DSL - Digital Subscriber Line


DSL is a technology capable of providing high bandwidth-information to individuals and business users by means of ordinary copper telephone lines. DSL installations began in 1997 and have since become increasingly popular with customers desiring a fairly inexpensive and fast connection. Depending often on ones distance from the telephone company, DSL may be able to achieve up to 6 or 7 megabits on the downstream, allowing users to experience continuous transmission of motion video, audio, and 3-D effects. Different types of DSL offer different types of rates. xDSL refers to the family or type of DSL, such as ADSL, HDSL, and RADSL. With DSL, you need only a single line for all your telecommunication needs.


Technology

Traditionally, phone lines have been used to transmit data such as a fax and phone using an analog signal. Since the Telephone Company already has existing analog systems set up, it’s easier to use simply use that system to transfer data from your computer modem to the Telephone Company. When data is transferred from your home or business modem, data is sent to and from the telephone company in an analog signal, it’s the responsibility of the modem to convert analog into digital data, or vice versa. The thing about DSL is that it assumes that data does not require this constant change from digital to analog and back. Therefore, digital data is transmitted to your computer directly as digital data, allowing the Telephone Company to use a wider bandwidth for transmitting. Essentially, DSL takes existing voice cables that connect customer premises to the phone company's central office and turns them into a high-speed digital link.

A DSL modem has a chip called a POTS splitter, which divides the existing phone line into two bands: one for voice and one for data. Voice travels on the first 4kHz of frequency. The higher frequencies—up to 2MHz depending on line conditions and wire thickness—are used for data. Another chip in the modem, called a channel separator, divides the data channel into two parts: a larger one for downstream Internet data and a smaller one for upstream Internet data.

At the other end of the phone line, 18,000 feet away at most, is another DSL modem, located at the phone company's central office. This modem also has a POTS splitter, which separates the voice calls from the data. Voice calls are routed to the phone company's public switched telephone network (PSTN) and proceed on their way as usual.

Data coming from your PC passes from the ADSL (most common DSL) modem to the digital subscriber line access multiplexer (DSLAM). The DSLAM links many ADSL lines to a single high-speed asynchronous transfer mode (ATM) line, which in turn connects to the Internet at speeds up to 1Gbps.


ADSL

ADSL, Asymmetrical Digital Subscriber Line, is the most familiar type of DSL. It’s called "Asymmetrical" because most of the duplex bandwidth is dedicated to the downstream direction. Both ANSI (American National Standards Institute) and ETSI (European Telecommunications Standards Institute) have already standardized ADSL. Only a small portion of bandwidth is available in the upstream direction. For example, some telephone companies may offer 640kps for the upstream, but give as much as 6.1megabits on the downstream. This high downstream means the telephone line can provide continuos motion and audio play. ADSL won’t compete with your neighbors for rates, like cable connections do. Some areas may need to upgrade resources to allow for this, but many companies already have existing services for DSL.

ADSL uses special modems called endpoints. Along with an endpoint, your PC needs a network interface card that treats the modem like a local device on the network. Many ADSL modems combine both functions into a single internal card.

ADSL comes in two competing varieties: Carrierless Amplitude/Phase modulation (CAP) and Discrete Multi-Tone (DMT). Each of these is a different modulation system, which is a method for putting data into a carrier signal on the ADSL wire and for reading the data at the receiving end. CAP uses a single carrier; DMT uses multiple carriers. Currently 90 percent of ADSL services use CAP. But that doesn't mean it will win out, as DMT is the basis of current ANSI and European standards for ADSL.


HDSL

HDSL (High bit-rate DSL) is another form of DSL equal to about a T1 in North America and an E1 in Europe. It is the earliest variation of DSL to be used in wide-band digital transmission within a corporate site and between the Telephone Company and a customer. Unlike ADSL, HDSL is a symmetrical form, which means the upload and download rates are equal. It uses less bandwidth and requires no repeaters. Using more advanced modulation techniques, HDSL transmits 1.544 Mbps or 2.048 Mbps in bandwidth depending upon the specific technique. HDSL provides such rates over lines up to 12,000 feet in length (24 gauge), the so-called Carrier Serving Area (CSA), but does so by using two lines for T1 and three lines for E1, each operating at half or third speed. Although HDSL has been the maturest form, it is more likely that it will give way to ADSL and SDSL.


What You Will Need

Most DSL require that a signal splitter be installed at either your home or business, with the Telephone Company giving you a nice bill for the service. Many companies will test your lines for free; however, you may need to be within two miles of the Telephone Company to ensure a good connection. Usually with the closer you get to the telephone company, the greater transfer of data can take place. Another factor may include the gauge of the copper wire. The heavier 24-gauge wire is capable of carrying longer distances than a 26-gauge wire. In general, the longest distance DSL can go without using repeaters is 5.5 kilometers. In addition, you may be required pay for a special modem and other equipment. And depending on who your ISP is, you will have to pay a monthly fee for the service, in addition to any cost from the telephone company.