data link layer

data link layer

The task of the data link layer is to convert the raw bit stream offered by the physical layer into a stream of frames for use by the network layer. Various framing methods are used, including character count, byte stuffing, and bit stuffing. Data link protocols can provide error control to retransmit damaged or lost frames. To prevent a fast sender from overrunning a slow receiver, the data link protocol can also provide flow control. The sliding window mechanism is widely used to integrate error control and flow control in a convenient way.

Sliding window protocols can be categorized by the size of the sender's window and the size of the receiver's window. When both are equal to 1, the protocol is stop-and-wait. When the sender's window is greater than 1, for example, to prevent the sender from blocking on a circuit with a long propagation delay, the receiver can be programmed either to discard all frames other than the next one in sequence or to buffer out-of-order frames until they are needed.

 Protocol 1 is designed for an error-free environment in which the receiver can handle any flow sent to it. Protocol 2 still assumes an error-free environment but introduces flow control. Protocol 3 handles errors by introducing sequence numbers and using the stop-and-wait algorithm. Protocol 4 allows bidirectional communication and introduces the concept of piggybacking. Protocol 5 uses a sliding window protocol with go back n. Finally, protocol 6 uses selective repeat and negative acknowledgements.

Protocols can be modeled using various techniques to help demonstrate their correctness (or lack thereof). Finite state machine models and Petri net models are commonly used for this purpose.

Many networks use one of the bit-oriented protocols—SDLC, HDLC, ADCCP, or LAPB—at the data link level. All of these protocols use flag bytes to delimit frames, and bit stuffing to prevent flag bytes from occurring in the data. All of them also use a sliding window for flow control. The Internet uses PPP as the primary data link protocol over point-to-point lines.