Modem

A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The name derives from this, standing for modulator-demodulator. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog signals, from driven diodes to radio.

The most familiar example is a voice band modem that turns the digital data of a personal computer into modulsated electrical signals in the voice frequency range of a telephone channel. These signals can be transmitted over telephone lines and demodulated by another modem at the reciver side to recover the digital data.

Classification
Modems are generally classified by the amount of data they can send in a given time unit, normally measured in bits per second (bit/s, or bps). They can also be classified by the symbol rate measured in bud, the number of times the modem changes its signal state per second. For example, the ITU V.21 standard used audio frequency-shift keying, a.k.a tones, to carry 300 bit/s using 300 bud, whereas the original ITU V.22 standard allowed 1,200 bit/s with 600 bud using phase shift keying.