【作者】 蒋道霞；

【导师】 刘凤玉；

【作者基本信息】 南京理工大学 ， 计算机应用技术， 2010， 博士

【摘要】 移动Ad Hoc网络是一种由一组带有无线收发装置的移动主机组成的多跳、临时性自治系统。网络中所有的节点地位平等,每个移动终端具有路由器和主机两种功能,其组网的无基础设施性和自组织性,使它在特定的军事和民用通信领域得到了广泛应用。但移动Ad Hoc网络中节点的移动性、介质无线性、MAC访问控制方式、网络层路由协议及路径的不对称性,会导致链路误码率高、网络分割和路由变化频繁等,这些都会导致网络丢包,若用传统的TCP拥塞控制方法,将所有丢包都作为拥塞指示而调用拥塞避免或慢启动,将严重影响Ad Hoc网络的TCP性能。本文对Ad Hoc网络的数据链路层、网络层和传输层工作机理进行了深入研究,并分别从数据链路层、网络层和传输层探索拥塞控制策略,主要研究内容如下：(1)分析数据链路层可能产生的拥塞机理,对现有AAR(Adaptive Auto Rate)协议进行改进,将POCC(Penalty function-based Optical Congestion Control)算法应用于AAR协议,提出了AAR-CC(Adaptive Auto Rate-Congestion Control)协议,从而在MAC层实现了拥塞控制的速率自适应机制。在AAR-CC协议中,源节点根据某时刻链路的总拥塞信息确定当时数据发送速率。新协议只对MAC帧稍作修改并增加一个字节的拥塞指示CI,具有可扩展性和低开销性。AAR-CC利用背靠背数据传输方式,充分发挥高质量通信信道利用率,有效缓解拥塞状况,从而提高网络的端到端饱和吞吐量。(2)分析MAC层退避机制,改进IEEE 802.11分布协调功能(Distributed coordinationfunction,DCF),提出了一种实时退避自适应拥塞控制(Real-Time Traffic Backoff Adaptive Congestion Control, RBA-CC)协议。RBA-CC协议有两项重要改进：1)将迅速交换和快速转发机制应用于多跳Ad Hoc网络,减轻了MAC层的流间竞争和流内竞争,从而提高实时业务的传输效率；2)依据节点队列的瞬间状态判断拥塞状况,让ACK帧携带下一跳节点的拥塞指示信息,以便实现上一跳节点的自适应退避拥塞控制。RBA-CC协议能够降低实时业务数据包的传输时延,并有效提高网络端到端饱和吞吐量。对RBA-CC协议进行仿真,验证了上述结论的正确性。(3)分析网络层路由协议,提出局部拥塞表概念,利用旁路、局部拥塞表和hello消息机制,对动态源路由(Dynamic Source Routing protocol, DSR)协议、Ad Hoc按需距离矢量(Ad Hoc on Demand Distance Vector, AODV)协议进行了分析,提出了拥塞自适应动态源路由(Congestion Adaptive Dynamic Source Routing, CA-DSR)协议、Ad Hoc拥塞自适应按需距离矢量(Congestion Adaptive Ad Hoc on Demand Distance Vector, CA-AODV)路由协议。在这两种拥塞适应路由协议中,主节点利用局部拥塞表,能在第一时间感知到网络拥塞,当拥塞发生时,拥塞节点的上一节点能按需建立绕过拥塞节点的旁路,当数据流到达已建立旁路的节点时,被按比例分流在主路由和旁路上,实现了路由拥塞适应功能。协议利用hello消息对局部拥塞表进行有效维护。仿真结果已验证,新提出的拥塞自适应路由协议的平均端到端延迟更短、数据包递交率更高、规格化路由开销更少。(4)分析现有Ad Hoc网络TCP性能改进方案,提出了一种基于端到端的Ad Hoc网络TCP拥塞控制改进算法(Improvement AD hoc network TCP congestion control, IADTCP)。提出了一种新的慢启动方案以解决现有策略中拥塞窗口增长不够平滑的问题；利用两连续数据包单向传输延迟差异IDD和短期吞吐量STT两个度量参数,联合判断网络拥塞状态；并用丢包率PLR和包错序率POR判断信道错误、路由改变等网络状态,然后通过回送的ACK数据包携带网络状态信息以便让发送端采取适当的控制措施。仿真实验已验证了该方案是有效的。更多还原

