Assuming that the IEEE802.11 Wireless Local Area Networks (WLANs) are based on a radio/infrared link, they are more sensitive to the channel variations and connection ruptures. Therefore the support for multimedia applications over WLANs becomes non-convenient due to the compliance failure in term of link rate and transmission delay performance. We studied link adaptation facets and the Quality of Service (QoS) requirements essential for successful multimedia transmissions. In fact, the efficiency of rate control diagrams is linked to the fast response for channel variation. The 802.11 physical layers provide multiple transmission rates (different modulation and coding schemes). The last 802.11g-version maintains 12 physical rates up to 54 Mbps at the 2.4 GHz band. As a result, Mobile Stations (MSs) are able to select the appropriate link rate depending on the required QoS and instantaneous channel conditions to enhance the overall system performance. Hence, the implemented link adaptation algorithm symbolizes a vital fraction to achieve highest transmission capability in WLANs. “When to decrease and when to increase the transmission rate?” Are the two fundamental matters that we will be faced when designing a new physical-rate control mechanism? Many research works focus on tuning channel estimation schemes to better detect when the channel condition was improved enough to accommodate a higher rate, and then adapts its transmission rate accordingly. However, those techniques usually entail modifications on the current 802.11 standard. Another way to perform link control is based on local Acknowledgment (Ack) information for the transmitter station. Consequently, two techniques where accepted by the standard due to their efficiency and implementation simplicity.

We propose a new dynamic time-based link adaptation mechanism, called MAARF (Modified Adaptive Auto Rate Fallback). Beside the transmission frame results, the new model implements a Round Trip Time (RTT) technique to select adequately an instantaneous link rate. This proposed model is evaluated with most recent techniques adopted by the IEEE 802.11 standard: ARF (Auto Rate Fallback) and AARF (Adaptive ARF) schemes. Simulation results will be given to illustrate the link quality improvement of multimedia transmissions over Wi-Fi networks and to compare its performance with previous published results.


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