Home Networking

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How the f*ck does a modem work? (alien.top)

submitted 11 months ago by [email protected] to c/[email protected]

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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.

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[–] [email protected] 1 points 11 months ago (3 children)

The straight answer is a modem bridges and translates between layers in the OSI model, typically layers 1&2 (physical and data) on one side (your coax for cable or fiber) and layers 3&4 (IP and TCP, the typical protocols for home networking) on the other side. We used to call modems the devices that would do that translation over a phone line, and we would call the devices that connected two different network types bridges (like coax or microwaves to Ethernet or token ring), but now we call what are really bridges modems for home networking deployments.

To get between ELI5 and PhD, dig into some good telecom textbooks and focus on the fundamentals like the OSI model, time-division multiplexing (TDM), frequency-division multiplexing (FDM), DOCSIS, how to send binary data over radio, etc.

[–] [email protected] 2 points 11 months ago (1 children)

But all these layers exist on both sides of a modem. Really the modem is translating between a type of physical/data layer on one side (e.g. twisted pair ethernet) to a different physical/data layer on the other side (e.g. docsis)

[–] [email protected] 1 points 11 months ago

That's why technically, it's a "bridge". When DOCSIS was deployed for the consumer market, marketing continued to sell the electronics box at the point of demarcation (DEMARC) as a "modem".

[–] [email protected] 1 points 11 months ago

Nice concise explanation. Would like to add that a modem originally were useful to be able to modulate and demodulate analog signals to and from digital signals due to limitations on most phone lines to allow for higher speeds than was practical if trying to transmit pure digital signals on them due to bad noise suppression.

[–] [email protected] 1 points 11 months ago

OSI Layers are bullshit leftovers from the past, they're no longer applicable in modern computing.