【作者】 黄波；

【导师】 张晓敏；

【作者基本信息】 山东大学 ， 通信与信息系统， 2012， 硕士

【摘要】 近年来,无线通信技术和局域网技术发展迅速。无线局域网具有：组网方便,移动性好,扩展性强等优点。随着IEEE802.11标准的不断扩充与完善,无线局域网变得越来越普及,其中以基础型网络的应用发展最为迅猛。目前越来越多的城市提供无线接入服务,人们使用个人电脑和PDA等无线产品通过接入点访问网络资源。然而无线局域网自身的一些特点也注定导致一些问题的产生,这些问题对网络的性能影响比较大,其中之一为隐藏站点。隐藏站点使得系统的潜在冲突概率陡然增大,越高的冲突概率,越低的系统性能。为了克服站点隐藏带来的不利影响,IEEE802.11定义了RTS/CTS四次握手机制,通过在数据交换之前发送RTS与CTS控制报文来减小每次冲突持续时间。然而控制报文的传输也会占用带宽资源,在不存在隐藏站点时候使用四次握手协议并不能提高信道利用率,反而使之降低。因此,在不存在隐藏时候使用基本机制,而存在隐藏站点时候使用RTS/CTS机制,这将有助于提高无线网络系统的传输效率。在基础型网络中隐藏站点不可避免,因此有效地检测隐藏站点对于提高无线局域网的性能具有重要意义和实用价值。为了解决无线局域网隐藏站点带来的问题,已有的检测算法利用站点在收到确认帧之前信道空闲时间的长短来判断是否存在隐藏。通过NS2仿真分析其在多种不同移动环境下的性能,发现该算法使用范围具有局限性。由于该算法在移动环境下无法捕捉到隐藏站点的变化情况,因此仅适用于站点静止环境,而在站点移动环境下不仅不能提高反而可能降低网络性能。为了能够在移动环境下捕捉到隐藏站点变化情况,本文根据基础型网络的特点,在该算法的基础之上,提出种既能适用于静止环境,又能适用于移动环境的隐藏站点动态检测算法,并通过仿真软件NS2验证其有效性。仿真结果表明：在站点静止和站点在服务区内移动的多种环境下,所提出的动态检测算法都能获得较好的性能。在此基础上,对动态检测算法在站点跨服务区移动,站点进入省电模式和站点退出网络的特定应用环境中所面临的问题进行了分析,提出了解决方案。更多还原

【Abstract】 In recent years, the Local Area Networks (LAN) and wireless communication technology have got rapid development. WLAN is being favored by many people for its convenience, good mobility and better expansibility. With the expansion and improvement of IEEE802.11standards, WLAN has becoming more and more popular, especially for infrastructure network. At present, more and more cities provide wireless access service. People use PC, PDA and other wireless products to access traditional wired network resources through Access Point.However, the wireless LAN’s characteristic is bound to lead to some problems. These problems have a bad impact on the performance of wireless network, especially hidden stations. Hidden stations make the system potential collision probability increasing. The higher the probability of collision, the lower the system performance is. In order to overcome the adverse of hidden stations, IEEE802.11defines four handshake protocols, which is RTS/CTS exchange. Send the RTS and CTS control packets before data exchange to reduce conflict duration. However, the transmission of control packets will consume bandwidth resources. When there are no hidden stations, using four-way handshake protocol lower channel utilization instead of improving. When the existence of hidden stations, using the RTS/CTS protocol and when no hidden stations using basic protocol, it will help to improve the transmission efficiency of the wireless network system. Moreover, hidden stations will be inevitable in infrastructure network. Therefore, effectively detecting hidden stations has a great significance and practical value for improving the transmission efficiency of wireless network system.In order to solve the problems caused by hidden stations in WLAN, current detection algorithm using the length of idle time of channel after the station receiving a acknowledge frame to judge whether there are hidden stations. Through NS2simulation analysis of its performance in a variety of mobile environments and found that the using of the algorithm has its limitations that It applies to the static environment, but cannot provide good performance in a mobile environment. In order to be able to capture the hidden station changes in a mobile environment, according to the characteristics of basic network, based on the above algorithm, present hidden station dynamic detection algorithm, which is not only applicable to the static environment, and can be used in mobile environment. It is through NS2simulation software to verify its validity. The simulation results show that the proposed algorithm can obtain better performance in a variety of environments. On this basis, analyze some problems such as the movement of hidden stations in Extended Service Set, hidden stations being in power saving mode and hidden stations exiting from the network, which are faced by dynamic detection algorithm, and propose solutions.更多还原