【作者】 于骊；

【导师】 舒炎泰；

【作者基本信息】 天津大学 ， 计算机应用技术， 2009， 博士

【摘要】 随着人类信息需求量的爆炸性增长,集语音、视频等实时业务和Web浏览、文件传输等数据业务为一体的综合多业务网络和无线网络,已成为网络发展的趋势。这就要求网络具有高速度、高质量、高可靠性的信息交换能力和文件传输能力。因此,拥塞控制就显得极为重要,也成为多年来的一个研究热点。本文针对不同类型网络的拥塞情况,应用跨层的方法设计了不同的拥塞控制方式。首先,本文设计了基于H∞优化控制理论的鲁棒AQM控制器,在保证控制器鲁棒性的同时,简化了参数整定方法。综合考虑系统的闭环性能和鲁棒性,通过理论分析得出控制器的统一表达式;并对控制器的稳定性和鲁棒性进行分析。本文以PID类控制器为例设计了鲁棒控制器R-PID,说明了参数整定方法。应用跨层的方法,并结合经验和仿真结果,实现了参数在线自适应调整。仿真结果证明R-PID在保持瞬时队列,降低排队延迟,提高链路利用率方面明显地优于传统AQM控制器;并通过参数自适应调整,使控制器能根据网络状况的动态变化在控制性能和鲁棒性之间做一均衡。其次,本文设计了适用于WLAN接入网络的拥塞控制和资源分配方法FairRA。基于最大最小公平性准则,提出一套适用于WLAN基站的公平调度规则集,并在此规则集的基础上设计了基于队列长度的调度器。采用跨层方法,应用MAC层信息计算WLAN的链路利用率,设计了基于WLAN可利用容量的拥塞控制方法。FairRA将调度算法和拥塞控制方法相结合,充分考虑WLAN的容量和节点的能力,在端节点和基站上均衡功能。仿真结果证明了在WLAN中,FairRA能更好地控制拥塞,平滑发送速率,从而达到无线资源的公平分配。最后,文章改进了DCCP/CCID-3协议,使其适合在无线Ad hoc网络中运行。改进后的CCID-3应用理论模型计算多跳Ad hoc网络中不进行重传的MAC延迟来修正传输层RTT估算方法;同时改进了分组丢失率计算和权重计算方法,并在发送节点处以ACK的序列号变化来探测丢失事件。仿真结果证明了改进后的CCID-3在网络吞吐率上有较大提高。更多还原

【Abstract】 With the explosively increasing requirement of information, people are hunting for networks combining real traffic, e.g., VoIP, online video, with data traffic, e.g., Web browsing, files transferring, which claim high speed, high quality and high reliability on information exchange and files transport. Therefore, it’s important and challenging to study congestion and congestion control in networks, which is also an attractive research topic in recent two decades. In this thesis, several congestion control schemes are designed with regards to different types of networks.First, a new robust AQM controller with a simplified parameter regulation method is designed based on H∞optimal theory. Taking the closed-loop performance and robustness into consideration, the uniform expression of the controller is developed through theoretical analysis. With these, analysis on stability and robustness of the controller is conducted. Further, a classical PID type controller, named R-PID, is taken as an example to show the controller’s robustness and parameter regulation. A traffic monitor is designed through heuristic and empirical approach. Further, a cross-layer-based adaptive parameters regulation method is implemented at router, making a trade-off between the performance and robustness accordingly. Simulation results show that R-PID has the significant robustness and outperforms the traditional AQM controllers on performance of queue length keeping, queuing delay and bottleneck link utilization. Moreover, through the adaptive parameter adjustment, the controller can make a trade-off between the robustness and performance with regard to network dynamic.Second, a fair resource allocation model, FairRA, with congestion control scheme is presented for WLAN accessed network. The Max-min fairness principle is extended to cater for the requirements of fairly scheduling at base station in WLAN, and then a queue-length-based scheduling algorithm is developed based on these extended principles. The link utilization of WLAN is calculated with the MAC layer information provided by cross-layer method, and then a congestion control scheme is designed based on available WLAN capacity. Integrating scheduling methods and congestion control scheme, the resource allocation model can provide fairness among the competing users in a great extent. Moreover, the model is a distributed system as it balances functions of congestion control among end points and base station. Simulation results show that FairRA provides a smooth sending rate, goodput, and a considerable fairness as well.Finally, focusing on performance enhancement of CCID-3 in DCCP, the thesis proposes two improvements on the equation of sending rate computation of CCID-3. One is to substitut the RTT estimated in transport layer with theoretical one, which is analyzed with IEEE 802.11 DCF protocol under the assumption that there is no retransmission at MAC layer. The other is modification on the packet loss rate computation, which can speculate the loss event from the ACK sequence number at sender by considering the ACK loss as a loss event and assigning the same weight as the packet loss. The weights of loss events are also modified to fit for the new computation method for the loss rate. The simulation results show that the modified CCID-3 has great improvement on throughput.更多还原