Stabilizing control of TCP/AQM networks with time delay in state variables

This paper studies how to design a congestion controller in TCP/AQM networks. In Hollot et al. (IEEE Trans. on Automatic Control, Vol. 47, No.6, 2002), the congestion controller is conservatively designed due to using the linearized model of the TCP/AQM networks in which a structured time-delay related term is treated as an unstructured uncertainty. Without treating any time-delay related term as uncertainty, we provide a necessary and sufficient a condition of existence of PI stabilizing controller for the system, and we propose a new design of the congestion controller via solving a stabilization problem of the system with a time delay in the variables of both state and input. The simulation results are provided to validate the proposed design method.