Multi-rate 802.11 WLANs
In IEEE 802.11 WLANs, stations can operate at different data rates. For example, in 802.11b, wireless stations transmit at four date rates 11, 5.5, 2 and 1 Mbps. A station changes its modulation type and transmits at a lower rate when it faces signal fading and interference. In this paper we introduce a model to capture the capability of the standard to operate at different data rates under both, finite load and saturation conditions. Many improvements are also made in order to make the model more consistent with the standard than existing models. The proposed analysis applies to both the basic access and the RTS/CTS access mechanisms. The results we obtained from the model also show the performance anomaly of the standard when it operates at different data rates that were previously observed in the literature, our experiments and simulation. We use the model to study a scheme we proposed earlier to avoid the degradation in the performance by changing the frame sizes of the stations according to their speeds. Throughput performance evaluation is provided and an analytical expression for average service time is given. Experiments were performed to validate the model