Research On MAC Layer Performance And System Reliability Of Wireless Lan

A wireless local area network (WLAN) links devices via a wireless distribution method, and usually provides a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network. IEEE 802.11 protocol has now become the predominant protocol within WLAN technologies, and is widely applied in daily life. However, the performance of WLAN is subjected to many more uncertainties compared to LAN; hence its performance analysis and enhancement have attracted the attention of industrial and academic researchers. Among them, the MAC layer protocol is one of the most intriguing research areas.The key idea of IEEE 802.11 MAC protocol is using the backoff scheme to reduce the collision probability. The main contributions of this thesis are listed below:As the basis of WLAN performance analysis, the thesis calculates the performance parameters under ideal channel condition, including saturation throughput, packet drop probability, average packet delay and packet drop time, and analyzes the impact of station numbers and data transmission method.Based on above analysis, the thesis proposes two new backoff schemes named as hybrid backoff and hybrid backoff with slow CW decrease. The proposed schemes reduce the percentage of failure transmissions in total transmissions by enlarging the contention window in different stages, and enhance the system performance. The influence of proposed backoff parameters on system performance is also discussed.Then, the thesis analyzes on the error-prone WLAN channel by constructing a new three dimensional Markov model. The model's accuracy is verified by comparing the numerical results with simulation. The influence of bit error ratio and packet length on system performance is studied with the new model. The results show that the inability to differentiate the reason of transmission failures causes the dramatic performance degradation in the error-prone channel.After parsing the data transmission procedure in the error-prone channel, the thesis proposes a method to differentiate the transmission failures, and proposes a new MAC scheme using back to back DATA retransmission. The new scheme can avoid the unnecessary backoff and RTS/CTS handshake, and minimize the waiting time before success transfer when the packet encounters bit errors. Modeling and numerical results prove that the new scheme can improve the system performance in the error-prone channel.The thesis also studies the system reliability modeling method. It derives the error bound of L-layer simplified model in term of the failure and repair rates of each component without creating the comprehensive reliability model. Then, the thesis proposes a reliability modeling method by adding states layer by layer to build a simplified model within required precision. The example proves the feasibility and effectiveness of the method.At last, the whole work of the dissertation is summarized and future research issues are discussed.