Computer Network: Network Congestion

Question

For a host machine that uses the Token Bucket Algorithm for congestion control, the token bucket has a capacity of 1MB and the maximum output rate is 10MBPS, Tokens arrive at a rate to sustain output at a rate of 5MBPS. The token bucket is currently full and the machine needs send 10 MB of data. The minimum time required to transmit the data is _____ sec.

Solution-

C + PS = MS,  C= Bursty Time, P= Token Arival Rate S= Token generate rate 
1MB + 5S = 10S
S = 1/5 = .2 sec

for .2 sec, data will flow at max rate, hence in .2 sec, .2*10 = 2MB 
Data to be sent = 10MB
Left data = 10-2 = 8MBPS

8MBPS data will be sent = 8/5 = 1.6 seconds

Total time to send 10 MB = 1.6 + 2 = 1.8 Seconds ( correct?)

Comments

yes i think??

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token arrival rate >= channel rate // then what is need of token bucket ?

but as a qsn point of view,

since, token arrival rate >= channel rate, it will always transfer at full burst rate (i.e 1MBPS)

time to complete transfer = 10/1 = 10s

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to transmit 1MB takes .2 sec

so for 8MB    8*0.2=1.6 sec

Total time =1.6+0.2=1.8 sec which is right

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no, Nitish it will switch transfer rate to 5 MBPS.

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@ Manu Thakur

for this question M=1MBPS not 10

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@Manu

you have channel rate as 1MBPS, isnt it ?

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@nitish @abhihesk sorry, typo!

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@ Manu Thakur  Token bucket capacity is 1mb now token arriving rate is 5mbps and max o/p is 10mbps
how for 0.2 sec 10mb data will flow unless your bucket has that much capacity?

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Please explain the solution ... i m not getting it

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What i think is:

Relative Rate = 10MBps - 5 MBps

= 5MBps

which means when both actions (token input and token output work) are happening simultaneously then every second 5 MB of data leaving the bucket.

1 sec -------- 5 MB

0.2 sec ----------- 1 MB

Now, it is given in the question that "The token bucket is currently full" so first it needs to transmit the currently held data which is 1 MB as given in the question that bucket capacity is 1 MB.

and for remaining 10MB --------- 2 sec

In total it is 2.2 sec.

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@ Shubhanshu Could you please clear my doubt token bucket will assign token to each packet and now we have a bucket of 1mb how its possible that effective output is 5 mb that you mentioned as relative rate.