We consider a channel aware scheduling for the downlink multi-service flows in orthogonal frequency division multiple access (OFDMA) based mobile WiMAX (Worldwide Interoperability for Microwave Access)system. We propose an intelligent medium access control (MAC) that adapts with the physical layer in response to the different QoS requirements of the diverse service flows in the system. This adaptation is represented by a joint packet scheduling and slot allocation scheme interaction in both MAC and PHY layers. Based on this interaction, a fair and efficient QoS guaranteed resource allocation for a mixed real-time and non real-time service flows is achieved. We provide extensive simulation results showing the performance aspects of the proposed cross-layer scheme.