The logical link layer deals with the transmission of frames. Each frame is a packet of data with a sequence number that is used to ensure delivery and a checksum to track corrupted frames. Several algorithms are available for use to acknowledge delivery of packages. The basic protocol works as follows.
The sending station

1. The sending station sends one or more frames with a sequence numbers them.
2. The sending station awaits acknowledgements of the sent frames before transmitting further frames.
3. If no acknowledgement for a particular frame arrives within a fixed time the frame is retransmitted

The Receiving Station

The receiving station acknowledges uncorrupted frames as they are received. Note that there must be a large enough range for the sequence numbers so that the receiving station can distinguish between resends (this will happen if an ACK gets lost) and new frames.Three protocols for the data link layer. For more details see either Tanenbaum’s book or Bertsekas and Gallager.

1. Stop and Wait. This requires only two sequence numbers
2. Go back n
3. Selective repeat

The last two allow several frames to be in transit.

After the creation of the OSI model it was found necessary to expand the data link layer to take account of shared communication channels in LANs. This requires a medium access layer