Understanding How a Cable Modem Works

A box called a cable modem is at the heart of Internet access over the cable TV network. The cable modem takes digital data from your PC's Ethernet card and puts in an unused block of frequency (think of it as another TV channel, but instead of pictures and sound, this channel carries digital data).

The cable modem places upstream data—data that's being sent from your PC to the Internet—in a different channel from that used for the downstream data that's coming from the Internet to your PC. By design, the speed of downstream data transfers is much higher than upstream transfers. The assumption is that people download far more stuff from the Internet than they upload. Probably true for most of us.

The coaxial cable that carries all those hundreds of cable TV channels to your home is a very capable signal carrier. In particular, the coaxial cable can carry signals cover-Secret ing a huge range of frequencies —hundreds of megahertz (MHz). Each TV channel requires 6 MHz, and the coaxial cable can carry hundreds of such channels. The cable modem places the upstream data in a small frequency band and expects to receive the downstream data in another frequency band.

At the other end of your cable connection to the Internet is a cable modem termination system (CMTS) that your cable company installs at its central facility. The CMTS connects the cable TV network to the Internet. It also extracts the digital upstream data sent by your cable modem (and by your neighbors as well) and sends them to the Internet. The CMTS also puts digital data into the upstream channels so that your cable modem can extract that data and provide it to your PC via the Ethernet card. Figure 13-4 illustrates how cable companies provide high-speed Internet access over their cable TV network.

Cable modems can receive downstream data at the rate of about 30 Mbps and send data at around 3 Mbps upstream. However, all the cable modems in a neighborhood share the same downstream capacity. Each cable modem filters out—separates —the data it needs from the stream of data that the CMTS sends out.

In practice, with cable modems you can get downstream transfer rates of around 1.5 Mbps and upstream rates of 128 Kbps.

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Cable Company Head End (the central distribution point)

Cable Modem Termination System (CMTS)

To Internet Backbone

Cable Company Head End (the central distribution point)

Cable Modem Termination System (CMTS)

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A neighborhood (one or more homes with Cable Modems)

A neighborhood (one or more homes with Cable Modems)

Another neighborhood (all homes with Cable Modems share same cable)

Figure 13-4: High-speed Internet Access Over the Cable TV network.

Another neighborhood (all homes with Cable Modems share same cable)

Figure 13-4: High-speed Internet Access Over the Cable TV network.

insider If you want to check your downstream transfer speed, go to http://bandwidth insight place.com/speedtest, and click the link to start the test. For my cable modem connection, the tests reported a downstream transfer rate of about 1.4 Mbps on good days.

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