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For achieving Quality of Service (QoS) for real-time applications in high-mobility wireless LANs (WLANs), IEEE 802.11e proposes a novel Media Access Control (MAC) protocol, namely Hybrid Coordination Function (HCF). HCF provides two channel access modes: (1) a contention-based Enhanced Distributed Channel Access (EDCA) and (2) a contention-free-based mode HCF Controlled Channel Access (HCCA). EDCA achieves random access among mobile nodes but suffers from low throughput under a high traffic load with two main reasons. First, EDCA uses the same minimum contention window (CW) for different classes of traffic, and thus increases the collision probability. Second, after successful transmissions the backoff stage is transited back to the minimum backoff stage, and may lead to collisions from the same class traffic. This paper thus proposes a distributed High-performance EDCA approach (H-EDCA) to partition collision domains of different classes of traffic. The goal is to differentiate CWs for avoiding access collisions from both the same class traffic and different classes of traffic. Moreover, the performance of throughput, average transmission delay and frame dropping probability, are mathematically analyzed in detail based on the discrete-time Markov chain model. Numerical results demonstrate that H-EDCA yields very competitive analytical results to simulation results, which justifies the accuracy of the H-EDCA analytical model. Furthermore, H-EDCA significantly outperforms EDCA of IEEE 802.11e in the compared performance metrics.

Keywords: partition collision domains, H-EDCA, differentiating backoff stage, contention window, Markov chain model

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Received May 14, 2009; revised August 8 & September 29, 2009; accepted October 15, 2009.
Communicated by Wanjiun Liao.
^* This research was supported in part by the National Science Council of Taiwan, R.O.C., under contract No. NSC-97-2221-E-224-072-MY3.