【作者】 谢丽英；

【导师】 韦岗；

【作者基本信息】 华南理工大学 ， 通信与信息系统， 2010， 博士

【摘要】 无线Ad Hoc网络的抗毁特性使得其被广泛应用于军事和民用领域。无论在军事通信领域还是商业领域,无线Ad Hoc网络的应用都需要能够为各种业务提供一定的服务质量(Quanlity of Service, QoS)保障。但是,由于无线Ad Hoc网络具有移动性、分布多跳性、带宽资源有限等特性,使得无线Ad Hoc网络的QoS技术面临着众多的挑战。QoS的保证需要Ad Hoc网络的各个层协议的支持,其中,媒体接入控制(Medium Access Control, MAC)协议运行在物理层上,是所有数据帧在无线信道上发送和接收的直接控制者,是提供QoS保障的重要一层,也是Ad Hoc网络的研究热点之一。本文主要针对Ad Hoc网络的MAC机制进行研究,包括对MAC协议进行建模,以及对MAC协议中的退避算法进行研究,具体体现在以下几个方面:1)利用G/G/1排队理论,针对一般场景下的非饱和无线网络,提出了一种MAC协议的非对称分析模型AMUWN(Asymmetry Model for Unsaturated Wireless Networks)。利用AMUWN模型推导了轻负载下的单跳无线网络和多跳Ad Hoc网络的MAC层的延迟及其抖动、吞吐量和丢包率的表达式,并通过NS2仿真软件验证了模型的准确性。2)利用AMUWN模型,分别分析了非饱和情况下MAC协议IEEE 802.11 DCF在单跳无线网络和多跳Ad Hoc网络中的平均延迟、抖动和丢包率等性能。分析表明,在多跳Ad Hoc网络中,增加节点的最大重传次数会显著地减小丢包率,同时对平均延迟和抖动的影响却很小。此外,增加某个节点的最小竞争窗口,会增加该节点的延迟及其抖动和丢包率,并且减小其传输范围外的其他节点的丢包率,但对这些节点的延迟及其抖动几乎没有影响。3)利用AMUWN模型,分别分析了非饱和情况下支持QoS的MAC协议IEEE 802.11e EDCA在单跳无线网络和多跳Ad Hoc网络中的吞吐量、平均延迟、抖动和丢包率等性能,并通过仿真验证了模型的有效性。该模型为研究无线Ad Hoc网络对QoS的支持提供了数学分析方法。4)提出了一种基于冲突概率和发送概率的退避算法CTPB(Collision and Transmission Probabilities based Backoff)。通过AMUWN模型对IEEE 802.11e EDCA在无线Ad Hoc网络中的性能进行分析可知,业务的延迟及其抖动和吞吐量由IEEE 802.11e EDCA的最小竞争窗口和分组到达速率所决定。但是,IEEE 802.11e EDCA的退避算法中的最小竞争窗口是固定不变的,并没有考虑网络的负载和信道的忙碌状况。为此,本文提出了一种基于冲突概率和发送概率的自适应退避算法CTPB,使得节点在网络较为空闲时充分利用信道资源,而在网络较为繁忙时避免拥塞的发生。仿真结果证明,CTPB算法能有效地提高Ad Hoc网络中EDCA的延迟性能,并且没有影响网络的吞吐量。5)退避算法的吞吐量性能评估。由AMUWN模型的分析可知,无线Ad Hoc网络中节点的冲突概率影响MAC协议的性能,而节点的冲突通常是由MAC协议中的退避算法解决。针对现有的主要退避算法,本文通过一维马尔科夫链分析了指数退避算法、线性退避算法和多项式退避算法以及各自对应的有最大重传次数限制的退避算法在饱和Ad Hoc网络中的吞吐量性能,为无线Ad Hoc网络的MAC机制的性能优化提供了理论依据。更多还原

【Abstract】 For its prospect of application in military and civilian fields, wireless Ad Hoc network has been a focus in current network research. Due to the prevalence of multimedia applications, Quality of Service (QoS) issue has been regarded as a hot spot in Ad Hoc networks. But it is a challenge to support QoS in Ad Hoc networks for its mobile characteristics, distributed multihop, limited bandwidth. Ad Hoc networks’support for QoS involves all protocol layers. Medium Access Control (MAC) layer works on the physical layer and directly controls the transmission and receiving of packets, so it is one of the most important layers for QoS and many researchers focus on MAC layer’s support for QoS.This thesis focuses on MAC schemes, including the model building and the backoff functions of MAC layer protocol. The main work is as follows:1) With G/G/1 queuing theory, aiming at the general circumstance of unsaturated wireless networks, AMUWN (Asymmetry Model for Unsaturated Wireless Networks), which is a MAC layer analytic model, is put forward. With AMUWN, the delay, delay jitter, throughput and packets drop probability in the MAC layer of unsaturated one-hop and multi-hop Ad Hoc networks can be deduced. The model is evaluated by NS2 simulation software.2) With AMUWN, the thesis analyzes the delay, delay jitter and packets drop probability in unsaturated one-hop and multi-hop Ad Hoc networks with IEEE 802.11 DCF as their MAC layer protocol. In multi-hop Ad Hoc network, increasing the maximum retry limit can dramatically decrease the packet drop probability, but there are few effects on the delay and jitter. Meanwhile, increasing the minimum contention window of a given node can increase its delay and jitter and the packet drop probability and decrease the packet drop probability of nodes that are out of its transmission range, but their delay and jitter are hardly affected.3) With AMUWN, the thesis analyzes the delay, delay jitter, throughput and packets drop probability in unsaturated one-hop and multi-hop Ad Hoc networks with IEEE 802.11e EDCA as their QoS supported MAC layer protocol. The NS2 is used to evaluate the performance of the proposed model. The model provides an analytic method of studying wireless Ad Hoc networks’support for QoS.4) An adaptive back-off function CTPB (Collision and Transmission Probabilities based Backoff) is proposed. According to the analysis of AMUWN, the delay and jitter and the throughput depend on the minimum contention windows of IEEE 802.11e EDCA and the packet arrival rates. But the minimum contention window of IEEE 802.11e EDCA is static without the consideration of the network load and the busyness of the channel. So, CTPB is proposed. Simulation results show that this function can improve the delay performance of EDCA without impairing the throughput in Ad hoc networks.5) The throughput evaluation of backoff functions. According to the analysis of AMUWN, the collision probabilities of nodes in Ad Hoc network influence the performance of the MAC protocol. For their randomness, the backoff functions are adopted to resolve the collision traditionally. In this thesis, a one-dimensional Markov chain model is used to analyze the saturation throughput in Ad hoc network for generalized exponential, linear and polynomial backoff functions and these backoff functions with maximum retry limit. The analysis provides a basis for the performance optimization of MAC protocol in wireless Ad hoc network.更多还原