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MODEM
(Modulator/demodulator). An electronic device for converting between serial data (typically RS-232) from a computer and an audio signal suitable for transmission over telephonelines. The audio signal is usually composed of silence (no data) or one of two frequencies representing 0 and 1. Modems are distinguished primarily by the maximum baud rate they support. Baud rates which can range from 75 baud up to 19200 and beyond. Data to the computer is sometimes at a lower rate than data from the computer 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. 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, 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).
See also acoustic coupler, adaptive answering, baud barf, Bulletin Board System, Caller ID, SoftModem, U.S. Robotics, UUCP, Usenet newsgroup: comp.dcom.modems.