Wow, so many lousy answers in here. …

The first question should be; Which kind of modem are you inquiring about as many different technologies exist.

There are many different types of Modems and each is unique, some examples include …

1. Radio frequency modulator demodulator (Modem)

2.Telephone Modems.Dual Tone Multi Frequency (DTMF)

3. Satellite Modems.

4. Digital Subscriber Line (DSL)

5. Cellular Modems.

6. Cable Television Modems.

ASSUMING you are referring to the one from a Cable TV ISP which hands off an RJ-45 Ethernet connection it’s known as a …

“Hybrid fiber-coaxial” (HFC) Network.

https://en.m.wikipedia.org/wiki/Hybrid_fiber-coaxial

The protocol which runs on such networks is known as the

“Data Over Cable Service Interface Specification (DOCSIS)”

Patented by: Robert Cruickshank III who built and led the CableLabs team that developed, tested, and certified the DOCSIS standard cable modem.

https://en.m.wikipedia.org/wiki/DOCSIS

As for the actual electrical/ electronic mechanism enabling the protocols algorithm, it’s known as …

“Phase-shift keying”

https://en.m.wikipedia.org/wiki/Phase-shift_keying

☆ The bottom line though,

Is that it’s really not even technically a modem, rather it’s actually a protocol converter which translates one digital standard into another.

How’s that for an answer 🤔

Happy Turkey!