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Chapter 17. How a Modem (Really) Works > Cable, DSL, and ISP Modems

Cable, DSL, and ISP Modems

The cable modem is a device that hooks a PC to a digital cable line. It is really an adapter because the digital data rate of your computer is not compatible with the cable system's digital system. A cable modem does not actually do any modulation or demodulation. It changes a digital signal from one format to another. A cable modem attaches through a standard Ethernet card in the computer. The cable modem attaches to the round coaxial cable (one that you typically attach to your television set). The cable modem will change the digital signal to a digital signal that the cable company's hardware can manipulate. At the cable company there will be other hardware to further refine and change the signal to interface with other services (i.e., telephone lines).

The digital network adapter for DSL has a variety of different technologies that also fall under the same nouveau moniker of modem. A DSL modem can be a discrete multitone technology (DMT), a carrierless amplitude modulation (CAP), or a multiple virtual line (MUL). The distinctions are minor, unless you're intensely interested in the subject. For the most part, all terminal adapters work the same, as they are all called DSL modems. A DSL modem is a device to convert a computer's digital signal into the type of digital signal used by the telephone company's equipment. They do not create an audible audio signal, but instead generate another digital signal.


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