Research On Adaptive MAC Technologies For Distributed Wireless LAN

As a convenient communication method, distributed wireless LAN (DWLAN) became more and more popular by the virtue of its inherent advantages on flexibly and rapidly networking. The main task of medium access control (MAC) protocol is to efficiently share limited wireless channel among distributed stations. Conventional MAC mechanism can hardly provide good performance in a complex environment of distributed networks. Consequently it is meaningful to develop MAC techniques with adaptive ability for network environment. Based on performance analysis and optimization of system, this dissertation focuses on adaptive MAC technologies for DWLAN.Chapter 1 introduces the features of wireless LAN (WLAN) and its related standards. Research works on MAC for WLAN are introduced, which includes objects, history, MAC mechanism in current standards, and recent research trends. The meaning and difficulties of MAC for DWLAN are also presented.Chapter 2 analyzes the throughput performance of MAC protocol for DWLAN and its optimization algorithms. Based on delicate mathematical model of IEEE 802.11 DCF, the average time spent in a successful and unsuccessful transmission is worked out. Then the relation of network saturation throughput and RTS threshold is formulated in a closed-form function. Analysis shows there is an optimal RTS threshold that can maximize network saturation throughput. According to the numerical method for finding out the optimal RTS threshold, an adaptive RTS threshold adjust algorithm to optimize network capacity (ARTA-OC) is proposed. A station with ARTA-OC can automatically adjust its RTS threshold to current optimum value; as a result, the network saturation throughput is maximized. Simulation results show that ARTA-OC has good performance.Chapter 3 studies adaptive backoff mechanisms for DWLAN and presents a priority backoff algorithm (PBA). The basic idea of PBA is that each station should collect statistics for other stations' transmission while sensing the channel, and maintain a sent-data table (SDT) for all stations in the network. When the station hasdata to transmit, it will calculate contention window (CW) with the statistics in SDT and its own priority. The principle of CW adjustment is: if the amount of sent data weighted by priority is higher than the network average value, a larger CW should be chose; otherwise a smaller CW should be chose. A station can adaptively select a proper CW in latest situation, thereafter collisions are reduced and performance is promoted, as well as network capacity is assigned among stations in proportion to their priorities. Simulation results prove that PBA has better performance than conventional backoff algorithm.Chapter 4 focuses on the fragmentation mechanism in wireless noise channel. The necessity of fragmentation in wireless channel with transmission bit errors is introduced, as well as fragmentation/defragmentation specified in IEEE 802.11. An analytical model is proposed to describe the states of a station in fragmentation transmission and their transitions. Then some performance metrics have been produced, which include the probability of a packet to be successfully transmitted after several retry attempts, the probability of a packet to be dropped, and the average retransmission times per packet. A parameter called transmission efficiency, which considers the channel time occupied by transmission and the successful probability of transmission, is introduced to evaluate the performance of fragmentation scheme. Furthermore, the impact of packet length and maximum retry limitation on transmission efficiency are analyzed respectively. Analysis shows that even in a same environment, packets with different length have different optimal fragmentation threshold corresponding to their maximum transmission efficiency. Larger maximum retry limitation can significantly increase the reliability of transmission while no benefits can be got on transmission efficiency in most cases. These research works are valuable in practical application because they can provide theoretical basis for optimization of fragmentation schemes and development of adaptive fragmentation algorithm.Chapter 5 discusses the energy consumption in transmission of stations in DWLAN and its optimization. IEEE 802.11 DCF is also chosen as basis of analysis. Firstly the energy consumption when a station transmits a certain amount of data in basic way and RTS/CTS way are analyzed respectively. Then the impact of RTS...