The maximum window size for data transmission using the selective reject…
2005
The maximum window size for data transmission using the selective reject protocol with n-bit frame sequence numbers is:
- A.
2n
- B.
2(n-1)
- C.
2n – 1
- D.
2(n-2)
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Correct answer: B
Answer: Maximum window size = 2^(n-1).
Explanation: To avoid ambiguity when sequence numbers wrap around modulo 2^n, the sender and receiver windows in Selective Repeat must be at most half the sequence-number space.
Total distinct sequence numbers = 2^n.
To prevent an old (previous) frame being mistaken for a new one after wraparound, the window size must be ≤ half of the sequence-number space, i.e., ≤ 2^n / 2 = 2^(n-1).
Example: if n = 3 then 2^n = 8 and the maximum unambiguous window size is 2^(3-1) = 4. Any larger window can cause overlap and ambiguity when sequence numbers wrap.
Therefore the maximum window size for Selective Reject (Selective Repeat) with n-bit sequence numbers is 2^(n-1).
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