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All service control nodes, for example Web sites or Mobile Switching Centers, can be modeled as server systems with one or more servers processing the incoming requests. To avoid overload at the service control node different types of admission control mechanisms are usually implemented. In this paper we discuss, from a control point of view, the modeling of a service control-node. The queue is assumed to be an M/G/1-system and is modeled by a nonlinear flow model and a simplified discrete-time model is used in the analysis and design of the system. An admission control system based on a PI-controller combined with an anti-reset windup feature is developed. The stability of the admission control system is analyzed and the stability of the closed loop system is proved. The results from the simplified queue model are verified through discrete-event simulations of the system.