Consider a network of 3 interconnected computers X, Y and Z arranged on a line, where the link bandwidth (bit-rate) between X and Y is 5Mbps, the link bandwidth between Y and Z is 2Mbps and the one-way propagation delays are 10ms between X and Y and 20ms between Y and Z. (X and Z are not directly connected).
How long does it take for X to send a 3KB packet to Z (via Y) and for Z to fully receive that packet, assuming that Y must receive all bits of the packet before it starts sending any bits to Z? (This is “store and forward” transport)

____________________ ms
(answer should be correct to one decimal place)

Sanjana Sri 16 Jan 2017 11:51 pm

from X to Y, Tt+Tp is = 4.9152 ms +10ms = 14.9152
from y to z Tt+Tp= 12.288+20= 32.288
total= 47.2

Habib Mohammad Khan 17 Jan 2017 11:32 am

As it is store and forward so all dat is sent to X to Y , then Y to Z ..As said by @anusha_motamarri , delays of both should be found exclusively..

So I also got 47.2 ns as answer..So I think it should be right ..

Pratyush Madhukar 17 Jan 2017 11:49 am

from X to Y, Tt  = L/B = 3*1000*8*1000 / (5*10^6) ms = 24/5 ms = 4.8 ms

so, Tt + Tp = 4.8 + 10 = 14.8ms

From Y to Z, Tt = 3*1000*8*1000 / (2 * 10^6) ms = 12 ms

so, Tt + Tp = 12 + 20 = 32 ms

Adding these two, total delay would be 46.8

Habib Mohammad Khan 17 Jan 2017 11:56 am

But as given in standard textbook terminology , as far as the system is concerned like packet size, frame size etc which are system oriented , so there 1 K = 1024 not 1000..1 M = 220...

But if we consider the medium characterisitics like bandwidth , clock rate etc , then 1 K = 103 , 1 M = 106 etc..

For reference , plz go thorugh the 1st chapter of Petersen Davie..It is mentioned there clearly.And in GATE questions also we follow same convention..

So it should be 47.2 not 46.8..Hope this clarifies..

Hence the answer should be corrected..

Tarak Maddukuri 20 Jan 2017 09:00 am

why L got 3*1000*8*1000 ???

3KB means 3*8*1024 only