Cable modems use the DOCSIS standard. On the downstream to everyone’s house, everyone’s data packets are combined by a CMTS into electrical signals using various modulation techniques. Modems extract the data addressed to that home and convert it to ethernet. For the upstream, modem’s are given specific time slots to broadcast (time division multiplex) so their signals do not collide with other modems.

It is more complex than that covering both MAC-PHY (layer 2 and 1), but over that it’s just IPs. If you want to know more, I would start looking for introductory material on DOCSIS.

So things are sent through the wire, that needs to be demodulated coming from the ISP, and modulated to go back to the ISP> Hence the title, module and demodulate. It takes the digital information, transforms it into an analog format (most often through coax cable where I live.

It used to be a lot simpler. The “Modem” received a stream of serial data (UART/RS232 + AT control codes).

The digital stream needed to be “encoded” and converted to electrical signals which could be sent on a physical wire… most often the PSTN network (Public service telephone network)… aka “the phone line”.

The audio/analoge technique used to produce the electrical signals was not as you might expect sending “blips” for 1s and silence for 0s. For way out of scope reasons that isn’t efficient. Techniques like PCM or PWM, Pulse code/width modulation. Beyond the scope. However this gets you the “MODulator”, “DEModulator”. Which is you “MoDem”

Today, these still exist. Most would refer to them as “dial up modems”. Rare. We tend to have “home broadband” technologies today which range from plain old twisted pair phone lines carrying 100s of times more bandwidth than an “analogu phone line”, but over the same wires. We have old “Coax” cable for “Cable TV” carrying high speed internet. We even now have pure optical fibre connections in to the home.

At this point, really, it can be argued there is no modem. Certainly not in the terminology used to previously describe one. What often gets refered to as the “Modem” is usually an ONT or similar ePon/docsis device controlled by an ISP which “converts” or “bridges” the optical (or other broadband signal) to “Ethernet”. Most, not all then additionally handle the authentication and “PPP” Peer to peer protocol used to transmit your data over this “You-ISP network”. In larger enterprise these components can be broken down into individual (or multitudes of) different boxes. ONT, PPPoE client, Router, Firewall, Switch, Wifi Access point. In most homes they will either be a single box provided by the ISP or 2 boxes with one provided by the “Network operator” and the other by your “Internet retailer.”

Machine Spirit.

How deep are you willing to go.

I’m a computer engineer, I design and develop embedded systems like modems, TCU, ECU and many others.

If you want to understand how a modem works, truly understand everything. You need to first understand some basic physics primarily electromagnetism and the property of waves.

From there you can jump to a higher abstracted level such as amplitude modulation, frequency modulation and phase modulation. From this you can try to read up on Quadrature amplitude modulation.

You can also use this to understand demodulation.

Then jump back to the lower level and study up signal theory and how these things work.

Honestly the level of information to fully understand how a device like a modem is huge.

If you love this stuff and you’re still young I would encourage you to become a computer engineer with a focus in asic. It’s not easy but the stuff you work on is rewarding.