【作者】 许昌春；

【导师】 张尧学； 杨宗凯；

【作者基本信息】 华中科技大学 ， 信息与通信工程， 2007， 博士

【摘要】 移动无线自组织网络(Mobile Wireless Ad Hoc Networks,MANET)是一种特殊的无线通信网络,是由多个带有无线收发装置的节点组成的一个多跳的自治系统,网络中的节点同时具有主机和路由器的功能。近十年来,无线自组织网络日益成为计算机网络和无线通信领域的研究热点,至今方兴未艾。由于不需要固定的基础设施并且具有极强的抗毁性能,无线自组织网络具有很高的军事应用价值。无线自组织网络的主要特征是无线信道共享、误码率高、多跳通信以及动态拓扑等,在这种情况下为多媒体通信提供服务质量(QoS)的支持具有很大的挑战性。媒体接入控制(MAC)协议位于MANET协议栈的下层,对网络的性能影响极大,在MAC层提供QoS支持是无线自组织网络的关键技术,本文围绕这一热点问题展开了深入研究。当前无线自组织网络最重要的MAC协议是IEEE802.11DCF,DCF是一种基于载波监听和冲突避免的随机接入协议。本文在分析802.11DCF协议的性能的基础上,深入分析了802.11无线自组织网络在MAC层实现服务区分和统计意义上QoS支持的能力。另一方面,考虑战术Ad Hoc网络的特殊情况,设计了一种基于多收发机的MAC协议。具体研究成果如下:(1)非饱和条件下IEEE 802.11 DCF协议性能分析在假设节点的发送缓冲区只能容纳一个数据分组的前提下,建立了一个二维马尔可夫链模型,准确分析了任意负荷条件下802.11DCF协议的性能包括吞吐量、平均分组时延、时延的方差等。结果表明,基本接入机制可获得与RTS/CTS机制几乎一样的最大吞吐量,并且时延性能大大优于饱和状态,从而初步表明802.11DCF具有支持统计意义服务质量的能力,也为全文的研究奠定基础。(2)非饱和条件下IEEE802.11e区分服务机制性能分析将针对非饱和条件下的马尔可夫链模型进行推广,分析结果表明,基于初始竞争窗口的IEEE 802.11eEDCA仅能在饱和条件下实现QoS(吞吐量、平均分组时延等)的相对区分,不能提供QoS的任何保证,而且当网络的总负荷低于最佳值时,基于初始竞争窗口的IEEE 802.11e基本上不能提供QoS的区分。(3)非饱和条件下的服务区分和QoS保证技术提出了直接基于业务负荷(即分组大小乘以到达率)的区分服务机制,设各个业务流具有不同的分组大小和到达率,分析结果表明,在控制总负荷的情况下,各个站点的吞吐量近似等于其负荷(即分组大小乘以到达率),而且平均分组时延比饱和状态小得多。(4)IEEE802.11MAC层排队性能分析上述创新点均假设站点的发送队列只能容纳一个数据分组,因此无法分析排队性能。新的分析模型由一个广义的马尔可夫链模型和M/G/1/K排队模型组成,能够准确地分析吞吐量、平均排队时延、MAC服务时间的均值及其标准差、分组阻塞概率等排队性能指标。(5)非饱和条件下802.11DCF协议的性能优化和准确统计意义上的QoS保证通过理论推导,得出最佳总负荷以及最优性能指标的解析表达式。当处于最佳总负荷点时,IEEE 802.11DCF可准确提供统计意义上的QoS保证,即吞吐量最大化、平均排队时延和分组阻塞概率趋近零、MAC服务时间的均值和标准差都大大低于饱和状态下的值,本文进一步推导得到了最大吞吐量的下界、MAC服务时间的均值和标准差的上界。(6)基于多收发机的MAC协议研究传统的Ad Hoc网络处于节能的考虑,每个节点只配备一个无线收发机,另一方面,IEEE802.11标准定义了多个频段而实际系统只使用其中一个。以车辆或舰艇为节点的战术Ad Hoc网络由于能量供应和节点尺寸上的优势,每个节点可配备多个无线收发机并充分利用802.11所定义的多个信道。本文设计了相应的MAC协议并分析其性能,该协议可大大提高多跳环境下的空分复用程度从而显著提高系统的吞吐量。更多还原

