In stop and wait control mechanism, one frame send and wait for acknowledgement then only can send the next frames. This is not efficient for transmit a large amount of data. Sliding window control mechanism is to enhance effectiveness of data transmission because several frames can be transmitted at a time before needing an acknowledgement.

            As its name suggest, sliding window use a window  to hold frames and provide a upper limit on the number of frames that can be transmitted before requiring an acknowledgement. The window  is actually an extra buffer created by both sender and receiver. To keep track of which frames have been transmitted and which received, sliding window introduces an identification scheme based on the size of the window. The frames are numbered modulo-n, which means they are number from 0 to n -1. For example, if n=8, the frames are numbered 0,1,2,3,4,5,6,7,0,1,2,3,4,……The size of the window is n-1 (in this case, 7). This mean the window cannot cover the whole module (8 frames); it covers one frame less. This is to avoid ambiguity in the acknowledgement of the received frames.

            At both sender and receiver side, there will be one window to keep track of which frames have been transmitted and which received. At sender side, as one frame is sent out, the left boundary of the window moves inward and the size of the window reduce by one. After acknowledgement arrived, then the window expand to allow in a number of new frames equal to the number of frames acknowledged by that acknowledgement. For example, given a window size is 7. If frame 0 to 4 is sent without acknowledgement received, then the window left only 2 frames (numbered 5 and 6). If an acknowledgement numbered 3 is arrived, then 3 frames are known to have transmitted undamaged and the sender window expand to include next 3 frames in its buffer. At this time, the sender window contains 5 frames (numbered 5, 6, 7, 0 and 1). Conceptually, the sliding window of the sender shrinks from the left when frames of data are sent and expand to the right when acknowledgements are received.

            At the receiver side, the window does not represent the number of frames received but the number of frames that may be received before an acknowledgement must send. When n frames received without returning an acknowledgement, then the size of receiver window reduce by n. And before the size of the window becomes zero, an acknowledgement must send as to continue receive incoming frames. And the acknowledgement sent is numbered by the next frames number expected to be received. For example, given the size of window are 7. If 3 frames are received, then the window size reduces by 3 frames (numbered 0, 1 and 2). And an acknowledgement with numbered 3 (the next frame expected to be received) is sent to sender. After the acknowledgement is sent, then the window size expands to 3 frames. Conceptually, the window of receiver shrinks from left when frames of data are received and expand to the right when acknowledgement is sent.