【作者】 张亮；

【导师】 舒炎泰；

【作者基本信息】 天津大学 ， 计算机应用技术， 2006， 博士

【摘要】 近几年,IEEE 802.11无线网络得到了迅速的发展,并已经被广泛地应用在人们的工作和生活中。同有线网络相比,无线网络的性能还有很大差距,这除了其物理传输介质的固有特点之外,MAC层协议是一个非常重要的因素。因此,对IEEE 802.11 MAC层协议的性能分析和改进具有非常重要的研究意义和应用价值。在带有接入点的无线局域网中,所有无线节点只能和接入点直接通信,每个数据帧都需要接入点进行转发,而且接入点的MAC层协议也是标准的DCF协议,这样在竞争信道时,接入点和其它无线节点具有相同的优先级,从而导致接入点成为无线局域网中的瓶颈,严重影响了无线局域网的性能。本文对该问题进行深入分析,并提出了两种改进策略:快速直接转发方案和提高接入点优先级方案。理论分析和仿真结果证明这两种方案都可以有效地解决无线局域网中接入点瓶颈问题,提高无线局域网的整体性能。基于IEEE 802.11b的无线网络支持多传输速率,无线节点可以根据环境动态的调整自己的传输速率来获得较高传输性能。本文分析了这种多速率特性对无线局域网性能的影响,在MAC层提出一种基于信道占用时间的公平调度算法,每个节点通过调整数据帧大小、每次发送的数据帧的数量或自身竞争信道的次数,来保证无线局域网在多速率情况下信道占用时间的公平性,同时也提高了无线局域网的系统吞吐率。本文在原有工作的基础上利用二维Markov模型进一步对DCF的平均介质访问延迟进行了研究。在此基础上,分析了RTS阈值和初始竞争窗口这两个参数对网络性能的影响,并针对无线局域网的特点提出了这两个参数的自适应算法,提高了无线局域网的性能。大量的小数据帧传输会严重的影响无线网络的性能,为了解决该问题,本文用小的数据帧来代替无线网络中的控制帧(如:CTS控制帧),这样既减少了无线网络中小数据帧的单独发送,也减少了MAC层控制帧的发送,从而能够充分利用无线信道,提高无线网络的性能。更多还原

【Abstract】 In recent years, IEEE 802.11 based wireless networks are becoming increasingly prevalent. Compared to the traditional wired networks, performance is expected to be worse in wireless networks in general. Yet, performance degradation is not only caused by inherent wireless characteristics, but also due to the inefficiencies within the Medium Access Control (MAC) protocol. As a result, to further improve the efficiency of IEEE 802.11 based networks, it remains necessary to study and improve the performance of IEEE 802.11 MAC protocol.In fact, since all WLAN data frames are routed through a centralized AP, and the AP shares the same priority as the other wireless stations in accessing the channel, the wireless AP is indeed a bottleneck in WLANs. This bottleneck effectively causes serious performance degradation within the network. In this thesis, we analyze the AP bottleneck problem in WLANs, and we proposed two approaches to alleviate this problem: cut-through forwarding method and a priority AP method. We performed various theoretical analysis and related simulations. The results shows that our proposed methods can indeed improve DCF performance in WLAN (compare to the original standard).IEEE 802.11 based wireless network can support multiple transmission rates. Stations in such network can adaptively adjust their transmission rate according to the environment. We also analyze the performance of WLANs under multiple transmission rates and propose an algorithm to guarantee fairness and to improve the aggregate throughput in WLANs. More specifically, we use the Channel Occupancy Time (COT) instead of the channel access probability in analyzing the fairness of WLANs. With our algorithm, all the wireless stations in the WLAN can achieve COT-based fairness by adjusting their packet length, sending the multiple back-to-back packets at one time, or giving up an opportunity to access the channel altogether. Our algorithm also leads to significant improvements to the aggregate throughput in multi-rate WLANs.Moreover, based on our previous work, we analyze the delay performance of IEEE 802.11 MAC layer protocol. Based on our analysis, we analyze impact of the RTS threshold and initial contention window size on the WLANs’performance. We further propose two algorithms that adaptively adjust these two parameter values to improve WLAN performance.Lastly, we recognize that transmitting very small packets worsens the performance of更多还原