A transform-free approximation for the finite capacity M/G/s queue

This paper develops a transform-free approximation for the steady-state queue-length distribution in an M/G/s queue with finite waiting spaces. The approximation is obtained by using a conservation law and some heuristics. It is shown that the approximation is exact for the cases with either no extra waiting space, exponential service-time distribution, or a certain two-parameter family of service-time distributions. It is also shown that the approximation has the same light-traffic properties as the known light-traffic limit theorem for the infinite capacity case when the number of waiting spaces is not less than one.