(Modulator/demodulator) An electronic device for converting between serial data (typically EIA-232) from a computer and an audio signal suitable for transmission over a telephone line connected to another modem. In one scheme the audio signal is composed of silence (no data) or one of two frequencies representing zero and one.
Modems are distinguished primarily by the maximum data rate they support. Data rates can range from 75 bits per second up to 56000 and beyond. Data from the user (i.e. flowing from the local terminal or computer via the modem to the telephone line) is sometimes at a lower rate than the other direction, on the assumption that the user cannot type more than a few characters per second.
Various data compression and error correction algorithms are required to support the highest speeds. Other optional features are auto-dial (auto-call) and auto-answer which allow the computer to initiate and accept calls without human intervention. 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 the two modems to renegotiate ("retrain") if the initial choice of data rate is too high and gives too many transmission errors.
A modem may either be internal (connected to the computer's bus) or external ("stand-alone", connected to one of the computer's serial ports). The actual speed of transmission in characters per second depends not just the modem-to-modem data rate, but also on the speed with which the processor can transfer data to and from the modem, the kind of compression used and whether the data is compressed by the processor or the modem, the amount of noise on the telephone line (which causes retransmissions), the serial character format (typically 8N1: one start bit, eight data bits, no parity, one stop bit).