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34 votes
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A computer on a $10\text{Mbps}$ network is regulated by a token bucket. The token bucket is filled at a rate of $2\text{Mbps}$. It is initially filled to capacity with $16\text{Megabits}$. What is the maximum duration for which the computer can transmit at the full $10\text{Mbps}$?

  1. $1.6$ seconds
  2. $2$ seconds
  3. $5$ seconds
  4. $8$ seconds
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9 Answers

Best answer
48 votes
48 votes

New tokens are added at the rate of $r$ bits/sec which is $2\;\text{Mbps}$ in the given question.

Capacity of the token bucket $(b) = 16\;\text{Mbits}$
Maximum possible transmission rate $(M) = 10\;\text{Mbps}$
So, the maximum burst time $= b/(M-r) = 16/(10-2) = 2 \;\text{seconds}$
Here is the animation for token bucket hope this will help us to understand the concept.

Correct Answer: $B$

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38 votes
38 votes
TIME TOKEN IN BUCKET TOKEN SEND LEFT IN BUCKET
First sec 16Mb+ 2Mb =18Mb 10Mb 8Mb
next sec 8Mb+2Mb 10Mb 0Mb

Hence for 2 seconds we can send the tokens at 10 Mbps.

20 votes
20 votes
It's 'kind of' aptitude question:

packets leaving bucket at 10Mbps   &&   packets entering bucket at 2Mbps.

So "actual rate" of bucket being empty is 10-2=8Mbps.

so (capacity/transmission rate) = 16/8 =2seconds.
5 votes
5 votes
  • I/P rate of tokens=2 Mega bps. 2 Megabits tokens come 1 sec
  • O/P rate of tokens=10 Mega bps. 10 Megabits tokens comes out in 1 sec
  • Capacity of the bucket=16 megabits
  • At t=0, bucket=16 Megabits
  • At t=1, 10 Megabits tokens comes out(as it is filled so first tokens have to pulled out then it will be filled), 2 Megabits tokens come in, (16-10+2)=8 Megabits tokens. At t=1 bucket has 8 Megabits tokens
  • At t=2, bucket has 8 Megabits tokens(it is not full no first tokens will come in then pulled out), 2 Megabits tokens comes in, bucket=8+2=10 Megabits. 10 Megabits tokens will come out. Now bucket has 0 Megabits tokens
  • At t=3, bucket has 0 tokens, 2 Megabits tokens comes in and 2 Megabits tokens goes out. So here tokens are send at the rate of 2 Mega bps and this will continue
  • In t=1 sec 10 Megabits tokens comes out
  • At t=2, 10 Megabits tokens come out
  • So till 2 sec we're able to send tokens at 10 Mega bps
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