Nowadays WiMax is considered as one of the main technologies for next generation high speed wireless access network. It provides a larger coverage compared to WiFi while supporting string QoS and security mechanism. However, this technology needs fine configurations tuning to achieve its performance, which implementation in different environments is very difficult for operators willing to deploy it. This paper presents QoS estimation methods and measurements over WiMax networks and highlights the complexity of this process. It presents performance measurements in term of achievable rate of an operational WiMax network to evaluate its capability to support different types of media transmission. Finally, it highlights the importance of an accurate time synchronization in the measurement process which is difficult to achieve and therefore proposes a time synchronization method to derive end-to-end delay measurements with appropriate accuracy.