Home Networking

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

submitted 10 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 10 months ago (1 children)

The modem device bridges between the local network and the WAN. The physical medium of the WAN is designed to cover longer distances.

Modems used to use audio signaling to send data over phone lines. Nowadays they generally use higher frequencies such as radio signals or optical to send data over radio link, DSL, cable, or fiber media.

The modem device encodes and error checks/corrects, converts and transmits the data -- between the different LAN-WAN protocols / physical layers.

MoDem stands for modulator-demodulator. That implies an audio or RF "baseband" signal being "modulated" (the carrier frequency shifted) to encode the data onto the physical layer.

However, modern modems use DAC/ADC (analog-digital/digital-analog converter) modules to transmit/receive the analog signals and directly convert to/from digital (logic level) signals. They don't need the intermediate step of generating a baseband carrier frequency and then modulating it. Hence the modern modem may actually be an SDR (software-defined radio). SDR can cover a huge frequency range and any encoding/modulation -- defined only by the software!

In fact, some newer radio transceivers use SDR for its flexibility. For example, Flex / Anan radio transceivers. Some designers have repurposed commodity modem chips to create less expensive SDR transceivers. There are now Ham Radio transceivers based on this concept.

[–] [email protected] 2 points 10 months ago (1 children)

A modem device bridges between the local area network (LAN) and the wide area network (WAN).

Nope. A modem can be a part of a box doing this but not always.

[–] [email protected] 1 points 10 months ago

Yes, of course. Feel free to explain it better!