Abstarct: Wide use of Transmission Control Protocol (TCP) in Internet and developing volume of sent data through the Web cause studying some issues like network congestion that is becoming more attractive for researchers in the past decades. Increasing packets lost rate and decreasing network efficiency make basic challenges for users and has decreased the reliability of network. In this thesis, at first congestion issue and importance of congestion control is explained and then discussed about suggested solution using control theory which means designing controllers by math concepts and place on routers which causes to decrease packet drop probability. To get the desired result, smith predictor is implemented as the best structure to guarantee stability and encounter delay in proper form and its operation will be evaluate to control the system. Subsequently Dahlin controller, PID controller and Robust PID controller has placed in this structure to compare the result of operations. Smith structure changes by Using concept of Equivalent Gain Plus Time Delay For robust design till entrance of parametric unknown uncertainties, existent PID controllers in this structure exhibit acceptable operation. The new control structure named Robust PID controller. Since this study related to network data system for more comprehension we supposed a topology with two nodes (routers) and some senders and receivers which has simulated in NS simulator to have more real results. The result shows Robust PID controller has the best operation in deferent condition of network compare to other controllers and preserve the queue length on desired value and use minimum capacity of router buffer for data transmission. To implement controllers in NS, analog to digital conversion technique is used. After preparation the related code, controllers have placed on related routers and the results has evaluated.