Requests arrive from a finite source of size N and are served by one of r(r ≤ N) servers at a service facility according to a First-Come-First-Served (FCFS) discipline. The service times of the requests are supposed to be identically and exponentially
distributed random variables with means 1/μ. After completing service, request i returns to the source and stays there for a random time having general distribution function Fi(x) with density fi(x). All of these random variables are assumed to be independent of each other.