An Efficient Back-off Mechanism for Simulation Study of VoWLAN Capacity Improvement using OPNET

Integration of delivering VoIP services over WLAN access networks (VoWLAN) increased the demand to support multiple of active simultaneous calls. Thus improving VoWLAN capacity has been addressed. The researchers are continuously contributing in improving VoWLAN capacity, (i.e. increasing the number of VoIP simultaneous calls), for various WLAN architectures. Although a lot of modifications have been developed for IEEE 802.11 standards toward improving its performance with real-time applications, all issued versions of this standard have a unique characteristic for infrastructure-based WLAN. That is MAC controls the access mechanisms for shared WLAN channels via distributed coordination function (DCF) entity. This paper aims to improve VoWLAN capacity by means of backoff time minimization for WLAN base station (i.e. Access Point – AP). AP MAC processing time (i.e. backoff overhead time) has significant effect on the downlink delay that all stations’ packets suffer from. Weighting factor ? is adopted for AP backoff time to minimize this downlink delay. This weighting factor points to the number of active VoIP clients during certain period. The new AP back-off time is the ratio of a uniformly distributed backoff random number to such weighting factor ?. VoIP calls generated using adaptive multi-rate voice codec AMR (12 Kbps). OPNET simulator has been used to validate proposed mechanism. Obtained results show notable improvement for VoWLAN capacity (i.e. 25% increment in simultaneous VoIP calls).