Explain the concept of deadlock

Yes. It can direct to deadlock. Suppose that a batch of frames arrived correctly and were accepted. Then receiver will advance its window. Now suppose that all acknowledgements were lost. The sender will eventually time out and send first frame again. The receiver will send a NAK. Suppose that this was lost. From that point on, sender would keep timing out and sending a frame which had already been accepted, but receiver will just ignore it. Setting auxiliary timer results in the correct acknowledgement being sent back finally instead, which resynchronizes