Priority Scheduling Based On The IEEE 802.11 DCF Mechanism

With the rising popularity of Wireless Local Area Networks (WLAN) and the increasingdemand for real-time multimedia applications, supporting Quality of Service (QoS) in WLANhas been an important challenging task. In comparison with the traditional best effort traffic,real-time applications impose stricter requirements on service parameters, such as data rate,delay and delay jitter. To provide reliable and guaranteed services for these applications,priority scheduling in the medium access control (MAC) layer is one of the key issues. The IEEE 802.11 standard is the most widely used WLAN standard today. It defines twoaccess mechanisms in MAC layer, the Distributed Coordination Function (DCF) and PointCoordination Function (PCF) respectively. A number of valuable QoS schemes have beenproposed based on the standard in recent years. Several of them, including the optional PCFdescribed in IEEE 802.11, rely on centralized control and the polling of backlogged hosts, whichgreatly constrains the operation of WLAN. Therefore, most QoS schemes adopt the moreflexible distributed control. However, most of them give less consideration on the delay boundof real-time traffic as well as the overall QoS. For real-time flows, it is often not enough with alow average access delay. We should also try to complete data transmissions within their delaybounds. In the mean time, it is important that QoS should be achieved with little overhead inview of the limited bandwidth of WLAN. And fairness among real-time flows also deservesattentions. In this thesis, we give a detailed introduction to the MAC layer mechanisms in IEEE 802.11standard. We also analyze the performance of real-time traffic with DCF mechanism. Thenvarious DCF based QoS schemes are discussed and a novel Distributed Priority Scheduling(DPS) scheme is proposed. Through a backoff strategy, the proposed scheme provides highpriorities for real-time traffic in terms of both throughput and average access delay. It alsoapplies to real-time flows with a quasi First Come First Serve (FCFS) scheduling order and anew collision-solving scheme. Through extensive simulations in NS-2, we showed that theDPS scheme was able to provide high throughput and low latency while solving the delay boundproblem for real-time flows. Moreover, it makes good use of the channel bandwidth. It offersfairer bandwidth allocation among real-time flows than DCF scheme.