A cross-layer mechanism for QoS improvements in VoIP over multi-rate WLAN networks

Anna Sfairopoulou
Dr. Boris Bellalta, Dr. Carlos Macián, Dr. Miquel Oliver

In IEEE 802.11 WLANs, Link Adaptation mechanisms, which choose the transmission rate of each node, provoke unexpected and random variations on the effective channel capacity. When these changes are towards lower bitrates, inelastic flows, such as VoIP, can suffer from sudden congestion, which results on higher packet delays and losses. In this thesis, a VoIP codec adaptation algorithm is proposed as a solution, based on a cross-layer feedback from RTCP packets and the MAC layer, which can adapt the codecs of active calls to adjust them to the multirate scenario. A combination of this algorithm with a call admission control mechanism is also studied. The results show an important improvement in terms of the QoS of the already active flows as also in the total hotspot's capacity. Additionally, by defining a new Grade of Service related parameter, the Q-Factor, which captures the trade-off between dropping and blocking ratio and perceived speech quality, the codec adaptation algorithm can be tuned to achieve maximum capacity without severely penalizing any of those variables, and hence satisfying both technical and user quality requirements. Finally, a new QoS-enabled AP, which implements these enhancements is designed.

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SIP, voice codec adaptation, multi-rate wireless networks, IEEE 802.11, VoIP, performance analysis, hotspot capacity, Call Admission Control (CAC), cross-layer, Quality of Service (Qos), MAC, RTP/RTCP, SIP, IEEE 802.11