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Modem

The word modem, a blend of "modulator" and "demodulator", refers to a device that modulates an analog "carrier" signal (such as sounds over a telephone) to encode digital information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Primarily used to communicate via telephone lines, modems can be used over any means of transmitting analog signals, from driven diodes to radio.

Early modems used frequency shift keying and simple frequency division duplexing. Later modems used first phase shift keying and then quadrature amplitude modulation with echo cancellation[?] to give progressively higher performance.

Modern audio modems (V.90 and V.92 standards) closely approach the Shannon capacity of the PSTN telephone channel.

Most PSTN dialup modems use the Hayes AT command set to initialise the modem.

ADSL modems are also a kind of modem, the main difference being that they are not limited to audio frequencies over the telephone line. Recent ADSL modems use coded orthogonal frequency division modulation.

Cable modems are also a kind of modem, this time using a range of frequencies originally intended to carry RF television channels. Multiple cable modems attached to a single cable can use the same frequency band, using a low-level media access protocol to allow them to work together within the same channel. Typically, 'up' and 'down' signals are kept separate using frequency division multiplexing.

See modulation for a fuller list of modulation techniques.

Modems are the most popular means of Internet access, UCLA 2001 study of American Internet users shows that 81.3% of them use telephone modem, and 11.5% cable modem, an order of magnitude more than any other method.

See also : 56k line



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