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I was just wondering, how many levels of the TCP/IP (OSI) Stack reside within a modem or a 3G data card and other similar networking devices?

These devices do have a physical (MAC) address, which indicates that they have a Data Link MAC sub-layer, but what about LLC? What about the rest?

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  • \$\begingroup\$ To be more precise, the devices that would connect to a computer over PCI to provide connectivity. \$\endgroup\$
    – Anshul
    Jun 10, 2013 at 9:10

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TCP/IP is not, strictly speaking, an OSI network - the OSI model only maps exactly onto the OSI network protocols (X25, X400 etc).

There's also an iceberg hidden in your question of which the data card is just the tip: the 3G data network the card connects to ("UTRAN") is not a simple thing and includes several sorts of link. It's almost entirely transparent to the user: from an end-user or operating system point of view, it provides a point-to-point link with an IP address on each end. The user's end may appear as a PPP modem-style link, or an ethernet-style link with a MAC address. The MAC address is there for convenience and may not be globally unique (I have several Sierra Wireless cards on my desk all of which report the same MAC address to Windows "ipconfig"). Although it looks like Ethernet to the operating system, it isn't anything like it at lower layers. This part of the link is essentially made up by the modem card's drivers pretending to be an Ethernet device.

All wireless modems have a substantial amount of link control mechanism ("LLC") in order to find a radio network, register on it, ensure billing security, allocate radio timeslots, and roam between cells. This is "out of band" from the point of view of TCP/IP.

The links from the modem to its base station, the base station to the operator core network, out to the data network operator (which may not be the same as the radio network operator - see "MVNO" such as GiffGaff), and onto the public internet are all encapsulated up into one "link" with an IP address on each end but not anywhere in between.

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  • \$\begingroup\$ Hey, thank you for that information, really helpful. This would mean that the device creates a form of abstraction by providing the device drivers which make it look like a NIC to the Operating System. So, the line between the device's native implementation and the point where the OS Stack takes over the packet processing is somewhere above the L2 layer, i.e. in the Network Layer? If any other wrapper to the device is to be added to add yet another abstraction level (or say, a sub-layer like packet analyser), it would be at this place, on a system-wide level? \$\endgroup\$
    – Anshul
    Jun 10, 2013 at 14:03
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    \$\begingroup\$ Yes, I think so; you'd attach a packet analyser at the driver or OS's "generic network device" layer. IP routing, responding to pings etc. is definitely the responsibility of the OS. \$\endgroup\$
    – pjc50
    Jun 10, 2013 at 14:40

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