Also known as: modulator-demodulator
A modem (modulator–demodulator) is a device that encodes digital data onto a carrier signal for transmission over a medium not built for it, and decodes it again at the far end.1
Overview
The name describes the job: modulate outgoing data onto a carrier — a tone on a phone line, an RF channel on a coax cable, light on a fiber strand — and demodulate the incoming signal back into bits. A modem is what terminates the link from an internet provider and hands clean Ethernet to a router. Variants include cable, DSL, fiber (ONT), and cellular modems that ride a mobile network. In consumer gear the modem and router are often combined in one box.
Where it fits
The modem is the boundary between a private network and the provider’s carrier medium; downstream of it the router acts as the gateway to the local LAN. The modulation/demodulation idea is the same one at the heart of an SDR — a GopherTrunk decoder demodulates an RF carrier into symbols, just as a modem recovers bits from a line signal.