Question

The Data Link layer of OSI model has four functions associated with providing service to LAN users. These functions are split into two groups treated by logical link control (LLC) layer and medium access control (MAC) of IEEE 802 reference model. Why? What function(s) is handled by LLC and what others are handled by MAC?

Answer 1

Data Link Layer helps sending the data in the form of frames over the media.

Thus it serves two services:

It accepts the packets from the layer 3 protocols and then convert those packets in the form of frames.

It controls media access controls and also helps in the error detection.

Thus each purpose is separated from other.

Logical Link control serves the first purpose while Media access control layer performs the second purpose.

Logical Link Layer performs addressing the stations over the transmission medium and also responsible for controlling the data exchanged between the sender and the receiver. It forms communications with the above layer and also gives error notification.

Media Access Control Layer tells who is allowed to access the media at any point of time else it can be used as simple frame transferring layer based on MAC address mentioned inside of the frame. This layer also performs frame synchronization that is where the data of one frame ends and next one starts. The frame can access the media by the processes defined by data link layer protocols.