modem

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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modem

Electronic device that converts digital data into analog (modulated-wave) signals suitable for transmission over analog telecommunications circuits (e.g., traditional phone lines) and demodulates received analog signals to recover the digital data transmitted. The “modulator/demodulator” thus makes it possible for existing communications channels to support a variety of digital communications, including e-mail, Internet access, and fax transmissions. An ordinary modem, operating over traditional phone lines, has a data transmission speed limit of about 56 kilobits per second. ISDN lines allow communications at over twice that rate, and cable modems and DSL lines have transmission rates of over a million bits per second.

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(MOdulator-DEModulator) Until the late 1990s, the term stood for a device that allowed a computer or terminal to transmit data over a standard dial-up telephone line. Since the advent of high-speed cable and DSL connections, modem may refer to devices for low-speed dial-up or high-speed broadband.

This definition pertains only to dial-up modems, which convert digital pulses from the computer to audio tones that an analog telephone line is set up to handle and vice versa. V.92 is the last dial-up standard, providing a data rate of 56 Kbps (see V.92). For high-speed broadband modems, see cable modem, DSL and cellular modem.

Like a Telephone
A modem dials the line and answers the call. It performs the digital-to-analog and analog-to-digital conversion while controlling transmission speed. Whatever the top speed, some lower speeds are supported to accommodate old modems or negotiate downward on noisy lines. Over the years, speeds evolved from 300 bps to 56 Kbps.

Built In Today
New computers geared to the home user generally have a built-in modem, while those targeted for the office may not. A modem can be added internally via a PCI card, or externally via a serial or USB port. In Windows, the Dial-up Networking "Make New Connection" wizard takes you through setting up your modem to dial your Internet provider.

Error Correction and Compression
Modems have built-in error correction (V.42) and data compression (V.42bis, MNP 5). On files that are already compressed, the hardware data compression adds little value, because it cannot make compressed files smaller. Modems also have automatic feature negotiation, which adjusts to the other modem's speed and hardware protocols.

The Hayes Standard
Most modems use the Hayes AT command set, which are machine instructions for modem control. The term modem has been used as a verb; for example, "I'll modem you later." See modem status signals and AT command set.

The Sportster

The Sportster was one of the hottest-selling products in the 1990s as people went online in record numbers. External units have the advantage of status lights, which help troubleshoot connections. Internal modems have since become the norm. (Image courtesy of 3Com Corporation.)

A Rack of Hayes Modems

Hayes was the pioneer in personal computer modems and set the standard for control commands. Hayes produced a full line of modems from singles to rack-mounted units such as this model with 16 modem cards. Allowing card replacement with power on, rack-mounted modems like these are found by the thousands at large ISPs. (Image courtesy of Hayes Microcomputer Products, Inc.)

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(hardware, communications)

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 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.