The Data Link Layer protects the higher layers from the characteristics of the physical transmission medium, and takes the raw transmission facility and transforms it into a line that appears error free to the network layer. Moreover, included amongst its functions is the synchronisation of data and flow control - ensuring a fast transmitter does not send to a slow receiver.

Another of the principal functions of this layer is to ensure that transmission errors are detected and to provide mechanisms from recovery from these - error control.

It achieves its tasks by having the sender break the input data up into data frames, transmit the frames sequentially, and process the acknowledgment frames sent back by the receiver. Obviously, since the Physical Layer merely accepts and transmits a stream of bits without any regard for meaning, it is up to this layer to create and recognise frame boundaries. In fact, at this level there are two forms of structure - byte-orientated protocols and bit-orientated protocols.

A noise burst on the line can corrupt a frame completely and in this instance 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. For example, a duplicate frame could be sent if the acknowledgment frame from the receiver was destroyed. It is the responsibility of the Data Link Layer to resolve the problems caused by damaged, lost and duplicate frames.

Another issue that occurs in this layer, and also higher in the hierarchy, is how to avoid a fast transmitter from overloading a slow receiver in data. Generally, this flow regulation and the error control are integrated.

Lastly, if the line is used to transmit data in both directions the problem of acknowledgment and data frames competing for use of the line arises piggybacking has been developed to avoid lessen this.

Services provided to the network layer

• Data link connection - The provision of data link connections between two network entities
• Data link service data units - In practice these are the frames of data exchanged over a data link connection
• Error notification - If an unrecoverable error is detected, the Network Layer is informed
• Flow control - The Network layer can dynamically control the rate at which it receives frames of data
• Quality of service parameters - These are optionally selectable and include residual error rate and throughput.

Below are some of the functions performed by the Data Link Layer:

• Set up and release of data link connections
• Delimiting and synchronisation - This is essentially the framing function which organises bits (the physical service data units) into frames (the data link protocol data units)
• Error detection - this function detects transmission, format and operational errors usually arising from deficiencies in the physical medium
• Error recovery - This function attempts to recover from errors
• Flow control

In conclusion, the second layer of the OSI Reference Model is where blocks of data are reliably transmitted over a transmission link.