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 witrh [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.

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.

See modulation for a fuller list of modulation techniques.