QoS Adaptation in SIP-based VoIP calls in Multi-rate IEEE 802.11 Environments

In this paper we study the impact of rate changes at the physical layer on VoIP flows in multi-rate IEEE 802.11-based environments. We determine how the network's rate changes affect the QoS of the active flows, and how these effects can be quantified and corrected using a mixture of RTCP and MAC layer information. We propose a decision mechanism that, upon hints of QoS degradation, triggers the transmission of RTCP packets to collect QoS feedback as perceived by the application, together with MAC layer information on the network's state. Subsequently, the VoIP flow parameters are changed, by adapting their codecs and choosing a more appropriate codec combination for the new conditions. Our results show a significant increase in network usage, without a significant QoS loss for the active flows in the cell.