How to tune VoIP codec selection in WLANs?

In IEEE 802.11e-based 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 bit-rates, inelastic flows, such as VoIP, can suffer from sudden congestion, which results on higher packet delays and losses. A VoIP codec selection algorithm has been proposed as a solution to this issue, which is triggered both by channel rate changes as well as in combination with a call admission control mechanism. The results show that an important improvement in terms of hotspot capacity for VoIP calls can be achieved by choosing the VoIP codec adaptively in a multi-rate scenario. By defining a new grade of service-related parameter, Qmacr, which captures the tradeoff between dropping and blocking probabilities and perceived speech quality, the codec selection algorithm can be tuned to achieve maximum capacity without severely penalizing any of those variables, and hence satisfying both technical and user quality requirements.