Patterson Chapt 1 Foudnation Q 13

Question

13. Suppose a 1-Gbps point-to-point link is being set up between the Earth and a new lunar colony. The distance from the moon to the Earth is approximately 385,000 km, and data travels over the link at the speed of light—3 × 10⁸ m/s.

(a) Calculate the minimum RTT for the link.

(b) Using the RTT as the delay, calculate the delay × bandwidth product for the link.

 (c) What is the significance of the delay × bandwidth product
computed in (b)?
(d) A camera on the lunar base takes pictures of the Earth and
saves them in digital format to disk. Suppose Mission Control
on Earth wishes to download the most current image, which
is 25 MB. What is the minimum amount of time that will
elapse between when the request for the data goes out and
the transfer is finished?
 

Answer 1

tp=385000*10^3/3*10^8=1.28 sec

1)RTT=2*1.28=2.56

2)bandwidth delay product=10^9*2.56=2560 Mbits

3)bandwidth delay product gives no of bits which can be sent in one rtt

4)2.56     10^9 bits

   ?           25MB

min time=25MB*2.56/10^9

=0.512 sec

Comments

for the last option I calculated- Tf = 25*8*10^6/10^9 = 200 ms. and Tp = 1283ms

So total time should be 1483 ms or 1.48s