I Didn’t Know That. . .

Q: How does a DSL connection to the Internet work?

A: A digital subscriber line takes advantage of the fact that copper telephone wires can carry a broad range of frequencies, up to as much as 1.5 million cycles per second (hertz). But the higher the frequency of an electronic signal, the shorter the distance it can propagate over copper wires without amplification. To maximize efficiency, the telephone system was designed to work with frequencies in the range of 300 to 3,300 hertz, which gives sufficient bandwidth for voices to be intelligible. That leaves a lot of unused bandwidth, and DSL takes advantage of it.

A special device at the phone company, called a multiplexer, takes the high-speed signal from a fiber-optic cable connected to the Internet and puts it into the phone line at a frequency ranging from about 240,000 hertz to 1.5 million hertz. This signal is detected by the modem in a home computer, which ignores signals in the frequency range used for conventional telephone service. Because the telephone doesn’t recognize signals higher than about 3,400 hertz, voice and data transmission can occur at the same time. The modem sends signals back to the phone company using frequencies between 25,000 and 160,000 hertz, and its data transmission rate is only 10% to 20% of the rate in the opposite direction.

The major disadvantage of DSL is that the user must be within about three miles of the phone company’s multiplexer. Beyond that distance, the signal deteriorates rapidly. The line between the computer and the phone company also cannot pass through amplifiers or fiber-optic cables at any point. Despite these limitations, experts predict that DSL will serve 9.3 million homes by 2003, compared to 9 million served by fiber-optic cables.