, so the same wire can carry both analog voice and digital data transmissions. The upstream rates range from 16 Kbits/sec to as high as 768 Kbits/sec. The downstream rates and distances are listed here.

Downstream Rate	Downstream Distance
1.544 Mbits/sec	5.5 km (18,000 ft.)
2.048 Mbits/sec	4.8 km (16,000 ft.)
6.312 Mbits/sec	3.6 km (12,000 ft.)
8.448 Mbits/sec	2.7 km (9,000 ft.)

VDSL (Very High-Data-Rate Digital Subscriber Line) VDSL is basically ADSL at much higher data rates. It is asymmetric and, thus, has a higher downstream rate than upstream rate. The upstream rates are from 1.5 Mbits/sec to 2.3 Mbits/sec. The downstream rates and distances are listed in the following table. VDSL is seen as a way to provide very high-speed access for streaming video, combined data and video, video-conferencing, data distribution in campus environments, and the support of multiple connections within apartment buildings.

Downstream Rate	Downstream Distance
12.96 Mbits/sec	1.4 km (4,500 ft.)
25.82 Mbits/sec	0.9 km (3,000 ft.)
51.84 Mbits/sec	0.3 km (1,000 ft.)

RADSL (Rate-Adaptive Digital Subscriber Line) This service is also similar to ADSL, but it has a rate-adaptive feature that will adjust the transmission speed to match the quality of the line and the length of the line. A line-polling technique is used to establish a connection speed when the line is first established.

DSL Lite (or G.Lite) DSL Lite is considered a "jump-start" technology that is meant to deliver DSL to the greatest number of users, as fast as possible. While it has a lower data rate than other DSLs, it does not require that the telephone company do anything to the lines. In addition, equipment to handle DSL Lite is becoming readily available at a low price.

A related technology is VoDSL (Voice Over DSL). This is discussed under the topic "Voice/Data Networks."