Scheduling in 802.11e: Open-Loop or Closed-Loop?

Abstract : Scheduling in 802.11e networks is managed by the Hybrid Coordination Function (HCF), located within the access point. HCF has access both to trafc descriptions (TSPEC) and to feedback information sent in every frame by stations. In this paper we discuss the use of open-loop or closed-loop scheduling; the rst one is based only on the TSPECs, while the second one relays also on the feedback information from stations and builds upon classical control theory design. We discuss how closed-loop scheduling can be used to manage both the QoS guaranteed trafc and the best-effort trafc with a non-marginal performance improvement compared to openloop scheduling algorithms. We propose a simple max-min fair scheduling algorithm based on a positional controller which measures buffer levels. The controller is up- and down-clipped to meet strict QoS guarantees, while optimally distributing stochastically guaranteed and spared resources. Simulation results based on ns-2 are presented to support the theory and the design, showing that the proposed scheduler is robust and performs always better than open loop scheduler in presence of trafc uncertainties.