Computer Networks · TEE · papers
Q6(a) · Slotted ALOHA arrival timeline
2023-01-27 · AI-only Sem V
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 slotted ALOHA, identify successful packets and justify collisions at the next slot boundary after each arrival. [10 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 | 1–2 | Alone in slot1 | Success |
| 2 | 2–3 | Same slot as3 | Collision |
| 3 | 2–3 | Same slot as2 | Collision |
| 4 | 3–4 | Alone in slot3 | Success |
| 5 | 5–6 | Same slot as6 | Collision |
| 6 | 5–6 | Same slot as5 | Collision |
Step 2. State successful messages
1, 4. The slot beginning at5 is the initial attempt of packets5 and6, not a retransmission.