On Active Queue Management Algorithms Based On Control Theory

TCP congestion control mechanisms, while necessary and powerful, are not sufficient to provide good services in all circumstances, especially with rapid growth in size and the strong requirement for QoS guarantee. It is needed to complement the end system congestion avoidance mechanisms. Active Queue Management, as one class of packet dropping/marking mechanism in the router queue, has been recently proposed to support the end-to-end congestion control in the Internet. The goals of AQM are to reduce the average length of queue in routers and thereby decrease the end-to-end delay experienced by packets, and ensure efficient use of network resources by reducing the packet loss caused by queue overflow. AQM highlights the trade-off between the delay and throughput.This research work is summarized as follows:1. The detail summary of some basic contents, issues, and recent research on congestion control is made, and the existed AQM scheme is analyzed, as well as the existent problems and improved measures of them are analyzed and summarized.2. In TCP/IP networks, the packet-dropping probability function is considered as a control input, a TCP AQM controller is modeled as a constraint linear time-delay systems.3. A state-space model is used to describe the dynamic properties of the network flow, and the discrepancy between the available link bandwidth and the nominal link bandwidth is considered as a disturbance. Then, the congestion control problem is solved by using the H_∞control approach.4. Based on the sliding mode control(SMC) method, an active queue management (AQM) scheme is designed to deal with the nonlinearity of the TCP model and the uncertainties in the networks, such as the TCP connections, the round trip time delay and the link bandwidth. The proposed algorithm is robust against the network modeling uncertainties and the parameters perturbations in the networks. The SMC based AQM controller can lead the actual queue length to the desired value and guarantee the asymptotic stability of the nonlinear closed-loop TCP system with saturated input. Simulation results are finally given to demonstrate the effectiveness and superiority of the proposed method.5. On the basis of summing up the work in this thesis, some comments are presented on the further research directions.