modem

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 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).

See also acoustic coupler, adaptive answering, baudbarf, Bulletin Board System, Caller ID, SoftModem,

U.S. Robotics, UUCP, whalesong.

Usenet newsgroup: news:comp.dcom.modems.

(2002-05-04)