Research Of Congestion Model In High Speed And Wireless Networks

With the development of society and the progress of technology, high-speed Internet service and wireless networks are widely used in people’s lives. Due to the increasing number of network scale and users, network load becomes heavy, and then congestion occurs. Network congestion causes a significant decrease in network performances such as the increase of packet dropout rates and transmission delay and so on. Network congestion control becomes a main means to improve network performance and increase the robustness of the system. Therefore, studying and solving the problem of network congestion control has important implications.The thesis focuses on stability analysis of congestion control algorithm and complex dynamics research. By means of dynamics method, we analyze nonlinear dynamic phenomenon in the congestion control system of high-speed network and the stability of wireless network. At last, the suitable strategies are designed to expand system stability range or increase the global stability.The main work and innovations of this study are summarized as followed:(1) The local stability conditions in the FAST TCP congestion control algorithm is analyzed. Combining the theory of nonlinear dynamics, dynamical behaviors which may be appeared are studied when the system loses stability. According to control theory, we propose the hybrid control of Hopf bifurcation in a FAST TCP mode. Then the direction and the stability of the bifurcating periodic solutions will be analyzed by using the normal form theory and the center manifold theorem. Theoretical analysis and numerical simulation are presented to show that the hybrid method can effectively delay and even eliminate bifurcation in a some parameter interval, which system stability range can be expanded.(2) The general approaches of sliding mode control are expressed. Considering the sliding mode characteristic, we apply the sliding mode control to congestion control model in a wireless network. Under the consideration of network delay, parameter change and unknown system parameters, a sliding mode controller with input delay compensation is designed. Sliding mode in the controller has strong robustness to perturbation and external interference. Theoretical analysis and experiments show that the sliding mode controller makes the system more stable during the whole control period.