Term:modem [Modulator Demodulator]
Modem is short for Modulator/Demodulator.
Technically, a modem converts an analog signal to a digital one, and vice-versa. Originally modems were the devices that connected a computer to a telephone line and converted signals between audible tones that could be transmitted on a normal telephone equipment (POTS), and the digital signals required by computers. The act of converting a digital signal to audio is “modulation”, and the reverse is “demodulation” – hence the term.
The term is being commonly misused to refer to almost any device that converts between ethernet (the digital signal) and the various ways that connectivity is delivered by ISPs. DSL, cable and cellular modems may not actually convert to and from analog/audio tones, but between differing types of digital signals – a process where modulation and demodulation is not actually required.
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A modulator-demodulator, or simply a modem, is a hardware device that converts data from a digital format, intended for communication directly between devices with specialized wiring, into one suitable for a transmission medium such as telephone lines or radio. A modem modulates one or more carrier wave signals to encode digital information for transmission, and demodulates signals to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded reliably to reproduce the original digital data.
Modems can be used with almost any means of transmitting analog signals, from light-emitting diodes to radio. A common type of modem is one that turns the digital data of a computer into a modulated electrical signal for transmission over telephone lines, to be demodulated by another modem at the receiver side to recover the digital data.