(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 commonly refers 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 56,000 bps (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 modem's data compression has no value. Modems also have automatic feature negotiation, which adjusts to the speed and hardware protocols of the modem on the other end.
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.
This was a hot-selling product in the 1990s as people went online in record numbers. External units have the advantage of status lights for troubleshooting connections. Internal modems since became the norm. (Image courtesy of 3Com Corporation.)
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 like this were installed by the thousands at large ISPs. (Image courtesy of Hayes Microcomputer Products, Inc.)
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