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In terms of the usual notation, for an M|M|1 queueing system, which of the following equations does not hold ?

  • A.\(L_{s} = \rho/(1 - \rho)\)
  • B.\(W_{q} = W_{s}\)
  • C.\(W_{s} = W_{q} + 1/\mu\)
  • D.\(L_{q} = \lambda W_{q}\)

Answer: B

The equation that does not hold is \(W_{q} = W_{s}\).

In an M/M/1 queue a customer's time in the system is the waiting time in the queue plus the service time, so

\[ W_{s} = W_{q} + \frac{1}{\mu} \]

which makes \(W_{s}\) strictly larger than \(W_{q}\). The two are never equal.

The other three are standard M/M/1 results: \(L_{s} = \rho/(1 - \rho)\) with \(\rho = \lambda/\mu \lt 1\); \(W_{s} = W_{q} + 1/\mu\) as above; and \(L_{q} = \lambda W_{q}\), Little's law applied to the queue.

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