CS352

1
CS352

• Leaky Bucket / Token Bucket
• Examples

2
Leaky Bucket

• Suppose that data arrives at 25 MBps in 40 msec
  bursts every second. To reduce the rate to 2
  MBps, we can use a leaky bucket. A leaky bucket
  is described by a constant output rate and a
  buffer capacity C. How big must C be to avoid
  that data is lost? How long does it take to
  transmit all the data in each burst?

3
Leaky Bucket

• The rate is 25 MBps (million BYTES / sec)
• Duration is 40 msec

4
Leaky Bucket

• The rate is 25 MBps (million BYTES / sec)

• Duration is 40 msec

5
Leaky Bucket

• If buffer capacity is C, how big must C be?

6
Leaky Bucket

• If buffer capacity is C, how big must C be?
• C 1 MByte

7
Leaky Bucket

• If the output rate is 2MBps, how long does it
  take to transmit all the data?
• If we have 1 Mbyte to send at a rate of 2MBps,
  then it will take ½ second or 500 millisecs

8
Token Bucket

• Assume instead that we use a token bucket with a
  capacity of 500 KB for the scenario in the
  problem above. Tokens arrive at a rate of 2 MBps.
  Assuming that the token bucket is full when the
  data burst arrives, how long does it take to
  transmit the entire burst?

9
Token Bucket

• Definitions
• S burst length in sec
• C token bucket capacity
• ? token arrival rate (bytes/sec)
• M maximum output rate (bytes/sec)
• Then
• C ?S MS

10
Token Bucket

• C ?S MS, or
• S C / (M - ?)

11
Token Bucket

• Our question How long does it take to transmit
  the entire burst?
• C 500KB 0.5MB
• M 25 MB/sec
• ? 2 MB/sec
• S C / (M - ?)
• 0.5 / (25 2)
• 0.022 secs 22 msecs

12
Token Bucket

• The maximum burst rate can be sustained for 22
  msecs,
• 25 MB/sec 0.022 secs 0.55 MB
• 1 MB 0.55 MB 0.45 MB
• Then,
• 0.45 MB 1 sec/2MB 0.225 secs
• So, 225 22 247 msecs

13
Token Bucket

• Now suppose the token bucket capacity is reduced
  to 250 KB.
• C 250KB 0.25MB
• M 25 MB/sec
• ? 2 MB/sec
• S C / (M - ?)
• 0.25 / (25 2)
• 0.011 secs 11 msecs

14
Token Bucket

• The maximum burst rate can be sustained for 11
  msecs,
• 25 MB/sec 0.011 secs 0.275 MB
• 1 MB 0.275 MB 0.725 MB
• Then,
• 0.725 MB 1 sec/2MB 0.362 secs
• So, 362 11 373 msecs