Worldwide interoperability for microwave access (WiMAX) based wireless metropolitan area networks have recently received wide attention as they support high data rates with great quality of service (QoS) capabilities. In order to support QoS, bandwidth request (BW-REQ) mechanisms are suggested in the WiMAX standard for resource reservation. In this letter we propose a simple and accurate analytical model for the performance analysis of contention-based bandwidth request mechanisms. The accuracy of this model is verified with simulation results.