• At the beginning of transmission, the receiver's window contains n-1 spaces for frame but not the frames.
  • As the new frames come in, the size of window shrinks.
  • Therefore the receiver window represents not the number of frames received but the number of frames that may still be received without an acknowledgment ACK must be sent.
  • Given a window of size w, if three frames are received without an ACK being returned, the number of spaces in a window is w-3.
  • As soon as acknowledgment is sent, window expands to include the number of frames equal to the number of frames acknowledged.
  • For example, let the size of receiver's window is 7 as shown in diagram. It means window contains spaces for 7 frames.
  • With the arrival of the first frame, the receiving window shrinks, moving the boundary from space 0 to 1. Now, window has shrunk by one, so the receiver may accept six more frame before it is required to send an ACK.
  • If frames 0 through 3 have arrived but have DOC been acknowledged, the window will contain three frame spaces.
  • As receiver sends an ACK, the window of the receiver expands to include as many new placeholders as newly acknowledged frames.
  • The window expands to include a number of new frame spaces equal to the number of the most recently acknowledged frame minus the number of previously acknowledged For e.g., If window size is 7 and if prior ACK was for frame 2 & the current ACK is for frame 5 the window expands by three (5-2).

                                 

  • Therefore, the sliding window of sender shrinks from left when frames of data are The sliding window of the sender expands to right when acknowledgments are received.
  • The sliding window of the receiver shrinks from left when frames of data are The sliding window of the receiver expands to the right when acknowledgement is sent.

There are three types of techniques available which Data-link layer may deploy to control the errors by Automatic Repeat Requests (ARQ):