I really searched a lot and couldn't find a detailed answer(s), what happens when the Network Interface Card sends a Ethernet frame to the modem? Does the frame directly go inside modem's cpu or ethernet interface send this frame to modem's ram and raise an interrupt? What is the role of cpu in modem? Where the Ethernet interface is located?

I know lots of questions but i really obsessed with hardware internals.

  • \$\begingroup\$ Single electronic design questions are required. \$\endgroup\$ – Leon Heller Jun 2 '15 at 19:33
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    \$\begingroup\$ You'll need to pay more attention to your question, as it is it makes no sense to me. which 'the Network Interface Card' and which 'modem'? Ethernet frames are generaly not send using a modem. How exactly a PC CPU communicates with a card depends on that cards interface (PCI?) and the (hardware and software) design choices the card manufacturer made. \$\endgroup\$ – Wouter van Ooijen Jun 2 '15 at 19:34
  • \$\begingroup\$ Hi Wouter, let me make it clear; suppose that u want to send data to tcp server, operating system's network stack recognize this data and if this data is larger than the Ethernet MTU, it splits this data to frames(Ethernet Frames) and than operating system pass these frames one by one to Network Interface Card, if u are in home network(small network), ur computer's NIC will send these ethernet frame to modem with Ethernet protocol,right? My point is how modem welcomes these packets? \$\endgroup\$ – spartacus Jun 2 '15 at 19:43
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    \$\begingroup\$ Op likely means modem in the general way, cable or fios or dsl modem. \$\endgroup\$ – Passerby Jun 2 '15 at 20:38
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    \$\begingroup\$ @spartacus: In the case you describe (some form of UTP network) there is no modem: the data is not modulated (modem means: mudulator / demodulator). It is sent in 'baseband' way: just as it is, probably with some encoding, which is done inside the NI. \$\endgroup\$ – Wouter van Ooijen Jun 2 '15 at 20:39

A modem, in general terms, has three parts. The first is modem part, changing one signal to another, like dialup modulating and demodulating analog signals, a cable modem demodulating rf frequency or a fios modem changing fiber optic signals into a locally usable form. The second is a router, and the third an Ethernet switch (even if single port).

When a computer sends a packet, it hits the switch. An Ethernet co-processor analyzes the packet, and if on the same network, sends it back out the correct port. It often doesn't even interact with the main cpu. If on a different network, it gets sent up to the routing processor, mainly the cpu of the device. If there is a route to that intended network it forwards it. If that means out through the modem part, then the cpu encapsulates the Ethernet packet into the communication method used for the connection, then encodes it into the physical layer of the network and sends it out, at which point the other end unpacks it, analyses where it should go based on the packet headers, and repack if needed to send out again.

Read this much more accurate and detailed E.E. times article on how Cable Modems work. Trust me, if you really want to know this is gold. http://www.eetimes.com/document.asp?doc_id=1255090

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    \$\begingroup\$ this is what i want,thanks god for sending clever people :) \$\endgroup\$ – spartacus Jun 2 '15 at 21:18
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    \$\begingroup\$ -1 You're totally ignoring GSM modems, Bluetooth modems, Wi-Fi modems, and telephone modems -- they all do. A Wi-Fi router combines a Wi-Fi modem with an Ethernet router. A standalone router, which accepts a Ethernet cable and has several Ethernet ports, is not a modem. Once again your emphasis on Ethernet details is relevant to has nothing to do with GSM, Bluetooth, or telephone modems. \$\endgroup\$ – tcrosley Jun 2 '15 at 21:41
  • \$\begingroup\$ Considering op was specifying consumer isp modems via Ethernet... @tcrosley and I never talked about stand-alone routers or wifi... \$\endgroup\$ – Passerby Jun 2 '15 at 22:35
  • \$\begingroup\$ @Passerby The title of the question is: "How a modem really works?" Modems don't care anything about the Ethernet or packets, they just transmit bits. Given the OP seems confused about this, and brings up Ethernet, doesn't mean Ethernet and packets need to be discussed. It's just adding confusion. \$\endgroup\$ – tcrosley Jun 2 '15 at 22:56
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    \$\begingroup\$ op said that was "exactly what I want". He wanted an explication on how a typical dsl/cable/fios to Ethernet works. @tcrosley \$\endgroup\$ – Passerby Jun 2 '15 at 22:59

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