The most important standard in wireless local area networks (WLANs) is IEEE 802.11. For this reason, much of the research work for the enhancement of WLANs is generally based on the behaviour of the IEEE 802.11 standard. This standard is divided into several layers. One of the important layers is the medium access control (MAC) layer. It plays an important role in accessing the transmission medium and data transmission of wireless stations. However, it still presents many challenges related to the performance metrics of quality of service (QoS), such as system throughput and access delay. Modelling and performance analysis of the MAC layer are also extremely important. Thus, the performance modelling and analysis have become very important in the design and enhancement of wireless networks. Therefore, this research work is devoted to evaluate and enhance the performance modelling of IEEE 802.11 MAC-distributed coordination function (DCF), which can lead to the improvement of the performance metrics of QoS. In order to more accurately evaluate the system performance for IEEE 802.11 DCF, a new analytical model to compute a packet transmission probability for IEEE 802.11 DCF has been proposed based on difference probabilities in transmission mechanism. The performance saturated throughput is then evaluated with the proposed analytical model. In addition, a new analytical model for estimating the MAC layer packet delay distribution of IEEE 802.11 DCF is also proposed. The performance results highlight the importance of considering the different probabilities between events in transmission mechanism for an accurate performance evaluation model of IEEE 802.11 DCF in terms of throughput and delay. To enhance the effectiveness of IEEE 802.11 DCF, a new dynamic control backoff time algorithm to enhance both the delay and throughput performances of the IEEE 802.11 DCF is proposed. This algorithm considers the distinction between high and low traffic loads in order to deal with unsaturated traffic load conditions. In particular, the equilibrium point analysis (EPA) model is used to represent the algorithm under various traffic load conditions. Results of extensive simulation experiments illustrate that the proposed algorithm yields better performance throughput and a better average transmission packet delay than related algorithms.