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. Because each medium has its own physics, the modulation scheme differs, but the role is always the same: turn a stream of bits into something the line can carry, and back again.
A modem is what terminates the link from an internet provider and hands clean Ethernet to a router. It speaks the provider’s line protocol on one side and standard local networking on the other, so everything downstream can ignore how the last mile actually works.
Variants include cable, DSL, fiber (an ONT, or optical network terminal), and cellular modems that ride a mobile network. In consumer gear the modem and router are frequently combined into one box, which blurs the boundary but not the two distinct jobs inside.
Kinds of modem
Each type is matched to the medium the provider runs to the premises:
| Kind | Carrier medium | Modulates onto |
|---|---|---|
| DSL | Telephone twisted pair | High-frequency tones |
| Cable | Coaxial cable (DOCSIS) | RF channels |
| Fiber (ONT) | Optical fiber | Pulses of light |
| Cellular | Radio (LTE / 5G) | RF carrier over the air |
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 very same one at the heart of an SDR — a GopherTrunk decoder demodulates an RF carrier into symbols exactly as a modem recovers bits from a line signal, which is why the two fields share so much vocabulary.