In this work, the performance of IEEE 802.11 WLAN scenarios is computed by evaluating Quality of Service (QoS) parameters such as medium access delay, end to end delay, retransmission attempts and throughput at data rate of 54Mbps by means of different mechanisms used in PHY- and MAC-layers. The Point Coordination Function (PCF) and Distribution Coordination Function (DCF) of MAC layer in conjunction with IEEE 802.11a (Orthogonal Frequency Division Multiplexing at 5GHz Bandwidth) and IEEE 802.11g (Extended data rate DSSS at 2.4GHz Bandwidth) used in physical layer are evaluated and compared to offer a better mechanism that can provide better QoS at high data rates of 54Mbps using OPNET MODELER 14.5 simulator.