# Q6(a) · Slotted ALOHA arrival timeline

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/papers/paper-2023-jan-ai-reexam/q6a

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/papers/paper-2023-jan-ai-reexam--q6a/

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.