2114 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 6, NO. 6, JUNE 2007 A Distributed Channel Access Scheme with Guaranteed Priority and Enhanced Fairness Hai Jiang, Member, IEEE, Ping Wang, and Weihua Zhuang, Senior Member, IEEE Abstract— Although the IEEE 802.11e enhanced distributed channel access (EDCA) can differentiate high priority traffic such as real-time voice from low priority traffic such as delay- tolerant data, it can only provide statistical priority, and is characterized by inherent short-term unfairness. In this paper, we propose a new distributed channel access scheme through minor modifications to EDCA. Guaranteed priority is provided to real- time voice traffic over data traffic, while a certain service time and short-term fairness enhancement are provided to data traffic. We also present analytical models to calculate the percentage of time to serve voice traffic and the achieved data throughput. Both analysis and simulation demonstrate the effectiveness of our proposed scheme. Index Terms— IEEE 802.11e EDCA, distributed channel ac- cess, quality of service, priority, short-term fairness. I. I NTRODUCTION T HE mandatory distributed coordination function (DCF) in the IEEE 802.11 [1] is the most popular medium access control (MAC) mechanism for distributed wireless access. It is based on the carrier sense multiple access with collision avoidance (CSMA/CA). In DCF, after sensing the channel being idle for a duration of distributed interframe space (DIFS), a node chooses a random backoff timer uniformly distributed in its contention window (CW ). The backoff timer counts down when the channel is continuously idle for more than DIFS, and freezes when a transmission from other nodes is sensed. The node transmits its frame when the backoff timer reaches zero. The CW is set to the initial value W min for the first transmission attempt for the target frame, doubled upon a collision until the maximum value W max is reached, and reset to W min upon a successful delivery. Each node can also use the optional request-to-send (RTS)/clear-to-send (CTS) dialogue before DATA frame (i.e., the information frame from the sender) transmission to alleviate the hidden terminal problem in a multi-hop environment. Due to the lack of a centralized controller, it is challenging to achieve quality of service (QoS) in terms of delay, delay jitter, and fairness in distributed channel access. To enhance the IEEE 802.11 MAC, IEEE 802.11e [2] proposes new Manuscript received September 20, 2005; revised March 10, 2006; accepted March 16, 2006. The associate editor coordinating the review of this paper and approving it for publication was Y.-B. Lin. This work was supported by a research grant from the Natural Science and Engineering Research Council (NSERC) of Canada. H. Jiang is with the Electrical Engineering Department, Princeton Univer- sity, Princeton, New Jersey 08544, USA (e-mail: haijiang@princeton.edu). P. Wang and W. Zhuang are with the Department of Electrical and Computer Engineering, University of Waterloo, 200 University Av- enue West, Waterloo, Ontario, Canada N2L 3G1 (e-mail: {p5wang, wzhuang}@bbcr.uwaterloo.ca). Digital Object Identifier 10.1109/TWC.2007.05746. features with QoS provisioning to real-time applications. As an extension of DCF, the enhanced distributed channel access (EDCA) in IEEE 802.11e has provided a priority feature among access categories (ACs) by classifying the arbitration interframe space (AIFS), and the initial (W min ) and maximum (W max ) contention window sizes. High priority traffic (e.g., real-time voice) has smaller AIFS, W min and W max values, and thus is more likely to get access to the channel than low priority traffic. However, EDCA provides only statistically rather than guaranteed prioritized access to high priority traffic [3]. Such statistically prioritized access is difficult to satisfy the delay requirement of each high priority packet 1 . Further- more, high priority traffic can suffer performance degradation due to low priority traffic offering heavy loads [4]. In addition, there is no QoS guarantee for low-priority traffic in EDCA. On the other hand, DCF and EDCA are characterized by inherent short-term unfairness [5]. A successful transmission from a node will set its CW to W min , giving its following packets a large chance to be served before packets in other nodes with a larger CW . It can be seen that the short-term unfairness results from the channel access policy of DCF and EDCA. The objective of this paper is to address these challenges. We base our work on IEEE 802.11e EDCA. With minor modifications to EDCA, we propose a scheme to provide guaranteed priority to voice traffic over data traffic and, at the same time, provide data traffic with a certain amount of service share and short-term fairness improvement. The rest of this paper is organized as follows. The related work is reviewed in Section II. Section III presents our proposed channel access scheme. In Section IV, we provide the performance analysis for voice and data traffic. Numerical results and discussion are given in Section V, followed by conclusion remarks in Section VI. As many symbols are used in this paper, Table I summarizes the important ones. II. RELATED WORK A. Priority Access To enhance the priority provisioning of IEEE 802.11 DCF, the approaches in [6], [7] scale the CW or the IFS according to the priority of traffic in the node. In [8], [9], after waiting for the channel to be idle for an IFS time, the node jams the channel by pulses of energy, called black burst, the length of which is determined by how long the node has been waiting or by the traffic priority. The node with the longest black burst 1 In this paper, a network layer packet is transmitted by a link layer frame. Hence the terms “packet” and “frame” are used interchangeably. 1536-1276/07$25.00 c  2007 IEEE