A cross layer scheme for maximising the combination of wireless VoIP capacity and quality

We present a novel mechanism for adjusting the combination of the number of simultaneous VoIP calls and voice quality, based on the monitored WiMax link parameters. We have implemented this mechanism which combines call admission control and setting of variable voice codec rate (and corresponding quality) dynamically in the Asterix PBX using the Speex voice codec. The mechanism includes dynamic parameter adjustments of currently active VoIP calls, with the capability to change quality on a per voice frame basis. It additionally limits acceptance of new calls until there is sufficient capacity to maintain a pre-defined minimum quality for the existing calls. We have also implemented the wireless link capacity estimator, which, based on the parameters measured in the receiver, estimates the data rate currently available on the wireless link. Our implementation is for a pre-WiMax service offered by UNWIRED in Australia. We show that the proposed cross layer optimisation enables a larger number of calls with improved voice quality, compared to a fixed scheme. We are currently working with a certified WiMax equipment supplier and we plan to extend the scheme to full WiMax implementation.