(Modulator/demodulator) - An electronic device for converting between serial data typically from a computer’s serial port and an audio signal suitable for transmission over POTS telephone lines.

Modems are distinguished primarily by the maximum baud rate they support. Baud rates can range from 75 baud up to 56000 and beyond. Data FROM the computer is often at a lower rate than data TO the computer. This is because an average computer user generates much less outgoing traffic than incoming traffic.

Various data compression and error correction algorithms are required to support the highest speeds. Most modern modems support a number of different protocols, and two modems, when first connected, will automatically negotiate to find a common protocol (this process may be audible through the modem or computer's loudspeakers). Some modem protocols allow two communicating modems to renegotiate ("retrain") if the initial choice of data rate is too high and gives too many transmission errors.