A Hybrid Coordination Function Scheme for WLANs

In this work, we propose a hybrid medium access scheme that can provide a virtually collision-free environment for an infrastructure high-speed WLAN. The proposed hybrid scheme may be viewed as the merger of the standard PCF and DCF schemes. It can efficiently provide the quality-of-service support like the PCF scheme, while utilizing the flexible on-demand CSMA mechanism similar to the DCF scheme. In the proposed scheme, every station employs the standard CSMA mechanism and the master station schedules backoff timers for every client station. The master station sends the backoff timer information together with the data or control frames destined to each given client station. A piggybacking frame exchange mechanism is provided to enhance throughput of the proposed system and a prioritized backoff-timer scheduling scheme is given to provide QoS support. Numerical results show that the proposed scheme outperforms the standard IEEE 802.11 DCF and PCF schemes, and can effectively support prioritized traffic services.