Sierra Tel Internet - How DSL Works

Digital Subscriber Line (DSL)
How It Works

Your traditional telephone service (sometimes called the "plain old telephone service" or POTS) connects your home or business to your phone company's central office over copper wires called "twisted pair" because they are wound around each other. Traditional telephone service was created to let you exchange voice information with other telephone users. The waveform signal used for this kind of transmission is called an analog signal. A terminal device, such as a phone set, converts an acoustical (natural analog) signal into its electrical equivalent in terms of volume (signal amplitude) and pitch (frequency or wave change).

However, computers use digital (as opposed to analog) signals. Digital signals are comprised of strings of zeros and ones (also called "bits"). Your computer's modem translates (modulates and demodulates) digital signals into analog signals so that they can be transmitted over telephone wires.

Modulating and demodulating data between your home and your phone company creates a bandwidth bottleneck. Because analog signal uses a relatively small range of the frequency available over copper wire, the amount of data transmitted by ordinary modems is limited to a maximum of 56 Kbps (thousands of bits per second).

DSL is a technology that does not require digital data to be changed into analog form and back. Digital data is transmitted over telephone wires as digital data! Because digital data uses a much wider range of frequencies than analog, it easily exceeds the 56 Kbps limitation of conventional modems. In addition, the signal can be separated so that some of the bandwidth is used to transmit an analog signal as well. Because the DSL line can carry both data and voice signals, you can use both your telephone and your computer on the same line at the same time!