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modem [modulator/demodulator], an external device or internal electronic circuitry used to transmit and receive digital data over a communications line normally used for analog signals. A modem attached to a computer converts digital data to an analog signal that it uses to modulate a carrier frequency. This frequency is transmitted over a line, frequently as an audio signal over a telecommunications line, to another modem that converts it back into a copy of the original data.

Synchronous data transmission uses timing signals in the data stream along with transmitted bits of uniform duration and interval. This permits the receiving modem to ignore spurious signals that do not conform to the anticipated signal. Asynchronous data transmission relies instead on various error-correcting protocols. Although most modems are either of the synchronous or asynchronous variety, some employ both methods of communication. Wireless modems send or receive data as a radio signal. A fax modem enables a computer to send and receive transmissions to and from a fax machine (see facsimile) or another fax modem.

Modems were first used with teletype machines to send telegrams and cablegrams. Digital modems were developed from the need to transmit large amounts of data for North American air defense during the 1950s. The first commercial modem was introduced in 1962. Dennis C. Hayes invented the personal computer modem in 1977, marking the emergence of the online and Internet era. In the beginning modems were used primarily to communicate between data terminals and a host computer. Later the use of modems was extended to communicate between hosts in networks. This required modems that could transmit data faster, leading to the introduction of compression techniques to increase data rates and error detection and correction techniques to improve reliability. However, still faster transmission speeds were required.

A traditional modem, operating over traditional—mostly analog—phone lines, has a data transmission speed limit of about 56 kilobits per second. A specification for an Integrated Services Digital Network (ISDN), which allows wide-bandwith digital transmissions using the public switched telephone network, was introduced in 1984. A phone call can transfer 64 kilobits of digital data per second with ISDN and 128 kilobits with dual-channel ISDN. ISDN connections are used to provide a wide variety of digital services including digital voice telephone, fax, e-mail, digital video, and access to the Internet.

Faster still are the Digital Subscriber Line (DSL) protocol, introduced in the early 1990s, and the cable modem, introduced in the late 1990s. Each of these has a maximum data transfer rate of 1.5 megabits per second. DSL provides a broadband digital communications connection that operates over standard copper telephone wires. The connection requires a DSL modem, which splits transmissions into a lower band for ordinary telephone calls and an upper band for digital data. The drawback of DSL is that connected computers must be within a few miles of the closest transmitting station. A cable modem modulates and demodulates signals like a telephone modem but it transfers data much more quickly over cable lines—primarily fiber-optic or coaxial cable. Broadband over Power Lines (BPL) modems work similarly but utilize electrical lines to transfer data; BPL modems are plugged into electrical outlets. BPL modems may be used to access an Internet service provider over the local power lines, or they may use the wiring within a building to create a network for the computers there.

See also baud; code; modulation.

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A combination modulator and demodulator at each end of a telephone line to convert binary digital information to audio tone signals suitable for transmission over the line, and vice versa. Also known as dataset. Derived from modulator-demodulator.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


A device that converts the digital signals produced by terminals and computers into the analog signals that telephone circuits are designed to carry. Despite the availability of several all-digital transmission networks, the analog telephone network remains the most readily available facility for voice and data transmission. Since terminals and computers transmit data using digital signaling, whereas telephone circuits are designed to transmit analog signals used to convey human speech, a device is required to convert from one to the other in order to transmit data over telephone circuits. The term modem is a contraction of the two main functions of such a unit, modulation and demodulation. The device is also called a data set.

In its most basic form a modem consists of a power supply, transmitter, and receiver. The power supply provides the voltage necessary to operate the modem's circuitry. The transmitter section contains a modulator as well as filtering, wave-shaping, and signal control circuitry that converts digital pulses (often input as a direct-current signal with one level representing a digital one and another level a digital zero) into analog, wave-shaped signals that can be transmitted over a telephone circuit. The receiver section contains a demodulator and associated circuitry that is used to reverse the modulation process by converting the received analog signals back into a series of digital pulses (see illustration). See Data communications, Demodulator, Electrical communications, Electronic power supply, Modulator, Wave-shaping circuits

McGraw-Hill Concise Encyclopedia of Engineering. © 2002 by The McGraw-Hill Companies, Inc.


Computing a device for connecting two computers by a telephone line, consisting of a modulator that converts computer signals into audio signals and a corresponding demodulator
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005


(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, baud barf, Bulletin Board System, Caller ID, SoftModem, U.S. Robotics, UUCP, whalesong.

Usenet newsgroup: news:comp.dcom.modems.
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(MOdulator-DEModulator) A device that adapts one type of signal to another. Until the late 1990s, the term referred mostly to analog modems, which allow a computer or terminal to transmit data over a standard dial-up telephone line. Since the advent of cable and DSL connections, the term commonly refers to other types (see cable modem, cellular modem, DSL and VoIP modem).

The remainder of this definition pertains only to analog dial-up modems, which convert digital data pulses from the computer to audio tones that analog telephones accept. New computers no longer come with an analog modem; however, one can be easily added via USB. For control, most analog modems use the Hayes AT instruction set (see modem status signals and AT command set).

From 300 to 56,000 Bits Per Second
At 56 Kbps downstream, the ITU's V.92 was the last standard for dial-up modems. Decades ago, the first modems transmitted 300 bps, and while 56 Kbps (56,000 bps) might seem like a huge leap, it is extremely slow for Web page retrieval. For example, a high-speed cable modem can support 100 Mbps (100,000,000 bps). See V.92.

Like a Telephone
A modem dials the line and answers the call. While performing digital-to-analog and analog-to-digital conversion, it also provides error correction and data compression. A modem's automatic feature negotiation adjusts speed downward to synchronize with a slower modem at the other end as well as to accommodate noisy lines.

The Sportster Modem
A hot-selling product in the 1990s, people went online in record numbers. Although internal modems became the norm, external units have the advantage of status lights for troubleshooting connections. (Image courtesy of 3Com Corporation.)

A Rack of Hayes Modems
The pioneer in personal computer modems, Hayes set the standard for control commands. This rack-mounted model allowed any of its 16 modem cards to be hot swapped. Devices such as this were installed by the thousands at large ISPs. (Image courtesy of Hayes Microcomputer Products, Inc.)
Copyright © 1981-2019 by The Computer Language Company Inc. All Rights reserved. THIS DEFINITION IS FOR PERSONAL USE ONLY. All other reproduction is strictly prohibited without permission from the publisher.
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