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The objective of this paper is study the small signal stability of congested TCP networks. The starting point for this analysis is a fluid model describing the interaction of TCP-controlled traffic with congested routers. Assuming integral action in the active queue management scheme, we compute the network's equilibrium point about which we determine the network's small -signal dynamics. Some variations on a linear model of Active Queue Management supporting Transmission Control Protocol flows are used to design classical controllers. Specializing to networks comprised of m link and n sources, we study closed loop stability, and we compare queue length in PID controller and PI controller and RED controller for n sources and m routers. The simulation results show that the integrated performance of the PID controller is obviously superior to that of the PI and RED controller.