In a sliding window ARQ scheme, the transmitter's window size is N and the…
2004
In a sliding window ARQ scheme, the transmitter's window size is N and the receiver's window size is M. The minimum number of distinct sequence numbers required to ensure correct operation of the ARQ scheme is
- A.
min (M, N)
- B.
max (M, N)
- C.
M + N
- D.
MN
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Correct answer: C
Answer: M + N
Explanation: The minimum number of distinct sequence numbers S must be large enough that sequence numbers in use by the sender and those within the receiver's acceptance window cannot be confused.
The sender can have up to N outstanding frames (sent but not yet acknowledged).
The receiver can accept up to M frames ahead within its window; these sequence numbers must not be mistaken for new transmissions.
To avoid reuse that could cause ambiguity, the sequence-number space must cover both ranges simultaneously, so S >= N + M.
Example: If the transmitter window N = 4 and the receiver window M = 3, you need at least 7 distinct sequence numbers (for example 0..6). Reusing a sequence number sooner could make the receiver confuse a new frame with an outstanding one, or an acknowledgement with a different frame.
Therefore, the minimum required number of distinct sequence numbers is M + N.
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