Cable modems: how do they work

A. Cable modems are devices that allow high-speed access to the Internet via a cable television network. They are similar to a traditional analogue modems, which connect to the Internet via an analogue telephone line, however, a cable modem is significantly more powerful, capable of delivering data approximately 500 times faster.

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A cable modem typically has two connections, one to the cable wall outlet and the other to a computer.

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Q. How do they differ from analogue phone modems

A. Like analogue modems, cable modems modulate and demodulate data signals. However, cable modems are more complex and have various extra functions.

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A cable modem sends and receives data in two slightly different ways. In the downstream direction, data travels from the network to the user, the digital data is modulated and then placed on a typical 6 MHz television channel. Data from the user to the network is referred to as upstream. This signal can be placed in a 6 MHz channel adjacent to TV signals on either side without disturbing the cable television video signals.

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Q. How can a cable modem carry Internet traffic as well as dozens of TV channels

A. Firstly, television signals actually take up very little space, ‘bandwidth’, on a cable. Each television signal is given a 6 megahertz (MHz) channel on the cable. The coaxial cable used to carry cable television can carry hundreds of megahertz of signals.

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Secondly, the downstream channel has a much higher bandwidth allocation (faster data rate) than the upstream, because Internet activity tends to be asymmetric. Image files and audio and video are very bandwidth intensive in the downstream direction. However at the user end, the upstream direction, mouse clicks and e-mail messages are not bandwidth intensive. The cable system puts downstream data into a 6 MHz channel, whereas upstream requires less of the cable's bandwidth, just 2 MHz.

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Q. Are cable modems internal or external to the computer

A. Cable modems can be either internal or external to the computer. In some cases, the cable modem can be part of a set-top cable box, requiring that only a keyboard and mouse be added for Internet access.

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by Lisa Cardy