Digital subscriber line (DSL)

Digital subscriber line (DSL) is a high-speed Internet technology that transmits digital signals over standard telephone lines. The technology allows both Internet and telephone connections to share the same line, though DSL uses a higher frequency than landline phone signals. This means that a computer can maintain an open connection to the Internet while allowing the line to be used for phone calls. Several types of DSL service exist, depending on the speed and frequency range of data transmission through the line-based connection. Although a DSL connection is faster than dial-up Internet service, its performance can be affected by the distance between the Internet service provider (ISP) and a user's home or business. DSL service is also not available in areas where the local infrastructure does not support the technology.

Background

Telephone technology was developed in the late nineteenth century as a means of converting sound into electrical signals and transmitting them via a wire medium. Inventor Alexander Graham Bell is credited with inventing the telephone in 1876 and was the first to patent a delivery system by using twisted pairs of copper wire to transmit the signals. The technology grew rapidly. By 1880, the United States had 49,000 telephones; thirty years later, that number had increased to 5.8 million. By the 1980s, more than 175 million phone lines were in use across the country. In the span of slightly more than a century, technological advancements had led to marked improvements in communications capabilities, but copper wire remained the standard medium for signal transmission.

By that time, voice communications were not the only transmissions being sent over telephone lines. In the 1960s, a joint venture between scientists and the United States military had developed the ancestor of the modern Internet. This technology had advanced enough by the early 1980s to give rise to the first home-based Internet connections and fueled rapid growth in the personal computer industry.

The Internet allowed computers to communicate across a network and share digital information and data with one another. Digital information is data, such as sounds and pictures, transformed into a series of numbers—0s and 1s—and then changed back into its original form at the receiving end. This "translation" is accomplished using a device called a modem, which is short for modulator-demodulator. Voice communication is an example of an analog signal, a transmission that uses sound waves to transfer information. The human voice operates in a relatively narrow frequency range—from 0 hertz (Hz) to about 3.4 kilohertz (kHz). A hertz is a measure of frequency and is equal to one cycle per second. That means a sound wave traveling at 5 Hz is moving at five cycles per second, while one traveling at 3.4 kHz is moving at 3,400 cycles per second.

Overview

In the early days of the Internet, most computers were connected over standard dial-up phone lines. A phone line could not be used to receive calls while it was being used by a computer, and it had a data limit of 56 kilobits per second (kbps). A bit is the smallest amount of information that can be used by a computer. A kilobit is the equivalent of one thousand bits. In the late 1980s, researchers began to develop a method of sending information over a single wire or cable using a wider range of frequencies. They realized that the copper wire used in most telephone lines could handle more frequencies than the limited range used by voice communications.

The first DSL service was created in the early 1990s and was called high bit-rate digital subscriber line (HDSL). This type of DSL was primarily used by businesses or in connections between telephone companies and customers. HDSL utilized equal data transmission speeds in both uploading and downloading information. Despite utilizing separate frequencies, data transmission occasionally caused interference with phone conversations. To fix this problem, researchers developed splitter technology to separate the analog voice signals from the digital data signals. Modern DSL lines operate at levels above 20 kHz—the highest frequency audible to the human ear. Frequencies between 25 kHz and 160 kHz are used to send data from the user to the server. Frequencies between 240 kHz and 1.5 megahertz (mHz)—1.5 million hertz—are used for downloading data from the server to the user.

Because home computer users typically download more data than they upload, most residential customers use a form of DSL known as an asymmetrical digital subscriber line (ADSL). ADSL splits the phone line to allow it to carry a wider range of frequencies for receiving data than for transmitting data. This means that residential ADSL users can download information at much higher speeds than they can upload it. For example, ADSL may be able to download at a rate of 8.4 megabits per second (mbps), or 8.4 million bits; upload speeds, however, may be just 640 kbps. A symmetrical digital subscriber line (SDSL) maintains equal data transmission space for both sending and receiving. An SDSL service is usually preferred by businesses that have more need to send outgoing information.

The speed capabilities of DSL can be affected by the distance between the user and the company providing the source of the connection. The farther the connection needs to travel, the weaker the signal quality gets and the slower the information loads. ADSL technology, for example, has a distance limit of 18,000 feet, or about 3.5 miles. Download speeds from a source about 1.7 miles away may be about 8.4 mbps, while those from a source 3.5 miles away may fall to about 1.5 mbps. As a result, people living in rural areas away from a DSL provider's headquarters may be unable to receive the service. DSL transmissions are also not compatible in areas that have installed fiber-optic cables. Fiber-optic lines use thin strands of glass instead of copper wire as a medium for transmitting signals.

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