Data Link Layer

The main task of the data link layer is to take a raw transmission facility and transform it into a line that appears free of transmission errors in the network layer. It accomplishes this task by having the sender break the input data up into data frames (typically a few hundred bytes), transmit the frames sequentially and process the acknowledgment frames sent back by the receiver. Since the physical layer merely accepts and transmits a stream of bits without any regard to meaning of structure, it is up to the data link layer to create and recognize frame boundaries. This can be accomplished by attaching special bit patterns to the beginning and end of the frame. If there is a chance that these bit patterns might occurs in the data, special care must be taken to avoid confusion.

The data link layer should provide error control between adjacent nodes.

Data Link Layer: Error Control

A noise on the line can destroy a frame completely. In this case, the data link layer software on the source machine must retransmit the frame. However, multiple transmissions of the same frame introduce the possibility of duplicate frames. A duplicate frame could be sent, for example, if the acknowledgement frame from the receiver back to the sender was destroyed. It is up to this layer to solve the problems caused by damaged, list and duplicate frames. The data link layer may offer several different service classes to the network layer, each of a different quality and with a different price.