# Is it possible to calculate the propagation delay if only the length of the link is known & not the speed at which a bit travels into the link?

Consider a $10^7$ bps link that is $400$ km long, with a queue large enough to hold $2000$ packets.Assume that the packets arrive at the queue with an average rate of $4000$ packets per second and that the average packet length is $2000$ bits.Find the average number of packets in the queue.

I am getting Average Queue length = 0.

In this question is it possible to calculate Propagation delay?
How can we calculate the propagation delay until we don't know about the speed at which a bit travels in the link?

• Why do you think your approach is wrong? Is there any specific part you are uncertain about? Generally we prefer conceptual questions rather than just solving your exercise or doing arithmetic for you. Can you edit your post to ask about a specific conceptual issue you're uncertain about, instead of asking us to grade your solution to this particular exercise? As a rule of thumb, a good conceptual question should be useful even to someone who isn't looking at the problem you happen to be working on. – D.W. Nov 26 '15 at 7:52
• Thank you for your concern. I edited the question.Actually I don't know the correct answer to this problem, and since the distance is also mentioned so I thought, have I missed the propagation delay? & hence I discussed it here. – Romy Nov 26 '15 at 8:09

## 2 Answers

I believe your approach is right. Since we don't have any other information given. Then this means the packets are getting consumed at the rate same as link bandwidth. So Avg packet waiting must be 0.

Yes, you can infer the propagation delay from the length of the link if you know something about the technology used to transmit the bits. As Wikipedia says, the propagation delay is equal to $d/s$ where $d$ is the distance and $s$ is the wave propagation speed. The wave propagation speed depends upon the technology and might be as fast as $c$ (the speed of light) or might be somewhat slower (e.g., for copper wire).