# Q6(b) · CSMA without collision detection

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/papers/paper-2024-ai-final/q6b

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/papers/paper-2024-ai-final--q6b/

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

### Q6(b). Seven messages arrive at 0.3,0.6,0.9,1.6,2.7,3.3,4.3, each with one-unit transmission time. CSMA has no collision detection; propagation takes 0.4 units, so transmission [2,3] is sensed busy at another node during [2.4,3.4]. (1) List each start time, or s if busy at arrival. (2) List successful packet numbers. No deferred retry is requested. [5 marks]

### Answer

### Given

Arrival times0.3,0.6,0.9,1.6,2.7,3.3,4.3;frame1;propagation0.4.

### To find

Starts/busy decisions and successfully received packets.

### Formula and why

A transmission starting at a is sensed by another node during [a+0.4,a+1.4]. Without collision detection, it continues for its full one-unit duration.

A node may start before an earlier distant transmission has arrived. Such starts can collide even though each node initially sensed idle.

### Solution

### Step 1. Inspect arrivals chronologically

| Packet | Arrival | Decision | Reason |
| --- | --- | --- | --- |
| 1 | 0.3 | 0.3 | No prior signal |
| 2 | 0.6 | 0.6 | Signal1 first arrives0.7 |
| 3 | 0.9 | s | Signal1 busy[0.7,1.7] |
| 4 | 1.6 | s | Signals1 and2 are busy |
| 5 | 2.7 | 2.7 | Both prior signals cleared by2.0 |
| 6 | 3.3 | s | Signal5 busy[3.1,4.1] |
| 7 | 4.3 | 4.3 | Signal5 cleared by4.1 |

### Step 2. Identify collisions

Frames1 and2 overlap and collide. Frames5 and7 have no overlapping reception intervals at another node. **Starts:0.3,0.6,s,s,2.7,s,4.3. Successes:5,7.**