<hardware, communications> (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
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).
See also acoustic coupler, adaptive answering, baud barf, Bulletin Board System,
Caller ID, SoftModem, U.S. Robotics, UUCP, whalesong.
Usenet newsgroup: comp.dcom.modems.
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