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Q6(a) · Pure ALOHA timeline

2024-02-06 · AI-only Sem V variant

Exact historical variant. Current-syllabus questions solved; excluded topics and source limitations stated.

Q6(a). Six messages arrive at different nodes at t = 0.3, 1.7, 1.8, 2.5, 4.2, 4.6. Each transmission takes one time unit; no retransmission is attempted until after t = 5. For pure ALOHA, identify which packets are successfully transmitted, with explanations. [5 marks]

Answer

Given

Arrivals0.3,1.7,1.8,2.5,4.2,4.6; one-unit frames; no retries before time5.

To find

Successful packet numbers and each relevant start/collision decision.

Formula and why

Pure ALOHA sends immediately. Slotted ALOHA waits for the next slot. CSMA/CD senses only signals that have had time to propagate.

Check intervals, not just whether arrival times differ. Any collision corrupts a frame; collision detection shortens the wasted transmission but does not make that frame successful.

Solution

Step 1. Trace every message

Packet	Transmission / slot	Reason	Outcome
1	0.3–1.3	No overlap	Success
2	1.7–2.7	Overlaps3 and4	Collision
3	1.8–2.8	Overlaps2 and4	Collision
4	2.5–3.5	Overlaps2 and3	Collision
5	4.2–5.2	Overlaps6	Collision
6	4.6–5.6	Overlaps5	Collision