A system (100) for evaluating performance of a network providing communication services by means of a generalized processor sharing (GPS) packet scheduling discipline. The services may include classes of services having real time and non real time (i.e. 'elastic') requirements, respectively, and the classes of services have relative priorities. The system (100) includes: - a performance evaluation module (120) configured for evaluating performance of a given class (r) of services as a function of the load (Σ1(r)) produced by other classes of services having a priority higher than or equal to said given class (r) as well as of perturbation effect (Σ2(r)) exerted on said given class (r) by other classes having priority higher than said given class (.r), and - a computation module (110) configured for determining said load (Σ1(r)) and said perturbation effect (Σ2(r)) as weighed sums of the contributions of each of said other classes to said load (Σ1(r)) and said perturbation effect (Σ2(r)).