# Q1(b) · Store-and-forward delay

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/papers/paper-2024-ai-final/q1b

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/papers/paper-2024-ai-final--q1b/

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

### Q1(b). A 2000-bit packet crosses three store-and-forward links, each 1 Mbps with propagation delay 2 ms. Ignore other delays. Find end-to-end delay. [5 marks]

### Answer

### Given

One packet, L=2000 bits;3 links;rate1000000 bits/s;propagation2ms/link;store-and-forward.

### To find

Total time until the final packet bit reaches the destination.

### Formula and why



$$
T=\sum_{i=1}^{3}(L/R_i+d_{prop,i})
$$



Each router must receive the complete packet before sending it onward, so all three serialization times count.

### Solution

### Step 1. Calculate serialization on one link

2000/1000000 seconds=0.002s=2ms.

### Step 2. Add propagation per link

2+2=4ms.

### Step 3. Add all three links

3×4=**12ms**. Queuing/processing are ignored as instructed.