On Active Queue Management Algorithms For TCP Networks Based On Saturation Control

With the development of network,the numbers of users and requirements have been increasing dramatically.All of these lead to the lack of Internet bandwidth and the network congestion which reduces the Quality of Service.As the nodes of Internet,routers could accept the congestion information happened in network at the first time.Combined with the congestion control algorithm based on the host,the algorithm based on routers can relieve Internet congestion well.Active Queue Management(AQM)algorithms based on routers becomes a hot problem in the research filed.In the TCP network,the control input denotes packet loss rate and its value is an arbitrary number between 0 and 1.Meanwhile,the output of system is the instantaneous queue length of router buffer which restricted by the max queue length.So the system has saturation feature that has a great influence on the stability of system.However,the literature about the saturation is few.In this thesis,controller based on saturation are proposed to control the queue length of router buffer.The main work can be summarized as follows:(1)A kind of AQM algorithm based on observer is proposed considering with input saturation and input time delay.Due to the saturation function,it is difficult to design controller.In the thesis,the saturation function is transformed in order to separate the controller from model.Because not all the states of the system are available,the observer is designed for system.Based on the observer,the feedback controller is proposed with the uncertainties.Finally,the numerical simulation shows that the proposed controller can make queue length converges to the expectation quickly.(2)A kind of AQM algorithm based on dynamic output feedback is proposed considering the input saturation and state time delay.Because of the saturation function,there is difference between the output of controller and the real input control.The difference is fed back to controller via anti-windup gain to weaken the saturation phenomenon.Then,based on the Lyapunov theory,the sufficient stability condition is given.Meanwhile,the definite solution of controller parameter is proposed.At last,the simulation shows that system output could converge to the expectation quickly under the proposed controller.(3)On the base of the nonlinear dynamic model of TCP network with both input saturation and output saturation,the AQM algorithm based on Back-stepping control method is proposed.Firstly,because the saturation function isn't derivable,the smooth differentiable function is chosen to replace it.Secondly,because the states are output and the first-order derivative of it,the restriction of output can be transformed into states saturation.Meanwhile,taking the disturbance into consider,the disturbance observer is designed to reduce the influence.Based on all of these,the Back-stepping controller is proposed.Finally,the simulation results show that the disturbance can be observed effectively and the queue length of router buffer can converge to the expectation rapidly by the proposed AQM algorithms.Finally,the main work of this thesis is summarized and some future research directions are put forward.