GATE 2016 CS (CS2) – Question 65

Computer Networks · Data Link Layer: Error Detection, Medium Access Control, Flow Control · 2 marks · Numerical answer

A 128×10³ bit/s satellite link has one-way propagation 150 ms. Selective-repeat ARQ sends 1 KB frames. Ignore ACK transmission time. Find the smallest sequence-number field width for full utilization.

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Correct answer: 4.0 to 4.0

Explanation

Frame time is $1024(8)/128000=0.064$ s. The required window is $\lceil1+2(0.150)/0.064\rceil=6$. Selective repeat needs sequence space at least 2W=12. The least b with 2ᵇ≥12 is 4 bits. Decimal-kilobyte interpretation also gives window six and the same result.

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