What are Cable Modems and How do they Work?

Many people have the same cable TV and internet provider, and use a single cable modem to transfer information from hundreds of cable television channels as well as websites to devices in their homes. Here’s how:

First of all, it’s important to remember that a television channel only takes up about 6-megahertz of bandwidth on a coaxial cable, which is designed to carry hundreds of megahertz of signals. In cable TV systems, a coaxial cable is the only means of transferring signals to your house, whereas in other systems a fiber-optic cable stretches out to different neighborhoods and then is connected to a coaxial cable for signal distribution to different houses.

coax cableThen that same cable company and infrastructure offers internet access, it’s offering to send information from the internet (downstream data) to individual houses through use of 6-MHz channel. It takes up no more space than a single TV channel. Upstream data (data from the individual back to the internet) requires even less of the cable’s bandwidth and generally comes out to about 2 MHz.

To send upstream and downstream data on cable TV infrastructure, you need a cable modem on the individual user’s end and a cable modem termination system on the cable provider’s end.

A cable modem is a piece of hardware that can either be internal or external to the computer. Often the cable modem can be part of a set-top cable box. If your cable system has been upgraded to digital cable, the new set-top box provided by your cable company will actually be capable of connecting to the internet regardless of whether you receive internet access through your cable TV connection.

A cable modem has a tuner, a modulator, a demodulator, a media access control (MAC) device, and a microprocessor.  The tuner connects to the cable outlet and receives the digital signal from the internet. Because it’s not a cable signal, the data comes in modulated and must pass through the demodulator. Some tuners have diplexers that allow them to use one set of frequencies for downstream traffic and one set of frequencies for upstream traffic. Otherwise the system will use the cable modem tuner for downstream data and a dial-up telephone modem for upstream data. Regardless, the tuner receives a signal and passes it to the demodulator.

The demodulator has four functions. First it receives a radio-frequency signal from the cable provider. It then varies the amplitude and phase of the radio waves so that they can be turned into a simple signal. This signal can be processed by the analog-to-digital converter, which turns it the radio signal into binary code. Finally, an error correction module checks the received information against a known standard so that it can detect any problems with the information transmission.

media access control deviceThe modulator does basically the same process in reverse in order to send data back upstream towards the cable provider.

The Media Access Control device sits between the upstream and downstream portions of the cable modem and act as an interface between the hardware and software portions of the various network protocols.

The microprocessor’s job varies based on whether the cable modem is meant to be a part of a larger system of computers and just to provide internet access without supporting other computers. This translates into variance with regard to how much the microprocessor picks up the slack for the MAC.

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