The Research on Performance Enhancement Mechanisms in IEEE 802.11 Based Wireless Local Area Networks

近年来，伴随无线通信技术的快速发展，无线局域网得到了越来越多的应用。本论文研究基于IEEE 802.11系列协议的无线局域网的性能改进机制。本文的主要贡献和创新点如下： 提出了一种分析EDCA接入方式下MAC接入延时的新模型，准确地给出了MAC接入延时的概率分布，为排队性能分析提供了依据。基于信号流图和母函数的研究方法，对EDCA接入方式下的MAC接入过程进行了详细分析，得到了MAC接入延时的概率分布和相关的数字特征，各类AC的发送概率、碰撞概率、挂起概率和延时均值随终端数目变化的规律，各类AC的AIFS值对MAC接入延时的影响，以及数据帧的丢失率变化规律，并得出结论——接入延时的变异系数大于1，因而采用负指数分布作为MAC接入延时的近似分布会错误估计系统的服务能力。仿真结果与模型的分析结果吻合良好，验证了模型和计算方法的正确性。 提出了一种EDCA的改进机制NEDCA，改进了网络的吞吐量和公平性。在NEDCA机制中，采用EEBA回退机制，根据网络传输过程中的时间相关性，充分利用无线信道的利用率信息，在持续的传输过程中，采用指数增加指数减小的方式自适应的设置新的回退窗口值，减少了不必要的碰撞冲突，提高网络的吞吐量；采用ATCU机制，利用无线信道的利用率信息，自适应的设置TXOP值，提高了各节点传输的公平性。通过仿真验证了吞吐量与公平性的改进。 提出了无线局域网间MAC层切换的改进机制以减少终端切换过程的延时。采用基于监测的多策略切换机制，根据业务流量的规格要求、AP的信号强度以及用户自定义的其它规则，判定移动终端是否需要切换，可显著减小探测阶段的延时；采用基于邻居图的同步扫描切换机制，利用邻居图获取AP的邻接关系，结合同步扫描机制，进一步减少终端在扫描过程中的切换延时，仿真结果表明相对同步扫描机制而言，该机制对正常通信过程的影响较小。 提出了一种无线局域网的自适应健壮报头压缩机制AAHC，可适应无线局域网的信道变化特性，在取得较高的压缩率的同时，具有很好的抗差错健壮性。AAHC机制中，采用三状态Markov模型建模无线信道，根据接收比特误码率划分信道状态，可屏蔽不同调制方式的影响；采用健壮的W-LSB编码方法对报头进行压缩；基于信道状态自适应调整可变滑动窗口VSW窗口大小，以取得压缩率与鲁棒性的动态平衡；传输层采用UDP Lite协议，减小信道出错的影响，提高信道的吞吐量。 提出了一种可有效保证实时业务性能并充分利用信道的准入控制机制。在新业务接入时，根据网络中已经存在的业务数量和流量特性，以及网络的服务能力，按照一定的规则实施准入控制，以保证接入业务的性能，并充分利用无线信道资源，提高网络的吞吐量。结合实验室所研制的社区宽带综合业务接入系统，详细介绍了准入控制机制在无线接入访问点设备WG10中的实现方法。 最后，总结了全文并指出了进一步的工作方向。With the rapid development of wireless communication technology, wireless local area networks (WLAN) have been extensively applied in the recent years. In this dissertation, we study the performance enhancement mechanisms of WLANs based on IEEE 802.11 protocols. The main works of this dissertation include the following content： Firstly, a new model is proposed to analyze the MAC access delay of EDCA mechanism of WLANs, which is based on the moment generating function and signal flow graph. Through this model we get the probability distribution function (PDF) of MAC access delay and several relative characteristic numbers, including the mean value, variance, standard deviation and deviation coefficient of MAC access delay. The results of the analytical model coincide with simulations, which validates the model proposed. A lot of analyses are presented based on the new analytical model. Secondly, a mechanism named NEDCA is proposed to promote the performance of EDCA. NEDCA modifies EDCA in the following two ways. The first way is that NEDCA adopts EEBA mechanism, which uses the exponential-increase exponential-decrease method to adapt the backoff window according to the channel utilization information of wireless channel. Through EEBA, some unnecessary collision can be avoided. The second way is that NEDCA adopts ATCU mechanism, which adaptively set the TXOP values based on the channel utilization information to promote the fairness index. Thirdly, mechanisms to promote the MAC handoff performance are proposed. In this dissertation, we propose two mechanisms: monitoring multi-policy handoff mechanism (MMH) and neighbor graph synchronizing handoff mechanism (NGSH). In MMH, the decision of handoff is based on the characteristics of the traffics, the signal strength of APs and other user defined criterions, which decreases the detection phase delay a great deal. NGSH combines the neighbor graph and synchronizing handoff, which can reduce the amount of wireless channels need to be scanned in handoff procedure and minimizing the impact to normal communication. Fourthly, an advanced adaptive header compression (AAHC) scheme is proposed, which adjusts the parameters of compression algorithm according to the exact estimation of wireless channel state and adopts UDP Lite as its transport layer protocol. This scheme can get high compression ratio and error-resistant robustness at the same time. The simulation results show that AAHC scheme outperforms the traditional ROHC scheme and CRTP scheme. At last, an access control scheme of infrastructured 802.11b WLANs is proposed, which controls the accepting of new traffics according to the existing traffics in the network and the characteristics of the new traffics. The realization of this access control mechanism in the WLAN AP equipment-WG10 is introduced

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measurement and analysis to wireless sensor network node energy consumption

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