Abstract: Active queue management (AQM) is essential to prevent the degradation of quality of service in TCP/AQM systems with round-trip time (RTT) delay. RTT delays are primarily caused by packet-propagation delays, but they can also be caused by the processing time of queuing operations and dynamically changing network situations. This study focuses on the design and analysis of an AQM digital controller under time-delay uncertainty. The controller is based on the Smith predictor algorithm and is called the SMITHPI controller. This study also demonstrates the stability of the controller and its robustness against network parameter variations such as the number of TCP connections, time delays, and user datagram protocol flows. The performance, robustness, and effectiveness of the proposed SMITHPI controller are evaluated using the NS-2 simulator. Finally, the performance of the SMITHPI controller is compared with that of a well-known queue-based AQM, called the proportional-integral controller.

Key words: Active queue management; Transport control protocol; Round-trip time delay; Smith predictor

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