IEEE 802.11 Wireless Local Area Networks (WLAN) supply broadband data accesses and evolve a growing demand of multimedia services for mobile users. To provide quality of service (QoS) for multimedia applications in WLANs, it is important to analyze the performance of 802.11 Medium Access Control (MAC) protocols under varying heterogeneous channel conditions. In this paper, we focus on 802.11 WLANs which adopt the MAC protocol of Distributed Coordination Function (DCF), and provide performance analyses to show the interlayer effect of physical-layer (PHY) channel diversities on DCF protocol as well as the consequent impact on multimedia QoS at application (APP) layer. We then discuss some feasible crosslayer designs for overall optimizing QoS performance in 802.11 WLANs. We also take example of the QoS unfairness problem due to the interlayer impact of PHY channel diversities and illustrate a solution of our cross-layer approach.