Under light traffic, we investigate the quality of a well-known approximation for first-moment performance measures for an M/G/c queue, and, in particular, conditions under which the approximation is either an upper or a lower bound. The approach is to combine known relationships between quantities such as average delay and time-average work in system with direct sample-path comparisons of system operation under two modes of operation: conventional FIFO and a version of preemptive LIFO. We then use light traffic limit theorems to show an inequality between time-average work of the M/G/c queue and that of the approximation. In the process, we obtain new and improved approximations.
