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Can somebody please help me understand this question:

"(c) A wants to send a 500 byte packet to D through B. B is supposed to follow the store-andforward model, that is, B will receive the whole packet from A and then start transmitting the packet to D. What is the end-to-end delay seen by the packet?"

A --> B (4 Mbps & 3000 km) and B --> D (10 Mbps & 900 km) This is also assuming all data is sending at the speed of light (3 * 10pow(5) km/s

Im just really stuck on this question like I get the calculations for the most part, however I have no idea how to determine any of this.

Salah Zahran
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2 Answers2

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It sounds like the questions aims at helping you understand different transit times of data of different speed/length links.

For A->B you should calculate how long the packet takes to transmit on a 4 megabit link. You then need to add the physical transit time, using the distance and the speed of light.

I.e. first you need to know how long it takes until the last bit is put on the link and then how long it takes for that bit to travel to the receiver.

When B has received that last bit it will forward the packet to D. You therefor need to repeat the calculation for the B->D part.

The sum of the two parts should be your answer.

I won't to the calculations for you, though.

Edit: Ok, I get the feeling you really tried yourself, so here goes.

Transmission time

A->B:

4 Mbps = 4 000 000 bits/s

500 bytes = 500*8 bits = 4000 bits

Transmission time = Packet size / Bit rate => 4 000 / 4 000 000 => 0.001 s

Distance = 3 000 km

Propagation speed = 300 000 km/s

Propagation time = Distance / propagation speed => 3 000 / 300 000 = 0.01 s

Total time = 0.001 + 0.01 = 0.011 s

Now you do B->D and add the two parts.

FishySwede
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  • I don't know what the calculations are though. How do I determine what I need to calculate exactly, like I dont have any formulas or anything. – Salah Zahran Feb 12 '20 at 15:46
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Thanks for the help, I was actually able to figure it out. So I had to calculate the delay from A -> B, which after calculations turned out to be 11ms. Then I had to add that to the delay of B -> D which was 3.4ms which makes the delay from A -> D 14.4ms. Thanks for the help.

Salah Zahran
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