The Data Link Layer - Sliding Window Protocols

Although we have seen some possible improvements in efficiency in our data link layer protocols to date, we still have one significant shortcoming. The sender must wait until an acknowledgment arrives from the receiver.

Over links with long propagation delays (such as a satellite link with a 540msec delay) this results in very inefficient use of the available bandwidth.

There is thus strong motivation to keep the sender and the medium 'busy'. We can achieve this by permitting the sender to send more than a single frame while waiting for the first acknowledgment.

In sliding window, or clock, protocols we have these properties:

• the sender has a sending window consisting of a list (array) of frames that have been sent but not acknowledged.
• The sender's window size grows as more frames are sent but not yet acknowledged.
• The receiver has a receiving window consisting of frames it is willing to accept. The receiver's window size remains constant.
• Each frame being sent has a sequence number from 0 to 2^n-1 (which fits in n bits). Stop-and-wait and PAR have n=1.
• A window size of 1 implies that frames are only accepted in order.

Sliding window protocols remain synchronized under conditions of premature timeouts, garbled frames and lost frames.