Research Article A IEEE 802.11e HCCA Scheduler with a Reclaiming Mechanism for Multimedia Applications Anna Lina Ruscelli and Gabriele Cecchetti TeCIP Institute, Scuola Superiore Sant’Anna, 56124 Pisa, Italy Correspondence should be addressed to Anna Lina Ruscelli; a.ruscelli@sssup.it Received 31 August 2013; Revised 9 January 2014; Accepted 6 February 2014; Published 20 March 2014 Academic Editor: Mei-Ling Shyu Copyright © 2014 A. L. Ruscelli and G. Cecchetti. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Te QoS ofered by the IEEE 802.11e reference scheduler is satisfactory in the case of Constant Bit Rate trafc streams, but not yet in the case of Variable Bit Rate trafc streams, whose variations stress its scheduling behavior. Despite the numerous proposed alternative schedulers with QoS, multimedia applications are looking for refned methods suitable to ensure service diferentiation and dynamic update of protocol parameters. In this paper a scheduling algorithm, Unused Time Shifing Scheduler (UTSS), is deeply analyzed. It is designed to cooperate with a HCCA centralized real-time scheduler through the integration of a bandwidth reclaiming scheme, suitable to recover nonexhausted transmission time and assign that to the next polled stations. UTSS dynamically computes with an (1) complexity transmission time providing an instantaneous resource overprovisioning. Te theoretical analysis and the simulation results highlight that this injection of resources does not afect the admission control nor the centralized scheduler but is suitable to improve the performance of the centralized scheduler in terms of mean access delay, transmission queues length, bursts of trafc management, and packets drop rate. Tese positive efects are more relevant for highly variable bit rate trafc. 1. Introduction Service diferentiation tailored to the type of applications and stations requirements is a key issue of Quality of Service (QoS) provided by a network. Te focus of the research on this topic is motivated by the spreading difusion of multimedia applications that users ask the network to conveniently support. Indeed, applications like Voice over IP (VoIP), video streaming, video conference, and High Digital TV (HDTV) have diferent features and must be appropriately handled. IEEE 802.11e Medium Access Control (MAC) Hybrid Coordination Channel Access Function (HCCA) [1], based on a centralized polling mechanism, has been proposed in order to introduce QoS support by means of diferentiation and negotiation of stations service parameters. In particular, a new protocol parameter, Transmission Opportunity (TXOP), has been introduced in order to guarantee a maximum transmission time for each station, whereas the polling period is managed by the Service Interval (SI) parameter. Te purpose of the other 802.11e MAC function, Enhanced Distributed Channel Access (EDCA), is the same but based on Carrier Sense Multiple Access/Collision Avoidance mechanism and on diferentiation of contention parameters. However, since these parameters are set by the reference scheduler as fxed values in time and for the diferent sta- tions, the network shows poor performances and the QoS management is not yet satisfactory for applications with variable bandwidth, data rate, packet size, and so forth [2– 5]. Indeed, in the case of Constant Bit Rate (CBR) trafc, the network is yet suitable to guarantee the required service, whereas, as far as Variable Bit Rate (VBR) applications are concerned, it is not able to conveniently follow the trafc changes. Terefore, in order to meet the QoS applications requirements, the MAC scheduling of stations and resources must be refned by means of more specifc diferentiation mechanisms. Te diferentiation of service, in the case of IEEE 802.11e HCCA, can be based on diverse approaches, such as (1) modifying the scheduling engine to provide variable TXOP and SI; (2) adopting a simple overprovisioning method (for instance, considering worst case conditions Hindawi Publishing Corporation Advances in Multimedia Volume 2014, Article ID 372693, 22 pages http://dx.doi.org/10.1155/2014/372693