Before revision:  In most cases it can be computed using either a recursion or a differential equation (for exponential job sizes with mean one). In this paper we study the value of the mean response time E[ R _\lambda] as the arrival rate \lambda approaches 1  (i.e. the system gets close to instability).
After revision:  In most cases it can be computed as the solution of a set of differential equations. Here we study the value of the mean response time E[ R _\lambda] as the arrival rate \lambda approaches 1 (i.e. the system gets close to instability).