The authors are interested in evaluating the performance of a cognitive radio network composed of secondary and primary mobiles, and look for optimising the decision process of the secondary mobiles when they have to choose between licensed or unlicensed channels. In fact, the system is composed of several channels where only one unlicensed channel is shared between all the secondary mobiles, when they decide to use this particular channel. As the secondary mobiles are equipped with cognitive radios, they are able to sense the licensed channels and use one of them if it is free. The authors consider first the global system and look for the optimal proportion of secondary mobiles that sense the licensed channels in order to optimise an average performance of the system. Second, the authors assume that each secondary mobile decides to sense or not the licensed channels and, are interested in an equilibrium situation as the secondary mobiles are in competition. After showing the existence and the uniqueness of equilibrium, the performance of this equilibrium is evaluated by looking at the price of the anarchy of the system.