【Abstract】 A mobile wireless ad hoc network (MANET) comprises of a cluster of mobile nodes without any fixed infrastructure of base station. Due to the transmission range constraint of transceivers, two mobile nodes may communicate with each other either directly, if they are close enough, or indirectly, by having other intermediate mobile nodes to forward their packets. MANET has become the one of the most significant hotspots in the fields of computer networks and wireless communications over the last two decades. Since MANET can be rapidly deployed, it has important merit for military applications such as armies in march, battle fields, fleets in oceans and other hostile scenarios. Because of the existing bit error rate in shared wireless channel and dynamic topology, it imposes big challenge on Quality-of-Service (QoS) supportting for multimedia communications over wireless ad hoc networks.The media access control (MAC) protocol, which lies at the low layer of the protocol stack for MANET, has significant influence on the performance of the network. The most important MAC protocol for MANET is IEEE 802.11 Distributed Coordination Function (DCF). DCF is a career sense multiple access with collision avoidance (CSMA/CA) protocol with binary exponential backoff. By modeling of IEEE 802.11 DCF, this dissertation focuses on providing service differentiation and statistical QoS gurantee in original IEEE 802.11 ad hoc networks. Furthermore, the tranditional single channel 802.11 MAC protocol is extended for the case that multiple transceivers and multiple channels available for a node in MANET. The main contributions in this thesis are organized as follows.(1)Performance analysis of IEEE 802.11 DCF in non-saturated traffic conditionsOn the assumption that each station can buffer just one date packet, a bi-dimensional Markov chain model is developed for modeling of 802.11 DCF in non-saturated conditions. The analytical model is quite simple but highly accurate. It’s shown that basic access mechanism can achieve almost the same maximum throughput as that of RTS/CTS mechanism in non-saturated conditions, at the same time, the average packet delay as well as its standard deviation is much lower than that in saturated conditions.(2)Performance analysis of IEEE 802.11e service differentiation mechniams in non-saturated conditionsBy extending the Markov chain model for 802.11 station in non-saturated conditions, this thesis further analyazes the QoS differentiation performance of IEEE 802.11e EDCA in non-saturated conditions. Unfortunately, IEEE 802.11e just can achieve relative service differentiation in saturated conditions. When the total load is less than the optimal point, 802.11e can hardly achieve significant serice differentiation.(3)On packet size and arrival rate diversity in IEEE 802.11 ad hoc networksPacket size and arrival rate diversity is very typical for practical traffics over 802.11 wireless links. The analytical model is useful for addressing the question of fairness between competing flows over IEEE 802.11 wireless links. When the total load is less than the optimal value, the throughput of each station is approximate to its load and the packet delay is much lower than that in saturated cases despite the packet size and arrival rate diversity. The results also indicate a MAC-independent scheme for 802.11 DCF to achieve not only throughput differentiation but also statistical QoS guarantee including throughput and packet delay.(4)Accurate queuing analysis of IEEE 802.11 MAC layerIn this thesis, the author develops a unified analytical model for IEEE 802.11 DCF with arbitrary buffer size in unsaturated conditions. The model comprises of a generalized Markov chain model and an M/G/1/K queuing model which enabling the thorough and accurate queuing analysis of IEEE 802.11 MAC layer. The performance metrics of queuing system include throughput, average queuing delay, and packet blocking probability, the average MAC service delay as well as its standard deviation.(5)Performance optimization and statistical QoS provisioning for IEEE 802.11 DCF in non-saturated conditionsIt’s shown that for practical 802.11 wireless ad hoc networks (i.e., buffer size larger than one and number of stations comparatively large) under the optimal offered load, the total throughput is maximized, the packet blocking probability (due to limited buffer size) and the average queuing delay tends to zero, the average MAC service delay as well as its standard deviation, which is much lower than that in saturated conditions, has its upper bound, furthermore, the optimal load is very close to the maximum achievable throughput regardless of the number of stations or buffer size. Hence the original IEEE 802.11 DCF protocol can accurately providing statistical QoS guarantee under optimal load.(6)Multi-transceiver multiple access (MTMA) protocol for mobile wireless ad hoc networksIn this thesis, the author proposes a multi-transceiver multiple access (MTMA) protocol for ad hoc networks in which each node has multiple sub-nodes equipped with independent wireless transceiver which can be tuned over all the available wireless frequency bands defined by IEEE 802.11 standard. Every sub-node dynamically reserves an idle traffic channel by RTS/CTS dialogue on the common channel that enable a node to perform parallel communications with other nodes. The MTMA protocol, which is suitable for vehicular based tactical ad hoc networks, greatly outperforms the single-transceiver RTS/CTS protocols with single channel or multi-channel.更多还原