Discussion
Imagine a flow specification that has a maximum packet size of 1000 bytes, a token bucket rate of 10 million bytes/sec, a token bucket size of 1 million bytes, and a maximum transmission rate of 50 million bytes/sec. How long can a burst at maximum speed last ?
Answer: D
A full-speed burst can last 25 msec.
During a burst at the maximum rate M, the bucket of capacity C empties while being refilled at the token rate \(\rho\), so the burst length S satisfies \(C + \rho S = M S\):
\[ \begin{aligned} S &= \frac{C}{M - \rho} = \frac{10^{6}}{(50 - 10) \times 10^{6}} \\ &= \frac{1}{40}\ \text{s} = 25\ \text{ms} \end{aligned} \]
Ignoring the refill and dividing 1 MB by 50 MB/s gives 20 ms, which is not offered; 25 sec and 50 sec are off by a factor of 1,000, and 50 msec does not follow from the formula.
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