What is the LLC sublayer?

The LLC sublayer (Logical Link Control) is the upper sublayer of the data link layer, responsible for logical connection management and interaction with the network layer.

Put simply: LLC is the "intermediary" between the data link layer and the network layer, which makes the data link layer's work more universal and independent of the specific technology (Ethernet, Wi-Fi, etc.).

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#Where the LLC sublayer sits

The data link layer is split into two sublayers:

• LLC (Logical Link Control) - the upper one, this one
• MAC (Medium Access Control) - the lower one

LLC:

• is closer to the network layer
• does not depend on the type of physical medium

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#Main tasks of the LLC sublayer

#1. Connection to the network layer

LLC:

• receives data from the network layer (for example, IP)
• passes it to the MAC sublayer
• in the reverse direction, passes data upward

Thanks to LLC, the network layer does not need to know which technology is used for transmission.

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#2. Identifying the upper-layer protocol

LLC indicates:

• which protocol is inside the frame (IP, ARP, etc.)

This is needed so that:

data is correctly delivered to the right protocol above

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#3. Logical transmission control

In its classic form, LLC can:

• manage the logical connection
• ensure ordering
• (optionally) participate in error control

In practice, on Ethernet most of these functions are almost never used, since higher layers (TCP) take them over.

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#4. Independence from the data link layer technology

LLC makes it so that:

• IP can work over Ethernet
• IP can work over Wi-Fi
• IP can work over other L2 technologies

Without any changes at the network layer.

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#What LLC does NOT do

Important for an interview: LLC:

• does not manage access to the medium
• does not work with MAC addresses
• does not interact directly with the physical layer

That is handled by the MAC sublayer.

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#Simple analogy

An office secretary:

• receives documents
• understands which department to send them to
• does not care exactly how they get delivered

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#MAC + LLC together (very briefly)

• MAC - how and over what to transmit
• LLC - what is transmitted and who to hand it to above

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#Short version for an interview

The LLC sublayer is the upper part of the data link layer that provides a logical link to the network layer, identifies upper-layer protocols, and makes data transmission independent of the specific L2 technology.