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]

31 comments fedilink hide all child comments

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

For cable internet there is a CMTS at the cable TV headendThe return path, your uplink covers from 5mhz to 42mhz. Your downlink 54 MHz to 860 MHz.

The CMTS modulates on the downstream using QAM this encodes on this analog signal via 256QAM.

To correct for Errors Reid Solomen encoding is used.

Your baseband data (digital) is converted into symbol bits than modulated in an broadband signal (analog) with 256 points of interest that are monitored for changes in frequency, amplitude and by extension phase.

Your Modem, takes these QAM signals demodulates the analog signal to extract the symbol bits. Then it turns it back into digital data and it comes out your ethernet port as baseband.

The reverse process is done on your upstream.

Analog is used for all but fiber optic data networks as its far more efficient per mhz of radio spectrum used. If you have fiber to the home its digital baseband the whole way like your ethernet cable.

Rather than broadband like dsl,catv, wifi, LTE, 5g, satellite, etc.

Multiple channels can be put together to enable upwards of 10 gigaits per second of capacity on a modern catv plant.

This all happens on layer 1 of the OSI model, (physical) until its handed off via a bridge to your port you plug your modem in.

American politicians have become confused between the meaning of broadband and baseband, rather confusingly trying to redefine it as a measure of speed rather than a technical term to differentiate between digital and analog signaling.

Technically an ONT for fiber to the home is a bridge (layer 2) or router (layer 3) if it doesn't have a coax port for television.

This is a grossly oversimplified technical explanation, if I went in depth this would be probably 3-4 chapters in a technical manual. 5+ if I included pictures and examples of how the framing bandwidth (analog in mhz) and signaling works.