How It Works: Cable Modems

Cable modems: devices that provide high-speed Internet access using cable television networks.

• Pro: fast data-transmission rates; always-on connection; least expensive broadband option.
  
  
• Con: no Internet service provider choice; bandwidth is shared with neighbors; not available in all areas.
  
  

These days, the cable that brings you MTV and CNN can also bring you the Internet. Some cable companies have expanded their services to include high-speed Internet connections for home users. Like its rival Digital Subscriber Line, cable modem service lets you get on the Net without having to dial in.

Cable access theoretically allows downstream speeds of up to 38 megabits per second, but actual rates are considerably slower. PC World test results, published in March 1999, showed that downstream speeds (from the Internet to you) over cable modems averaged 382 kilobits per second, while upstream rates (from you to the Internet) averaged 315 kbps. But those speeds are still about seven times faster than what you get with standard modems.

One reason you'll rarely see the top speeds: Cable modem users must share bandwidth with everyone else on the network. The more people online at any moment, the slower your access will be. Unlike standard modem access, which creates a one-to-one circuit between your PC and your Internet access provider, cable access uses what's called a branching network. In such a network, a device called a cable modem termination system at your cable company's central office sends out signals along a main line. That main line then splits into "feeder lines." All subscribers along a given feeder line must share its overall bandwidth. Your cable modem converts the incoming signals into data your computer can understand and then sends it to your computer through an Ethernet card.

Two types of cable systems are currently in use: coaxial and fiber optic. The older coaxial cables max out at 350 MHz and can't send signals upstream. To get around the latter problem, the cable company uses the phone line to send data upstream. The newer fiber optic networks support rates up to 750 MHz and allow two-way traffic, in which the upstream data typically uses some portion of the 5- to 40-MHz frequency range. Whichever system is used, cable companies split all available bandwidth into 6-MHz channels and assign those channels to individual networks, such as ABC or NBC. For Internet access, one of these 6-MHz channels is dedicated to moving data downstream; fiber optic systems dedicate another channel to upstream signals.