terminology – Is an ONT a modem? – Network Engineering Stack Exchange

Short answer: no, an ONT is no modem.

Modem is short for MOdulator/DEModulator and the term is only used for three device classes: telephone modems, DSL modems, and cable modems (thx Cody). These modulate digital data on to voice-grade phone lines or (possibly former) television cable. DSL and cable “modems” often include a router, but technically these are routers that include a modem.

An ONT is an Optical Network Terminator, it connects the ISP’s (passive) optical network with your LAN. Normally, it includes a router as well but in theory it could just be media converter.

Optical networks work by sending light pulses over optical fiber, so you actually could argue that they use amplitude modulation of the used wavelength. Accordingly, calling an ONT “modem” isn’t completely wrong from the physics POV but in a networking context, that term isn’t used by people in the know.

The rule of thumb is that “modem” is only used for devices coding digital data onto an “analog” medium – in the sense of its normal or traditional use. Of course, technically all media are actually analog in their physical nature…