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DSL Connection

DSL (Digital Subscriber Line) is a technology for bringing high-bandwidth to homes and small businesses over ordinary copper telephone lines. xDSL refers to different variations of DSL, such as ADSL or SDSL. Assuming your home or small business is close enough to a telephone company central office (CO) that offers DSL service, you may be able to receive data at rates up to 6.1 megabits/second. Typically, individual connections will provide from 1.544 Mbps to 512 Kbps downstream and about 128 Kbps upstream. DSL can carry both data and voice signals and the data part of the line is continuously connected. DSL is expected to replace ISDN in many areas and to compete with the cable modem both in price and performance.

How It Works

Traditional phone service called POTS (Plain Old Telephone Service) connects your home or small office to a telephone company office over twisted copper wires. This service was created to let you exchange voice (analog) information with other phone users. Since the telephone company's signaling is already set up for this analog transmission, it's easier for it to use that as the way to get information back and forth between your telephone and the telephone company. A device called modem (modulator demodulator) connected to a computer, can exchange digital information to be carried over analog lines.

Because analog transmission only uses a small portion of the available amount of information that could be transmitted over copper wires, the maximum speed of ordinary modems is about 56 Kbps. The speed is constrained by the fact that the telephone company converts information that is received in digital form into analog form for your telephone line, and requires your modem to change it back into digital form. In other words, the analog transmission between your home or business and the phone company is a bandwidth bottleneck.

Digital Subscriber Line is a technology that assumes digital data does not require conversion into analog form and back. Digital data is transmitted to your computer directly as digital data and this allows the phone company to use a much wider bandwidth for transmitting the data to you. Meanwhile, if you choose, the signal can be separated so that some of the bandwidth is used to transmit an analog signal so that you can use your telephone and computer on the same line at the same time.

Splitter vs. Splitterless DSL

Most DSL technologies require that a signal splitter be installed at a home or business, requiring the expense of a phone company visit and installation. However, it is possible to manage the splitting remotely from the central office. This is known as Splitterless DSL or Universal ADSL and has recently been made a standard.

Factors Affecting the Experienced Data Rate

DSL modems follow the data rate multiples established by North American and European standards. In general, the maximum range for DSL without a repeater (amplifier) is about 5.5 km (18,000 feet). As distance decreases toward the telephone company office, the data rate increases. Another factor is the gauge of the copper wire. The heavier 24-gauge wire carries the same data rate farther than 26-gauge wire. If you are located beyond the 18,000 feet range, you may still be able to have DSL if the phone company has extended the local loop with fiber optic cable.

The Digital Subscriber Line Access Multiplexer (DSLAM)

To connect multiple DSL users to a high-speed backbone network, the telephone company uses a Digital Subscriber Line Access Multiplexer. Typically, the DSLAM connects to an asynchronous transfer mode (ATM) network that can aggregate data transmission at gigabit data rates. At the other end of each transmission, a DSLAM demultiplexes the signals and forwards them to the appropriate individual DSL connections.

Types of DSL

aDSL

ADSL (Asymmetric Digital Subscriber Line) is the form of DSL that will become most familiar to home and small business users. ADSL is called "asymmetric" because most of its two-way or duplex bandwidth is devoted to the downstream direction, sending data to the user. Only a small portion is available for upstream bandwidth. However, most Internet data, especially graphics and multimedia-intensive Web data need lots of downstream bandwidth. User requests and responses are small and require little upstream bandwidth. Using ADSL, up to 6.1 megabits per second of data can be sent downstream and up to 640 Kbps upstream. The high downstream bandwidth means that your telephone line will be able to bring motion video, audio, and 3-D images to your computer or web-enabled TV. A small portion of the downstream bandwidth can be devoted to voice rather data, so you can have phone conversations without requiring a separate line. Unlike a cable modem, you won't be competing for bandwidth with neighbors in your area. In many areas, the existing telephone lines will work with aDSL. Some areas may need the existing telephone lines upgraded.

sDSL

SDSL (Symmetric DSL) provides an equal amount of bandwidth in both directions. For this reason, the maximum sDSL downstream data rate is lower than the aDSL downstream data rate. sDSL can carry as much data on a single wire of twisted-pair as can be carried on a T1 line. It's symmetric because the data rate is the same in both directions.

vDSL (may be available in the future)

VDSL (Very high data rate DSL) is a developing technology that promises much higher data rates over relatively short distances (between 51 and 55 Mbps over lines up to 1,000 feet or 300 meters in length). It's envisioned that VDSL may emerge somewhat after ADSL is widely deployed and co-exist with it. The transmission technology and its effectiveness in some environments are not yet determined. A number of standards organizations are working on it.

DSL Summary Table

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