An Indoor Tracking-based Handoff Mechanism for VoIP Applications in IEEE 802.11 WLANs

IEEE 802.11 WLAN (wireless local area network) provides high data rate up to 55 Mbps, but WLAN lacks supporting latency sensitive applications, e.g., VoIP (voice over IP), for mobile stations (STAs) to perform handoff between service ranges of multiple access points (APs). In order to provide seamless connectivity, the STAs trigger MAC (medium access control) layer handoff to the optimal AP but they should suffer from severe handoff latency in at least 300 msec which is not enough to guarantee the requirement of VoIP applications. Large handoff latency affects the service disruption of the VoIP applications, especially the VoIP user experiences voice discontinuance while he or she makes a call over WLAN. If the user has knowledge about the location of neighboring APs, the MAC layer handoff can be performed with diminished latency enough to support the VoIP applications. Therefore, in this paper, we propose an indoor tracking-based handoff mechanism which aims at reducing the handoff latency under 20 msec to guarantee the requirement of the VoIP applications. More specifically, we devise a received signal strength (RSS)-based indoor tracking system (RITS) which maintains the location information including the mapping between APs' coverage and RSS measured from the APs. The RITS allows STAs to obtain the accurate AP location according to the measured RSS. Our RSS measurement is focused on listening to beacon frames sent not only from the operated channel of the serving AP but also from overlapped channels of neighboring APs. Furthermore, utilizing the RITS enables STAs to measure their location in very highly precision, so that the STAs can determine the optimal handoff timing to avoid the service degradation. The results of our experimental studies show that the RITS based localization can find the STA's actual location with 95% accuracy. Also, the proposed handoff mechanism outperforms typical handoff approaches of WLAN standard in terms of the MAC layer handoff latency.