modem
(Modulator/demodulator) An
electronic device for converting between serial data
(typically {EIA-232}) from a computer and an audio signal
suitable for transmission over telephone lines. In one scheme
the audio signal is 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 can range from 75 baud up to 28800
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 {algorithm}s
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 ("stand-alone"), connected to one of the
computer's {serial port}s. 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: {news:comp.dcom.modems}.
(1996-02-09)