Under the constraints of limited bandwidth and exponentially rising user demand, there is a dire need to maximize throughput. Maintaining Quality of Service (QoS) in a communication network demands congestion control with high accuracy. This paper focuses on this challenging task that incorporates design of effective congestion controller to reduce packet loss in cognitive radio networks. Limited buffer capacity and bandwidth impose restriction on the performance when number of requests increase. The controller is designed on the notions of sliding mode which is a variable structure control technique known for its robustness and disturbance rejection capabilities. An optimal design strategy is used in development of the controller. The efficiency of the controller is confirmed by numerical simulations. Keywords-Quality of Service (QoS), Cognitive Radio Net works (CRN), opportunistic spectrum utilization, fluid flow model, non stationary Poisson process, quadratic minimization, infinite switching, sigmoid functions.