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What differs the LLC sublayer from the MAC sublayer?

From what I've understood
- The MAC sublayer does what the PHY layer should be doing: detection (error, collision), channel listening, analog-to-digital and digital-to-analog translation, etc.
- While the LLC sublayer decides what to logically do with that information, e.g. retransmit the frame if an error occurs.

I know that the MAC sublayer also provides addressing on broadcast networks.

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The MAC sublayer does what the PHY layer should be doing: detection (error, collision), channel listening, analog-to-digital and digital-to-analog translation, etc.

No. The physical layer moves bits from one end of a cable to the other end. Channel arbitration (together with MAC), line code, analog stuff etc is all physical layer.

What differs the LLC sublayer from the MAC sublayer?

The MAC sublayer belongs to the data link layer. Its main purpose is framing and to direct those data frames across a network using some kind of addressing (many protocols use MAC addresses). Commonly, the MAC sublayer also handles error detection.

The LLC sublayer sits on top of the MAC layer within the data link layer. Its main purpose is to multiplex the protocols that may concurrently use the MAC layer (like IPv4, IPv6, IPX, Ethertalk, ...). It may also be used to retransmit error frames detected by MAC.

Mulitplexing the data link layer (L2) means that multiple upper-layer protocols can share the same network - especially network layer ones like IPv4, IPv6, IPX, but also other protocols using L2 like xSTP, LLDP, FCoE, PPoE, etc).

That sharing works on a frame-to-frame basis by indicating each frame's payload association. On plain Ethernet, usually just the EtherType field does that job but other L2 protocols or variants require a distinct LLC sublayer.

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  • Your explanation is, in my opinion, very useful in the sense that I can understand what are the attributes of the LLC and MAC individually. However, how does MAC perform the addressing if the destination MAC address comes from software (the ARP cache is stored in RAM)? Also it would be great if you could detail a bit on multiplexing, which I'm personally having issues understanding. Does multiplexing mean that two different network layer protocols can go on the same frame (at the same time), or just that they access the media in the same way through the NIC?
    – Neox
    Commented Dec 28, 2022 at 18:28
  • The destination MAC address (e.g. from ARP) is put in the frame and the data link layer forwards the frame to its destination. That forwarding can be done in either hardware or software, but it's mostly done in hardware by switches. Multiplexing enables a single data link layer protocol to carry different next-layer protocols concurrently (not within a single frame though) - for Ethernet and the like check out Ethertype.
    – Zac67
    Commented Dec 28, 2022 at 19:17

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