As usual we need some notations. In steady state let us denote by
• N number of customers in the system,
• P(z) the generating function of N, that is P(z) = E(zN),
• B∗(s) =R ∞ 0 e−stdB(t) the Laplace-Stieltjes transform of the service time S,
• W∗(s) the Laplace-Stieltjes transform of the waiting time in the system, or response time T ,
• W∗ q (s) the Laplace-Stieltjes transform of the waiting time in the queue Tq,
• C2S= V ar(S) E2(S) squared coefficient of variation of service time S,
• L = E(N), Lq = E(Nq) average number of customers in the system, queue, respectively,Pollaczek and Khintchine derived them in the period 1930-50.