In this paper, we propose and analyse a new scheme to handle ranging requests in IEEE802.16e networks based on differentiated backoff parameters and a novel hierarchical code partitioning principle. Our analysis is of the form of a fixed-point equation which characterizes the system operating points and which allows us to compute the attempt rate of each QoS class as well as the distribution of the number of ranging bandwidth requests received successfully by the base station as well as their average delay. Both analysis and simulation confirm that our scheme yields high throughput and low collision probability for real-time connections without altering the collision probability for non-real-time connections.