摘要
随着信息技术的发展,提供便捷访问Internet的IEEE802.11无线局域网(Wireless Local Area Networks, WLAN)正广泛应用于人们的工作和生活中。无线局域网仅涉及空中接口的物理层(PHY)和媒质接入控制层(MAC), MAC层协议性能成为影响无线局域网性能的重要因素,对IEEE802.11MAC协议的性能分析和改进具有重要的研究意义,本论文主要研究点如下：
1、针对MAC层DCF协议对多媒体业务的承载性能进行分析：在WLAN中传输视频和数据帧具有明显的差别,由于视频流需要给用户提供连续并且清晰的画面,因此需要重点关注视频帧的传输时延和吞吐量。在传输过程中,较长的视频帧会被分割并填充到服务数据单元(Service Data Unit, SDU)中。不同的帧率和帧长会影响到MAC层的传输性能(传输时延和吞吐量)。结合已有的Markov模型,针对IEEE802.11WLAN中的视频帧传输,提出了一个更精确的方法分析传输性能,在不同帧率条件下,推导了不同帧长和视频流数的关系,并且精确地计算了视频流的吞吐量。
2、针对MAC层DCF协议的业务多速率性能进行改进：IEEE802.11协议中规定了物理层所支持的多种接入速率,为了分析实际应用中低速接入节点对网络整体性能的严重影响,提出了DCF协议中基于Markov模型的帧时延的精确算法,推导出低速节点对WLAN网络整体吞吐量性能的影响,并用NS2进行仿真验证。在此基础上进一步提出低速节点的优化方法,为WLAN网络的规划和优化提供有效的理论参考依据。
3、针对MAC层DCF协议的业务公平性进行改进：在IEEE802.11DCF网络中,接入点(Access Point, AP)传输的数据量远大于其他站点,然而AP与其他站点接入无线信道的优先权相同,由此导致了上下行数据流吞吐量的不公平性。针对公平性问题的解决,提出了两种机制：一种机制是通过基于网络负荷动态调整AP的最小竞争窗口来实现,推导了不同上下行数据流条件下保证公平性的AP最小竞争窗口值,由AP负责监控实时数据流,当网络处于较重负荷时,AP通过动态调整最小竞争窗口的值来实现公平性；另一种机制是通过基于概率发送机制调整AP发送概率来实现,推导了不同上下行数据流条件下保证公平性的AP最优发送概率,主要通过调节AP的数据传输概率来实现公平性。理论分析和仿真结果显示,两种新机制均能够明显改善网络中上下行数据流的公平性,并在一定程度上提升网络整体吞吐量。
With the improvement of information technology, IEEE802.11wireless local area network (WLAN) has been widely used in our work and daily life. WLAN involves only physical layer (PHY) and medium access control layer (MAC) of the air interface. The performance of MAC layer protocol becomes an important factor to affect WLAN performance. Thus, the performance analysis and improvement of IEEE802.11MAC protocol are of great importance and practical value. This thesis mainly includes the following researches:
(1) The performance of DCF protocol for the multimedia services transmission is analysed in MAC layer. There are obvious differences between video and data frame transmission in WLAN, for the video streams need to provide people with consequent and distinct scene. Thus we should concern with the transmission delay and throughput for the video frame. In transmission, the long video frame would be segmented, and then fill into the SDU. Different frame rate and size lead to different MAC layer transmission performance such as delay and throughput. In this paper, a precise scheme is proposed to analyse the video transmission performance in IEEE802.11WLAN. With different frame rates, the relationship between frame size and video numbers has been conducted, and the video throughput has been preciously calculated.
(2) The performance of DCF protocol with multi-rate transmission is analysed in MAC layer. The function of multi-rate support in PHY is prescribed in IEEE802.11protocol. To analyze the serious affects to the performance of whole networks caused by the low-rate access nodes, a precise algorithm is proposed based on the improved Markov model in DCF protocol. The throughput of WLAN is conducted with low-rate access nodes, and the simulation results with NS2prove the conclusion. After that the optimization methods are proposed for the low-rate access nodes, and all these are the effective references for WLAN planning and optimization.
(3) The fairness of DCF protocol is analysed and improved in MAC layer. The data traffic needed to be transferred by AP is much more than the other STAs in the IEEE802.11DCF networks. However, AP should access the wireless link with the same priority as STAs, and thus the throughput between uplink and downlink is of great injustice. To solve the problem of unfainess, two schemes have been proposed. One sheme is acheieved by dynamically adjusting the minimum contention window of AP based on the network load. The AP minimum contention window value has been derived to ensure fairness under the conditions of different uplink and downlink data streams. AP monitors the growing real-time data traffic, and when the network system is in heavy traffic, AP could adaptively adjust the contention window to achieve fairness. And the other sheme is realized by adjusting the transmission probability of AP based on the method of transmission with probability. The optimal transmission probability of AP has been derived to ensure fairness under the conditions of different uplink and downlink data streams. The fairness could be acheieved by adjusting the transmission probability of AP. Detailed theoretical analysis and simulation results show that both schemes can effectively achieve fairness between uplink and downlink flows, with the total throughput increased.
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