A Modem (MODulator-DEModulator) is a device that converts computer data to modulation of a carrier signal for transmission. Modems were used on phone lines (with speeds from 110 to 56,600 bits per second) for many years to access online services, bulletin board systems, and dialup Internet connections, and still are in some places, though most people use broadband and wireless networking now instead. Cable modems are used to connect to the Internet service offered through cable TV wires.
Many varieties of modems are controlled by computers via the Hayes command set.
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