Google searches have only yielded 5th grade level examples (“the modem talks between your ISP and your home network!”) or articles I would need a degree to understand. Can anyone provide an explanation that’s somewhere in between the two? I understand the fundamentals of how the Internet works, and how LAN works regarding a router and individual devices, but I’m curious to know more about the link between those.
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.”
I’ll give it a shot.
In the past, before the internet existed, homes were connected to the national phone system. The national phone system is a bunch of interconnected electric lines that are designed to transport electricity in the frequencies used for speech / sound.
When internet became a thing, people needed electric lines to transport data from their internet providers to their home. Instead of building new electric lines, which would be very expensive, tech companies created a technique called “modem”.
A modem transforms computer data into sound data, which can travel through phone lines. Once the sound data reaches the internet provider, another modem turns it back into computer data.
As others have noted a modem is a modulator / demodulator.
Most modems are designed into other things. Like cable coax modem/router/switch/wi-fi access point. Or other things. Or in a cell phone it converts bits internal to the phone into radio signals or back the other way. Many things people call a modem is really a more complicated box with a modem built into it.
In the basic definition of the term it is bits <-> analog wave forms. Back in the day it was land line modems and the wave forms could be heard by most people if they listened “in”. Actual FAX machines (not email pretending to be a FAX) still do this. Connected to the “bits” side of a modem is usually a circuit that takes those bits and organizes them in agreement with the other end of the connection. Mostly these days Ethernet.
Most Fiber setups do NOT have a modem as they have networking bits already in the light in the fiber. They do “media conversion” between fiber and copper.
And if you want to dig deeper look into how DSP (Digital Signal Processing) works, Fourier Transforms, and how Wi-Fi radios encode things, and so on.
It modulates and demodulates. It’s right in the name 🤷 duh /s
Take a look at old issues of Byte Magazine where they will explain how to make a cassette tape backup system for your 1975 computer. Basically, you need to turn ones and zeros into tones that can be recorded on cassette tape, and you need to do it in a way that strings of 111111’s and 00000’s get the right count of '1’s and '0’s. There are a bunch of different ways of mapping tone changes to '1’s and '0’s, (frequency shift keying, phase shift keying, etc) and it’s pretty amazing all the ways to pack bits into tones (how many tones do you want to distinguish - 2? 4? more?).
You could also look up how the old 300 baud (essentially bits per second) modems encoded bits over telephone lines that only provided 3Khz of bandwidth, and how more sophisticated encodings allowed 1200 baud, 2400 baud, and ultimately 56K baud.
