With the rising popularity of IEEE 802.11 wireless networks, and the explosive increase of multimedia traffic (such as video, audio) in the Internet, the research on performance and QoS of wireless networks becomes more and more important. When the medium access control (MAC) protocol is taken into consideration, it is one of the important factors in performance and QoS problem of wireless networks. So it is significant to study the performance of IEEE 802.11 MAC protocol, and the QoS problem that relate to MAC protocol in wireless networks.We analyzed the delay performance of IEEE 802.11 MAC protocol. Based on the exponential backoff mechanism of DCF (Distributed Coordination Function), a two-dimensional Markov model with consideration of the freeze state is presented. Using this model, we analyzed the average medium access delay of DCF under saturation. We also analyzed the average queue delay of MAC layer by modeling the interface queue of each node with M/D/1 model. The simulation results show that the models are very accurate.Based on the analysis results of medium access delay, we proposed an approach to improve the delay performance of DCF. In the improved DCF, by sensing the frames transmitted by other nodes, each node maintains an active node table to estimate the real-time number of active nodes in the network. When a node contends the channel, it will set the value of initial contend window according to the current number of active nodes. The simulation results illustrate that the improved DCF outperform the standard DCF on both delay and throughput performance.To alleviate the impact on reserved bandwidth caused by channel contention at MAC layer in ad hoc networks, we proposed a cross-layer mechanism, neighborhood reservation (NR), which can provide guarantee for the bandwidth reservation. When a node reserves bandwidth for a QoS flow, corresponding bandwidth should be reserved in all its neighboring nodes through controlling the transmission rate of best effort flows at MAC layer. Thus the reserved bandwidth can be guaranteed.By integrating the in-band signaling system INSIGNIA with NR, we proposed an in-band signaling system with NR – NR-INSIGNIA. Compared with INSIGNIA, the simulation results show that NR-INSIGNIA can improve the reserved packet ratio, throughput, packet delivery ratio, and the end-to-end delay of QoS flows.