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modem

(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.)






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