Ambiguity Example
Suppose that we are using a 3-bit sequence number, such as is typical for HDLC. This gives N=2³=8. Since wr=1, we must limit wt≤7. This is because, after transmitting 7 packets, there are 8 possible results: Anywhere from 0 to 7 packets could have been received successfully. This is 8 possibilities, and the transmitter needs enough information in the acknowledgment to distinguish them all.
If the transmitter sent 8 packets without waiting for acknowledgment, it could find itself in a quandary similar to the stop-and-wait case: does the acknowledgment mean that all 8 packets were received successfully, or none of them?
Read more about this topic: Sliding Window Protocol, Examples, Go-Back-N
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