LLC Layer (Logical Link Control): Data Link Layer Of OSI Model

Short Bytes: If the MAC layer is responsible for the medium access then LLC layer is responsible for the logical link control for the data link layer. This is the property that makes LLC an important layer in the data link layer. In this article, we will take a deeper look into the LLC layer.

As we talked earlier in the article describing OSI Model and its 7 layers, the data link layer is the penultimate or the second lowermost in the OSI model.

The data link layer is made up of two sublayers:

MAC (Media Access Control) Layer
LLC (Logical Link Control) Layer

Both of these two sublayers are responsible for different functions for the data link layer. Today, we are going to talk about the LLC layer in details.

LLC layer:

LLC layer is also known as the logical link control. As it is evident from the name itself that for the data link layer, the LLC layer serves the purpose of providing end to end flow, error control, and multiplexing different protocols over the Mac layer of the data link layer.

For example:

Let’s assume that I am running Windows 10 and I want to do a video chat with my friend who is sitting across oceans in Germany. Now, here is what happens when we start the video chat:

Multiplexing

So when I start video chatting, my application, that maybe Skype, starts generating video data packets. However, the data link layer has some restrictions like a packet size cannot be greater than a particular size, let’s say X bytes in total. That implies that the data packets have to be broken down into different chunks and those data packets have to be transmitted chunk by chunk.

Why so?

Let’s say you send data packets of longer sizes. Then, if that data packet is lost, you will have to send those packets once again. And the larger the size of the packet is, the larger bandwidth will be required to resend those data packets. That’s why an optimized frame size has been defined for the size of a data packet for data link layer.

So, now the data packets have been broken into frames and they will be sent chunk by chunk across the internet. So when my data link layer breaks the videos in chunks, it is smart enough to mark those frames with some cues. This allows my friend’s data link layer to exactly know the order of the frames so that it can re-arrange the frames back in order.

Flow control

Another thing that LLC layer is responsible for is flow control. Let’s take a look at the flow control this way.

I am a not-so-rich person so I cannot afford a laptop and I am still working on old Windows PC with 256 Mb RAM. On the other hand, my girlfriend can afford a brand new laptop that was just launched in the market.

So when we video chat, because of the HD quality camera from her laptop, the video size I receive outsizes my video sent to her from my poor camera. Does that mean that we should run out of sync? No, that does not happen, right?

And that’s where exactly flow control comes into the picture. LLC layer makes sure that one fast computer does not overrun a slower one. For that purpose, the concept of ‘Ack’ acknowledgment has come into the picture. For every frame received or sent, I must send/receive an ack respectively to be in sync.

Error control

LLC layer uses CRC (Cyclic Redundancy Check) to check frame errors at data link layer. This is more like providing a preview of a bigger picture to your friend that where he/ she is going wrong.

If you have any comments or thoughts related to it, feel free to ask and correct us.