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 l- - atency.
- [Show abstract] [Hide abstract] ABSTRACT: Recently, many researchers have investigated the Voice over IP (VoIP) service over Wireless Local Area Networks (WLANs). One of the important challenges is to reduce the handoff delay in WLANs. Especially, the channel scan delay, which takes the largest portion of the WLAN handoff delay, is too long to support the delay-sensitive VoIP service. We propose a selective channel scan based on the pre-collected channel information of the neighboring access points (APs) to reduce the channel scan time. Mobile Stations (MSs) collect the channel information of neighboring Access Points (APs) when the VoIP is not used. When the VoIP service is used, MSs perform the selective channel scan mechanism based on the collected channel information. We use the handoff trigger based on the Inter-Arrival Time (IAT) of VoIP packets instead of Received Signal Strength Indication (RSSI). We evaluated the performance of the proposed scheme through computer simulations. The simulation results show that our scheme reduces the handoff delay enough to support the VoIP service over WLANs.0Comments 1Citation
- [Show abstract] [Hide abstract] ABSTRACT: The efficient handover in the IEEE 802.11 networks is the key requirement for real-time multimedia support in such networks. The handover delay is significantly limited with the Fast Transition protocol introduced in 802.11r amendment. However, the whole handover process remains too disruptive for the real-time services. Network scanning is the most important handover delay factor. The article introduces a novel Neighbor List protocol that eliminates the requirement for channel probing by adding restrictions on network topology. In consequence the average handover delay can be reduced to a value below 50 milliseconds. The article presents and discusses simulation results of the proposed algorithm.0Comments 0Citations
- [Show abstract] [Hide abstract] ABSTRACT: The distance between two wireless nodes can be obtained by taking time-of-arrival (ToA) timestamps during a round-trip measurement. Such a two-way ranging approach does not depend on accurate clock synchronization between the nodes. However, the timestamps in each node are taken with respect to the local oscillator which is subject to time-variable clock skews influencing the round-trip time and distance estimate. The symmetric double-sided ranging protocol is able to minimize this ranging error, but requires a second round-trip measurement, thus increasing the energy consumption of the nodes. This paper presents a novel carrier-aided clock skew estimation approach, which estimates the skew based on the carrier frequency estimate of uplink frames, thus removing the necessity for a second round-trip measurement. We show the feasibility of our approach by an FPGA-based prototype for IEEE 802.11 WLAN. The measurements show that the skew estimation error is about 40 ppb for a 1 ms frame. Given that the mobile devices requires 30 ms to respond to a ranging frame, the skew estimation error and the oscillator instability account for a ranging error of below 13 cm for the 80 % percentile.0Comments 0Citations