Functions of data link layer

The data link layer is layer 2 of the OSI model. It provides reliable node-to-node (hop-to-hop) transfer of frames over a physical link.

Major functions

1. Framing: divides the bit stream from the network layer into manageable frames, adding a header and trailer. Methods: character count, byte stuffing, bit stuffing.
2. Physical (MAC) addressing: adds the source and destination MAC addresses in the frame header to deliver the frame to the correct device on the same network.
3. Error control: detects and corrects damaged, lost or duplicate frames using CRC, checksum and parity, with acknowledgements and retransmission (ARQ).
4. Flow control: keeps a fast sender from flooding a slow receiver using stop-and-wait or sliding window protocols.
5. Access control (MAC sublayer): decides which device may use a shared channel at a time (CSMA/CD, token passing).
6. Link management: setup, maintenance and release of the data link.

Sublayers

• LLC (Logical Link Control): flow and error control, interface to network layer.
• MAC (Media Access Control): addressing and channel access.

 Network layer packet
        |
  [H | packet | T]   <- frame
        |
  Physical layer bits