exponential backoff

Question

Nodes A and B are connected with 100 Mbps ethernet segment with 6 microsec pop.delay between them.Suppose A,B send frames at t=0 and frames get collided.after first collision A draws k=0 and bdraws k=1.if jam signal is ignored and timeout is 1 RTT then at what time A's packet gets completely delivered to B...assume packet size 1000 bits.

• A. 28 microsec
• B. 16 microsec
• C. 22 microsec
• D. 38 microsec

Comments

What changes can we expect in the result if the jamming signal has not been ignored?

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@  dan31 

What changes can we expect in the result if the jamming signal has not been ignored?

then jamming signal transmission time also be added before sending packet again . 

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ok..Can you please confirm if we need to add jamming signal for min frame size calculation?

As there are some previous gate question, where it is included. But I don't think we need to add it.

ref- https://gateoverflow.in/1397/gate2005-74

Answer 1

TT= 10 micro second

pd = 6 micro seconds.

as the value of k determines after how many time slot they will again transmit.

a is saying i will immediately send the packet while b is saying i will send after one time slot.

soo a will directly transmit without a delay so it will be 16 micro second

Comments

ans is D

Answer 2

Both the station start at the time t=0

At t=3us collision occurs than 3us again will required to detect the collision so at t=6us collision will be detected

Now 1tp that is 6us will require to clear the link as collision has been occurred so we must clear the link before starting so now at t=12us link will be ready to again transfer.

Now 10us is time to transmit the frame so at t=22us at will finish transmitting

Further tp=6us will require it to propagate so toata time taken to deliver the packet is 22+6us=28us.

Comments

Now 1tp that is 6us will require to clear the link as collision has been occurred so we must clear the link before starting so now at t=12us link will be ready to again transfer.

Clearing the link means?

Answer 3

 

ans ) 38 microsec

Comments

in last line it should be 22+16=38.

what will be changes if there is modification that K=1 and B choose k=0.

is it 22+12+16???

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@magma why would both retransmit the frames after 1RTT? A should transmit the frame immediately after detecting collision(since jam signal is ignored here) and B should wait for 1RTT before transmitting..isnt it so?

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why are you using extra Tt time after collision?

since data is already placed in the link

I think only Tp should be used

I'm a bit confused here can you please explain