Abstract
Fairness is a key issue in the Resilient Packet Ring (RPR). The current RPR's congestion control scheme is simple, but exhibits oscillations in the case of unbalanced traffic scenarios. These oscillations result in a bandwidth loss. In this paper, the Control Theory is used to resolve the congestion control issue. The Proportional Differential controller is proposed to assign the bandwidth fairly among the participating flows at the congested link. The PD controller is successful, but exhibits oscillations and become unstable when the ring delay is large. Hence, the impact of the delay on the stability of the control system is investigated analytically and through simulation. Moreover, the Smith principle is used to remove the influence of the delay. The proposed controller assures stability and ring utilization.