switching

Question

A sends 2 frames of 1000 bit each to B via switch S.Bandwidth=10Mbps  propogation delay  over links=5us find time when second packet reaches B completely

Comments

is ans 305 ?

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similar Gate question

https://gateoverflow.in/8495/gate2015-3_36

Answer 1

Answer should be 310 micro-seconds.

Tt = 1000 / 10Mbps = 100 micro-seconds.

Tp = 5 micro-seconds.

Since The Store and Forward delay is not mentioned, we can assume it to be negligible.

It is a case of pipelining in Packet Switching, so, The Propogation delay will only be considered once for the first packet since the rest of the packet follow it !

Also, the Transmission delay will be considered = No. of hops for the first packet only, and for the subsequent packets, after each Transmission delay time, 1 packet will be delivered.

So,

First packet = 2*Tt + Tp( from A to S )  + Tp ( from S to B )  = 2*100 +5 + 5 = 210 micro-seconds.

Second packet = Tt = 100 micro-seconds.

Total time at which the second packet is completely delivered to the destination = 210 + 100 = 310 micro-seconds.

Comments

@manu00x you have done the correct analysis ...your answer is different from the selected answer only because you have taken 5microsec delay for each link while in above answer he has taken 5 microsecond for complete link

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@Vaibhav Thanks a lot! yes you're right!

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yes @ manu00x 

your diagram is correct and your point is correct too .. 

i understand , the main point here is :two time Tp is to be consider ,

because we consider propagation time only for first packet not for first link ..so from A to S one time Tp is consider then again from S to B another time Tp need to be consider.

@hem chandra joshi  see the edited answer, it is 310 correct answer due to that above point.

Answer 2

Frame 2 will be recieved at reciever after ≈ 310  µs ,if switch uses a store and forward mechanism.

http://inst.eecs.berkeley.edu/~ee122/fa09/notes/04-Performancex6.pdf