Approximations for the waiting time in the GI/G/s queue

The paper provides some two-moment approximation formulas for the mean waiting time and the delay probability in a GI/G/s queue. These formulas are certain combinations of the exact mean waiting times for the M/M/s, M/D/s and D/M/s queues and the first two moments of the interarrival times and service times. To see the quality of the approximations, they are numerically compared with exact solutions and other approximations for some particular cases.