Definition: modem

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

This definition pertains only to analog dial-up modems, which convert digital data pulses from the computer to audio tones that analog telephones accept. The first modems transmitted 300 bits per second (300 bps). Although many early computers had built-in analog modems, one could be added via USB. For control, modems used the Hayes AT instruction set (see modem status signals and AT command set).

Dial the Line and Answer the Call
Like a telephone, an analog modem dials the line and answers the call. While performing analog-to-digital and digital-to-analog conversion, the modem also provides error correction and compression. Its automatic feature negotiation adjusts speed downward to synchronize with a slower modem at the other end, as well as to accommodate noisy lines.

The Hot-Selling Sportster

A very popular modem in the 1990s as people went online in record numbers. Although analog modems were built into many computers, external units had the advantage of status lights for troubleshooting. (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. Huge quantities of this equipment were installed at large ISPs. (Image courtesy of Hayes Microcomputer Products, Inc.)

From 300 to 56,000 to 500 Million and More

Debuting in 1999, V.92 was the last ITU standard for dial-up modems. At 56,000 bps, V.92 was a huge leap from the first modems at 300 bps. Today, such speed seems archaic when compared to this Xfinity residential cable service at 564.65 Mbps. Gigabit speeds (Gbps) are also available from various Internet providers. See ISP and V.92.