A Dynamic Link Adaptation for Multimedia Quality-Based Communications in IEEE_802.11 Wireless Networks

Abstract

Assuming that the IEEE 802.11 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 such WLANs becomes non-convenient due to the compliance failure in term of link rate and transmission delay performance. Voice and broadband video mobile transmissions (which normally have strict bounded transmission delay or minimum link rate requirement) entail the design of various solutions covering different research aspects like service differentiation enhancement (Rebai et al., 2009), handoff scheme sharpening (Rebai, 2009a, 2009b, 2010) and physical rate adjustment. The core of this chapter focuses on the last facet concerning the link adaptation and the Quality of Service (QoS) requirements essential for successful multimedia communications over Wi-Fi networks. 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 original 802.11 standard operates at 1 and 2 Mbps (IEEE Std. 802.11, 1999). Three high-speed versions were added to the original version. The 802.11b supports four physical rates up to 11 Mbps (IEEE Std. 802.11b, 1999). The 802.11a provides eight physical rates up to 54 Mbps (IEEE Std. 802.11a, 1999). The last 802.11g version, maintains 12 physical rates up to 54 Mbps at the 2.4 GHz band (IEEE Std. 802.11g, 2003). 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 to 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 adapt their transmission rate accordingly (Habetha & de No, 2000; Qiao et al., 2002). However, those techniques usually entail modifications on the current 802.11 standard. In (del Prado Pavon & Choi, 2003), authors presented a motivating rate adaptation algorithm based on channel estimation without any standard adjustment. However, this scheme supposes that all the transmission failures are due to channel errors and not due to multi-user collisions.