QoS Guarantee For Real-Time Traffic In IEEE 802.11 PCF Mode

With the rapid development of Wireless Local Area Network (WLAN), IEEE802.11 standard is widely accepted, and the ability of WLAN to support real-timetraffic with stringent quality of service (QoS) requirements has come into fore. IEEE 802.11 standard specifies two different channel accessing mechanisms,namely, the Distributed Coordination Function (DCF) and the Point CoordinationFunction (PCF). DCF is designed to provide contention service for asynchronous datatransmission using CSMA/CA (Carrier Sense Multiple Access with CollisionAvoidance), while PCF is designed to provide contention free service for real-timetraffic using a polling scheme. Regarding QoS, many works have been done on DCFto provide priority differentiation service, but fewer researches on PCF, because PCFis an optional mode, and seldom commercial product supports it. Nevertheless, PCF isa better way to provide parameterized QoS for interactive real-time applicationsbecause of its connection-oriented character. In this thesis, we introduced IEEE802.11 standard and its various improvementmechanisms, studied the combined performance of real-time and non real-timetransmission with PCF using Network Simulator (NS). We found that PCF performsrelative good with light load, which means a small number of real-time connectionsand not too much contention nodes with best effort traffic. But when increasing thenumber of real-time connections, PCF performs poorly. Furthermore, PCF onlysupports two priorities, contention and contention free service, because of its roundrobin scheduling. In order to support voice and video traffic with parameterized QoSwhile keep the non real-time transmission performance at an acceptable level, weproposed a new scheduling algorithm and an admission control scheme. In the proposed scheduling algorithm, the PC (Point Coordinator) allocates thewireless channel time in CFP (Contention Free Period) according to the delay andbandwidth requirements of real-time traffics, and polls them with calculated intervals.By doing this, the polling overhead is decreased and the service capacity is enlarged.Our admission control scheme is based on the delay requirements of real-time traffics,the admission condition is changed dynamically according to the best-effort trafficload, and the beacon delay is considered. We implemented them in NS, and simulationresults show that real-time traffics with different data rates and delay bounds can beserved within their QoS requirements, while the minimum throughput requirements ofthe non real-time traffic can be satisfied.