# Q6(a) · CSMA/CD arrival timeline

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/papers/paper-2022-ai-final/q6a

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/papers/paper-2022-ai-final--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 CSMA/CD, identify successful packets. Propagation between nodes takes 0.2 units. If a node senses busy or experiences collision, it waits until after t=5 before retrying. A collision stops transmission immediately, but the last bit still propagates for 0.2 units. Explain carefully. [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 | 0.3–1.3 | Signal clears other nodes at1.5 | Success |
| 2 | 1.7–2.0 | At1.7 channel idle; detects3 at1.8+0.2=2.0 | Collision |
| 3 | 1.8–1.9 | Cannot sense2 until1.9; then stops | Collision |
| 4 | 2.5–3.5 | Last collision signal clears by2.2, so idle | Success |
| 5 | 4.2–5.2 | Previous signal cleared by3.7 | Success |
| 6 | No transmission | At4.6 senses5, whose signal arrives4.4 | Busy; waits |

### Step 2. State successful messages

**1, 4, 5**.