Modems
An illustration of data sent using a modem and a regular telephone
line.
General
A modem is a device that converts digital data originating from a terminal
or computer, to analog signals used by voice communication networks such
as the telephone system. At one end, modems convert the digital pulses
to audible tones and convert audio tones back to digital pulses at the
other.
The word "Modem" stands for "MOdulator-DEModulator".
Characteristics
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Transmission speed
Modems are available in different transmission speeds, which are measured
in BPS (bits per second) also called BAUD rate.
Standard modems speeds: 9600 baud, 14400 baud, 28800 baud, 33600 baud,
56800 baud.
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Internal/External
Internal modems are electronic cards. An internal modem is installed
in one of the computer's expansion slot.
External modems are fully functioning external devices. The external
modem is connected to a computer using a serial cable to one of the computer's
serial ports, and draws power from an external power source.
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Error detection and correction
Nowadays most of the modems support advanced error detection protocols
at the hardware level. Detection and correction is achieved using
such protocols as:
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MNP (Microcom Networking Protocol) - a protocol developed by Microcom for
full-duplex, error free communication. MNP was developed to detect,
and correct, errors resulting from noisy telephone connections, and have
five variants.
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ITU-T (The International Telecommunication Union) is a home for various
other protocols, known for their names which start with a V (otherwise
known as the V-Dot Standards).
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V.42: An error control standard for 9600 baud modems. This standard
supports MNP-2 through MNP-4.
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V.34: An error control standard for 28,800 baud modems/14,400 baud
faxes.
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Compression
Due to the the low transmission speeds achieved by modems, most modems
support a variety of data compression protocols. Using these protocols,
the data throughput can rise by an average of 50%.
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MNP-5: Maximizes data transfer rates, and provides compression.
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V.42bis: An extension to V.42 that includes data compression. Also
supports MNP-5.
Related topics
See Also
