GATE 2021 CS (CS2) – Question 64

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

A pure ALOHA network uses 1000-bit frames and a 1 Mbps link. The aggregate transmission rate, including retries, is a Poisson process of 1000 frames/s. Find successful throughput in frames/s, rounded to the nearest integer.

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Correct answer: 130 to 140

Explanation

Frame duration is 1000/10⁶=0.001 seconds. The normalized offered load is G=1000×0.001=1 attempt per frame time. Pure ALOHA succeeds only when no other attempt occurs in the two-frame vulnerable interval, probability e⁻². Throughput is 1000e⁻²≈135.335 frames/s, rounded to 135, within the official interval.

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