How It Works: DSL

DSL connects your computer to the Internet at speeds as fast as 52 megabits per second, using the copper lines that already bring you phone service. In addition to offering significantly better download and upload times than traditional modems, DSL offers the benefit of always being on--you don't have to dial up your Internet service provider every time you want to get on the Net. And unlike cable modems, DSL connections are dedicated, so you don't have to share bandwidth with other users in your neighborhood.

There are several varieties of DSL, but all achieve their high speeds the same way: By sending data over previously unused frequencies in phone lines. Regular voice signals travel over phone lines at frequencies ranging from 0 kHz to 4 kHz. Standard modems use the same frequencies as voice. But DSL uses frequencies between 25 kHz and 1 MHz. That extra bandwidth also means it can send more data.

This broadband connection requires special hardware at both ends. On your end, a DSL modem modulates digital information from your computer to send it along phone lines. These signals are then translated by a Digital Subscriber Line Access Multiplexer located at the phone company's nearest central office. The DSLAM separates the voice from the data signals, sending the latter to an ISP (which is frequently the phone company itself) and from there to the Internet at large.

DSL has one significant downside: The farther you are from the central office, the slower your connection is. As you move away from the central office, more distortion enters the line and the signal deteriorates. To counter this, the phone company slows down transmission rates, from 1.5 mbps to 384 kilobits per second, for example. But slowing the speed only works up to a point--if you live more than two miles from the nearest central office, you can't get DSL at all. According to the industry trade group ADSL Forum, about 60 percent of United States telephone customers live within areas that could support DSL.