The main function of the Data Link layer is to receive data packages from network layer and convert them to the frames that are ready to be transmitted by the first layer in OSI model – the Physical layer. The frame would include not only data packages, but also the sender’s and the receiver’s network addresses, as well as error checking and control information. The checking and control would ensure that frame has been delivered without troubles. If the part of the frame was lost during transmission, the Data Link layer protocols of the receiver would see that and would request data retransmit from the sender.
Before data is being sent the error checking mechanism is creating the unique four-byte frame check sequence (FCS) number. The FCS number is being created based on the values of all of the preceding fields in the frame. The receiver’s end receives data and the FCS value. The FCS is converted back using the same algorithm and is compared to the frame’s fields. In unlucky scenario of failed comparison, the data is assumed corrupted and receiver would request data resend. If the comparison is a success, the receiver would send the message of successful data transmission back to the sender. However, if receiver did not receive the “success” message during specified time, it would automatically resend the data frames.
Another function of Data Link layer protocols is to control the flow when many different nodes are sending data at the same time. This allows network card process data without errors.
The Data Link layer has been divided into two sublayers by the IEEE. The top sublayer is called Logical Link Control and it is communicating only with Network layer providing reliability and flow control functions. The bottom sublayer called Media Access Control is responsible only for adding physical address to the frame and does the communication with the Physical layer. Physical address is the unique ID assigned to every network interface in the node. In some cases it might be called MAC address, hardware address, or Data Link layer address.
