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What's the type of multiplexing used by Ethernet (LLC layer)?

It says in Wikipedia "The LLC sublayer provides multiplexing mechanisms that make it possible for several network protocols (e.g. IP, IPX and DECnet)". I don't understand this multiplexing principle. What do they mean by "multiplexing"? I can't find the information.

Thanks

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  • You should read up about EtherType and SNAP and then come back when you've got a real question. Homework is explicitly off-topic here, see the help center.
    – Zac67
    Feb 3, 2022 at 19:38
  • That's actually not homework. I'm improving my knowledge of Ethernet and I could not find any information about multiplexing. Anyway, thanks for the information
    – ragnar
    Feb 3, 2022 at 19:46
  • What you mean by multiplexing used by Ethernet (LLC layer)? - The LLC exists for only one reason - to know if the upper layer is IPv4 or IPv6.
    – new_py
    Feb 3, 2022 at 20:30
  • It says in Wikipedia "The LLC sublayer provides multiplexing mechanisms that make it possible for several network protocols (e.g. IP, IPX and DECnet)". I don't understand this multiplexing principle. What do they mean by "multiplexing" ?
    – ragnar
    Feb 3, 2022 at 20:36
  • I'm going to update my question because it's indeed not clear
    – ragnar
    Feb 3, 2022 at 20:55

1 Answer 1

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Multiplexing is the use of a single communication channel (medium, resource) for multiple, mostly independent purposes. An example from the analog world is a traffic light that could be considered to use time-division multiplexing (TDM) to enable the use of a street crossing for vehicles going in intersecting directions. There are many other forms of multiplexing.

Ethernet frames carry a payload. For multiple types of payload (IPv4, IPv6, IPX, ARP, BPDUs, AppleTalk, PPPoE, ...), their type needs to be specified in a universal way (rather than looking at the payload and trying to interpret it). That way enables some kind of deflector to redirect each frame received by a NIC to the service that handles that type. Without multiplexing, you'd need a dedicated NIC and network for each type of traffic.

With Ethernet, the EtherType field is used for that purpose or, even more universally, a SNAP header.

For example, an IPv4 instance sets the EtherType value for its frames to 0x0800. It passes those frames to a NIC to transmission. On the destination, the frames are received and their EtherType value indicates that they need to be passed to the IPv4 instance there, enabling the IPv4 stacks to talk to each other, without exclusive access to Ethernet.

The same happens with IPv6 (EtherType 0x86DD), PPPoE (EtherTypes 0x8863 and 0x8864), or any of the other protocols that can run on top of Ethernet (or another protocol using the same mechanism).

On the other hand, imagine a mail service that transport envelopes between two commercial buildings. As long as there's no writing on the envelopes, the sender may differ but the receiver is always the same. That receiver would have to open the envelope and figure out where to pass the letter on. That might require a lot of guesswork and if the letter is in a foreign language, they'd be lost.

Instead, the sender marks each envelope with a number/name/whatever that unambiguously identifies the intended company in the other building, where the mail person can easily identify the purpose and destination of the letter.

Of course, using a fully qualified address on a letter envelope in real life enables us to multiplex a single instance of mail service between arbitrary senders and recipients worldwide. Perhaps you could compare the EtherType to writing the destination country on the envelope, enabling delivery even when the actual address is in Chinese.

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  • I read from wikipedia this text """"The LLC sublayer is primarily concerned with multiplexing protocols transmitted over the MAC layer (when transmitting) and demultiplexing them (when receiving)"""'-----what does mean by demultiplexing?
    – S. M.
    Apr 9, 2022 at 11:20
  • Multiplexing (here) means that you use a single link-layer instance but you can use multiple protocols (or instances) on top, (IPv4, IPv6, IPX, LLDP, MSTP, LACP, ...).
    – Zac67
    Apr 9, 2022 at 12:44
  • one host could use more than one layer3 protocol at the same time, for example one host could have ipv4 address as well as ipv6 address?
    – S. M.
    Apr 9, 2022 at 14:21
  • Yes, any host can use any L3 protocol and any L3 protocol can be used by any subset of hosts. Running IPv4 and IPv6 in parallel is called dual stack and pretty much standard today.
    – Zac67
    Apr 9, 2022 at 15:11
  • But multiplexing mean more than Ethertype value could be used in one Ethernet frame?
    – S. M.
    Apr 9, 2022 at 15:36

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