【Abstract】 Mobile Ad Hoc network is a multi-hop, temporary autonomy system which is constituted by a group of mobile hosts with wireless receive-send equipment. All nodes in the network are equality in status. Each mobile terminal has two functions of router and host. It is widely applied to special military field and civil field thanks to its features of no-infrastructure and self-organization. But its features of node mobility, media wireless, control mechanism of MAC, route protocol in network layer, path asymmetry are likely to induce poor performance of Ad Hoc network such as high bit error rate (BER), network partition, route change frequently, et al. These may result in packet loss. The TCP performance will be affected badly under the adoption of the TCP congestion control mechanism that applied in wired network, which treats a packet loss as an indication of network congestion, and then invokes congestion avoidance or slow start. This dissertation researches the Ad Hoc network operation mechanism of data link layer, network layer, transport layer, and explores congestion control strategy from them respectively. The main research content is as follows:(1)Analyzes the mechanism of MAC layer congestion, and proposes an adaptive auto rate-congestion control (AAR-CC) protocol. The AAR-CC improves the AAR protocol with POCC algorithm and realizes the adaptive rate congestion control mechanism. It chooses the source node’s data sending rate according to the state of link congestion. The AAR-CC protocol just adds one byte for CI (Congestion Indicator) and makes the MAC frame only a small change, so it is a scalable low-overhead algorithm. The AAR-CC can enhance the utilization of channel quality by back-to-back data transmission, and it can effectively alleviate congestion level and improve end-to-end network saturation throughput.(2)Analyzes the backoff mechanism of data link layer, improves IEEE 802.11 Distributed Coordination Function (DCF), and proposes a real-time traffic backoff adaptive congestion control (RBA-CC) protocol. The RBA-CC protocol has two important improvements. Firstly, it applies Quick-exchange and Fast-forward mechanisms to multi-hop Ad Hoc network. This alleviates MAC layer inter-stream contention and intra-stream contention, and improves the transmission efficiency of real-time traffic. Secondly, it gets congestion information (CI) from queue instantaneous state of node, and then uses ACK frame piggybacking to transfer the CI to the preceding node, realizes adaptive backoff congestion control mechanism. The simulation results show that the RBA-CC protocol can decrease the delay of real-time traffic, remarkably increase the network end-to-end saturation throughput. (3) Analyzes the route protocols in network layer, presents a concept of local congestion table, improves Dynamic Source Routing (DSR) protocol and Ad Hoc on Demand Distance Vector (AODV) protocol on the bases of bypass, local congestion table, hello message systems, and then proposes two routing protocols of Congestion Adaptive Dynamic Source Routing (CA-DSR) and Congestion Adaptive Ad Hoc on Demand Distance Vector (CA-AODV). In the two protocols agreement, a primary node can be aware of network congestion immediately by using local congestion table. When congestion happens, the previous node on the primary route tries to find a bypass path on-demand to go around the congestion node. When data flow gets to the node with a bypass, it will be distributed in primary path and the bypass proportionally. As a result, it can realize the congestion adaptive function. They update their local congestion table by hello message. The simulation results show the two protocols can obtain less average delay from end to end, higher data delivery ratio, less normalized overhead.(4)On the basis of analysis of present improvement schemes of Ad Hoc network, this dissertation proposes an improving TCP congestion control algorithm for mobile Ad Hoc networks through employing end-to-end identification (IADTCP). A new method of slow-start strategy is proposed to resolve the problem that the congestion window grows unsmooth. The IADTCP uses two metrics IDD(Inter Delay Difference, the delay difference between consecutive forward packets) and STT (Short Term Throughput) to perform multi-metric joint identification for congestion detection, and uses PLR (Packet Loss Ratio) and POR (Packet Out-of-Order Delivery Ratio) to identify the network states of CHANNEL ERR and ROUTE CHANGE. Then the sender side will take advisable measures in light of network state information carried by back ACK package. The simulation results have validated the feasibility and efficiency of the proposal.更多还原