# Q1(a) · Store-and-forward delay

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/papers/paper-2022-ai-final/q1a

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/papers/paper-2022-ai-final--q1a/

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

### Q1(a). A 1000-bit packet crosses three store-and-forward links. Every link has rate 2 Mbps and propagation delay 1 ms. Ignore other delays. Find the end-to-end delay. [5 marks]

### Answer

### Given

One packet, L=1000 bits;3 links;rate2000000 bits/s;propagation1ms/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

1000/2000000 seconds=0.0005s=0.5ms.

### Step 2. Add propagation per link

0.5+1=1.5ms.

### Step 3. Add all three links

3×1.5=**4.5ms**. Queuing/processing are ignored as instructed.