Modem

modem, (from “modulator/demodulator”), any of a class of electronic devices that convert digital data signals into modulated analog signals suitable for transmission over analog telecommunications circuits. A modem also receives modulated signals and demodulates them, recovering the digital signal for use by the data equipment. Modems thus make it possible for established telecommunications media to support a wide variety of data communication, such as e-mail between personal computers, facsimile transmission between fax machines, or the downloading of audio-video files from a World Wide Web server to a home computer.

Most modems are “voiceband”; i.e., they enable digital terminal equipment to communicate over telephone channels, which are designed around the narrow bandwidth requirements of the human voice. Cable modems, on the other hand, support the transmission of data over hybrid fibre-coaxial channels, which were originally designed to provide high-bandwidth television service. Both voiceband and cable modems are marketed as freestanding, book-sized modules that plug into a telephone or cable outlet and a port on a personal computer. In addition, voiceband modems are installed as circuit boards directly into computers and fax machines. They are also available as small card-sized units that plug into laptop computers.