Let X{t) be the number of customers at time t ≥ 0 in a queueing system with s servers and finite capacity c, for which the time τ between two consecutive arrivals is a random variable such that 
We assume that the times between the arrivals of customers are independent and identically distributed random variables. Similarly, the service times are independent random variables having, for each server, the same probability density function as r. We define the stochastic process 
