【作者】 任娟；

【导师】 裘正定；

【作者基本信息】 北京交通大学 ， 信号与信息处理， 2010， 博士

【摘要】 随着无线通信技术的发展以及移动设备的盛行,提供高质量的无线宽带接入Internet服务已成为人们日益增长的需求。然而,目前相对比较成熟的WLAN技术仅能实现单跳无线接入,其信号覆盖范围较小且只能布设在有线网络附近,不能满足更大范围的无线网络覆盖需求。作为一种新兴的下一代网络,无线Mesh网络利用无线多跳技术实现更大范围的无线接入网络覆盖,并以覆盖区域广、低成本、接入便利等诸多优势,成为无线宽带接入Internet的关键技术。由于无线Mesh网络采用无线多跳技术,而数据流在无线网络中经过多跳传输后,其性能就会急剧下降。因此,如何降低跳路间的竞争和干扰、提高网络性能是无线Mesh网络研究的重点。本文围绕这一问题展开讨论,重点研究了基于IEEE802.11的无线Mesh网络信道分配策略、多信道路由协议、以及单信道和多信道网络中的拥塞控制机制。论文主要工作和创新如下:首先,对无线Mesh网络的基本原理进行概述,提出目前无线Mesh网络面临的一些主要问题,并分析了现有相关成果所存在的不足。对基于IEEE 802.11的Mesh网络节点进行层次结构介绍,总结各层面相关的研究热点,同时还对国际上著名的研究项目、解决方案进行综述。其次,对无线Mesh网络的路径内及路径间干扰问题进行分析,介绍了单接口Mesh网络和多接口Mesh网络中实现跨信道通信的不同原理及优缺点,概述现有的动态分配、静态(或准静态)分配、混合分配三类信道分配方案。针对无线Mesh骨干网络特点,提出了一种准静态的集中式信道分配方案,以实现信道资源的灵活分配和有效利用。实验证明,采用提出的信道分配算法后网络的吞吐量及稳定性均得到了有效提高。再次,概述了相关ad hoc网络路由协议及现有的Mesh网络路由协议,分析了现有多信道Mesh路由协议通过建立路径质量评测尺度实现信道多样化、高吞吐量的路由原理。通过分析路由协议与信道分配方案之间的影响关系,同时结合多径路由技术的负载平衡功能,为Mesh骨干网络流量提出了一种与信道分配关联的多信道负载平衡路由策略。仿真表明采用负载平衡路由算法后,网络的整体吞吐量远胜过单信道路由,而且较一般多信道路由也有明显改善。然后,分析了传统TCP和UDP传输协议在无线多跳网络中面临的问题,综述了现有基于窗口和基于速率两种无线多跳网络拥塞控制算法的原理和特点。通过对单信道Mesh网络的空间复用性的研究,以及对大量实验数据的分析,建立了多跳传输的链路层丢包模型。同时结合Mesh网关的高处理能力和流量汇聚特点,提出一种自适应速率的网关拥塞控制算法,用于提高单信道网络的空间复用性,避免网络拥塞。实验证明,所提出的算法较已有控制算法更有效提高了网络的吞吐量和公平性。最后,分析了多信道网络与单信道网络的多跳传输区别,提出多信道网络空间上和频域上的双重复用特性,并指出该特性下现有拥塞控制机制存在的MAC层传输协调问题。针对这些问题,提出了一种竞争感知的跨层拥塞控制机制,该机制采用连续多包定时发配的方式来取代传统拥塞控制机制中的单包定时发配方式,同时根据节点的可用带宽计算合适的发包间隔,从而达到既减轻MAC层竞争也提高信道利用率的目的。实验数据充分说明,所提出的拥塞控制机制对提高网络吞吐量和公平性、降低网络延迟都有很好的效果。更多还原

【Abstract】 With the development of wireless communication technologies and the prevalence of mobile equipments, to provide high quality services of wireless broadband access has become people’s daily increasing demand. However, the relatively mature techniques of WLAN can’t satisfy people’s requirement for large coverage of wireless network since it can only realize single-hop wireless access. As a new next generation network, the wireless mesh network provides more extensive coverage of wireless access networks, and it has become a key wireless broadband technique according to its various advantages such as large coverage and low cost.Because the wireless mesh network uses multi-hop wireless technology, the performance of flows will decrease rapidly when they traverse through the network by multiple hops. So how to reduce the interference and improve the network performance are the key issues of wireless mesh networks. This paper aims to resolve these problems by researching on the channel assignment, multi-channel routing and congestion control for the IEEE 802.11-based wireless mesh networks. The major works and innovations of this paper are described as below:Firstly, the principle of wireless mesh networks is summarized. Some key problems for current wireless mesh networks are proposed and the shortages of existing researches are analyzed. The hiberarchy of node in IEEE 802.11-based wireless mesh networks and the related hot research issues are introduced. Meanwhile, the well-known international research programs and existing solutions are also summarized.Secondly, the problems of intra-flow and inter-flow contentions are analyzed. The principles and characteristics of realizing cross-channel communication in single-radio mesh networks and multi-radio mesh networks are introduced respectively. Three kinds of current channel assignment schemes are classified which are dynamic assignment, static (or quasi-static) assignment and mixed assignment. According to the characters of backbone mesh networks, a centralized quasi-static channel assignment scheme is presented which is able to provide flexible and efficient ways for utilizing the channel resources. Simulation results show that after using the proposed channel assignment algorithm, both the network throughput and stability get effectively improved.Thirdly, related ad hoc routing protocols and existing wireless mesh network routing protocols are summarized. The principle of existing multi-channel mesh routing protocols is introduced which realize channel-diverse and high-throughput routing by establish path metric which can evaluate path qualities. Via analyzing the interactions between channel assignment and routing and combining with the load-balancing function of multi-path routing technique, a load-balancing multi-channel routing scheme is proposed to provide high-throughput paths for the backbone traffics in wireless mesh networks. Simulation results validate that with the load-balancing routing algorithm, the aggregate network throughput surpasses those of the single-channel routing as well as the common multi-channel routing.Then, the problems for traditional TCP and UDP protocols to be faced in wireless multi-hop networks are proposed and two kinds of existing congestion control schemes for wireless multi-hop networks are classified, which are respectively the window-based schemes and the rate-based schemes. Through studies on the spatial channel reuse of single-channel mesh networks, a link-layer packet drop model for multi-hop transmission is established. Meanwhile, considering the high processing capacity and the traffic aggregate character of the mesh gateway, a rate-adaptive gateway congestion control scheme is proposed, aiming at improving the spatial channel reuse and avoiding congestion for single-channel mesh networks. Performance evaluations prove that the proposed scheme outperforms existing control scheme in both the network throughput and fairness.Finally, the paper studies the multi-hop transmission differences between multi-channel networks and single-channel networks, and presents dual channel reuse characteristic in spatial and spectral aspects for multi-channel networks. Existing congestion control schemes suffer MAC-layer transmission coordination problems due to this characteristic. So a new contention-aware congestion control scheme is proposed, which introduces the way of dispatching multiple packets at each transmission interval instead of single packet in traditional rate-based control scheme. The proposed scheme computes a proper transmission interval according to node’s available bandwidth. As a result, it’s able to relieve the MAC-layer contention as well as improve the channel utilization. Simulations prove that the proposed scheme has quite good effects in improving the network throughput and fairness as well as reducing the network delay.更多还原