modem

(MOdulator-DEModulator) Until the late 1990s, the term referred to a device that allows a computer or terminal to transmit data over a standard dial-up telephone line. Since the advent of cable and DSL connections, the term may also refer to high-speed broadband modems (see cable modem, DSL and cellular modem).

This definition pertains only to dial-up modems, also known as "analog modems," which convert digital pulses from the computer to audio tones that analog telephone lines can carry, and vice versa. V.92 is the last dial-up standard, providing a data rate of 56 Kbps (see V.92).

New computers geared for home users may or may not have a built-in analog modem, while those targeted for the office typically do not. However, a modem can be added internally via a PCI card, or externally via a USB port.

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 number of lower speeds are also supported to accommodate old modems or negotiate downward on noisy lines. Over the years, speeds evolved from 300 bps to 56 Kbps.

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.)

Download Computer Desktop Encyclopedia to your iPhone/iTouch

modem	N56 Modem
Cellular Modem	On_board_modem