Control & Systems Eng., University of Technology, Baghdad, Iraq


Abstract – Internet represents a shared resource wherein users contend for the finite
network bandwidth. Contention among independent user demands can result in
congestion, which, in turn, leads to long queuing delays, packet losses or both.
Congestion control regulates the rate at which traffic sources inject packets into a
network to ensure high bandwidth utilization while avoiding network congestion. In the
current Internet, there are two mechanisms which deal with congestion; the end-to-end
mechanism which is achieved by the Transmission Control Protocol (TCP) and the
intermediate nodes algorithms such as Active Queue Management (AQM) in routers.
In this paper, a combined model of TCP and AQM (TCP/AQM) is formulated and
first simulated without a controller. The results show that it is unable to track the desired
queue size. So, to get better tracking performance, an adaptive PID controller based on
wavelet network (AWNPID) is used as AQM in the router queue. The non-adaptive PID
controller is also demonstrated, and its weakness to the network dynamic changes is
compared to the robustness of adaptive controller (AWNPID). The analytical results for
linearized TCP/AQM model are presented in MATLAB version 7.0.