:: The working of a cable modem

A phone modem modulates a digital signal from the computer into an analog signal for the phone lines, and demodulates the analog phone signal into a digital one.

This is not the case with cable modems. The initial cable modems were on these lines, but now cable modems are turning towards a fully digital network.

The key components of a cable modem include a tuner, a modulator, demodulator, MAC and the interface. The tuner is a device for frequency matching with the radio frequency signal being received from (or sent into) the cable network. The tuner has a biplexer since it has to handle separate frequencies for the upload and download data. The tuner connects the modulator/demodulator units to the cable. The demodulator receives radio frequency signals, converts them from analog to digital signals, performs error correction and synchronization and transmits this to the computer. The modulator works in the reverse direction transforming the digital data from the computer into the radio frequency analog signal. The Media Access Control (MAC) is a mechanism between the upload and download paths that is used by the service providers to control the bandwidth being used. The interface can be PCI, Ethernet or USB and this connects your cable modem to your PC. A set top box is used for a direct connection from the cable to your TV.

Besides external modems, one can also use internal modems that are usually fitted inside a desktop PC. Set top boxes can provide you access to the Internet using your TV set and a keyboard.

In the downstream direction, the digital data is modulated and then placed on a 6 MHz television channel, between 50MHz and 750 MHz. Currently, 64 QAM (quadrated amplitude modulation) is the preferred downstream modulation technique, offering up to 27 Mbps per 6 MHz channel. This signal can be placed in a 6 MHz channel adjacent to TV signals on either side without disturbing the cable television video signals. The upstream is transmitted between 5 and 42 MHz. This tends to become very noisy, due to RF interference and impulse noise. So most manufacturer use QPSK (Quaternary Phase shift keying), as it is more robust. But it is slower than QAM. QAM and QPSK are methods to modulate digital signals to radio frequency signals.