An Indoor Tracking-based Handoff Mechanism for VoIP Applications in IEEE 802.11 WLANs
ABSTRACT 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-
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ABSTRACT: Allowing users to discover and communicate their positions in the physical world has long been identified as a key component in emerging mobile computing applications. Pervasive positioning is required for the wide scale adaptation of location awareness. Vehicular communication in a campus wide hotspot plays a major role in establishing this. To provide seamless roaming between a subnet in a hotspot and to track the position of the mobile client is the main objective of this paper. wireless local area networks (WLANs) have become the state-of-the art campus networking option in many academic and corporate campuses. As “Wi-Fi” technology becomes ubiquitous, it is important to understand trends in the usage of these networks. As the mobile clients are moving from one base station/access point to another, the conventional layer-2 handoff consumes more time in the channel-scanning process. It takes around 310ms, which leads to a heavy packet loss in real-time applications. The proposed dynamic handoff mechanism for mobility management is designed to minimize the handoff latency in IEEE 802.11 wireless local area networks, which in turn reduces the data loss and the signalling overhead in real-time applications. In this mechanism, threshold points are considered to provide better solution for fast and ping-pong type moving clients in a campus wide network.In our approach the base station (BS) range is divided into two zones namely Z1 – Good and Z2 – Average zone based on the signal strength as well as the distance. To predict the location of the mobile client in a campus wide network, a location prediction is introduced. Based on the location of the mobile client, signal strength and capacity of the BS, the minimal required resources are proactively reserved for the mobile client. A pre hand-off initiation algorithm is triggered when the mobile unit (MU) enters the minimum threshold (Thmin) point. The threshold points are used to provide the handoff region (signal) notification to the mobile user. The MU receives the information about the candidate BS with QoS parameters. The dynamic channel scanning is incorporated in all the access routers and the switching centre (backbone router) to provide various QoS parameters to enhance the layer-2 handoff process and thus to reduce the layer-2 handoff latency. The experimental results are compared with the conventional layer-2 handoff latency for a campus wide network. The latency time is reduced from 310 to 33ms, which is more suitable for real-time applications in a campus wide network.Computer Communications. 01/2008; 31:2781-2789.
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ABSTRACT: Deployment of Voice-over IP (VoIP) and other real-time streaming applications has been somewhat limited in wire- less LANs today, partially because of the high handoff la- tencies experienced by mobile users. Our goal in this work is to eliminate handoff latency by exploiting the potential of multiple radios in WLAN devices. Our proposed ap- proach, called MultiScan, is implemented entirely on the client-side, and, unlike prior work, MultiScan requires nei- ther changing the Access Points (APs), nor having knowl- edge of wireless network topology. MultiScan nodes rely on using their (potentially idle) second wireless interface to opportunistically scan and pre-associate with alternate APs and eventually seamlessly handoffongoing connections. In this paper we describe our implementation of MultiScan, present detailed evaluations of its effect on handoff latency and evaluate performance gains for MultiScan-enhanced wireless clients running Skype, a popular commercial VoIP application.Proceedings of the 5th Conference on Internet Measurement 2005, Berkeley, California, USA, October 19-21, 2005; 01/2005
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ABSTRACT: IEEE 802.11 based wireless networks have seen rapid growth and deployment in the recent years. Critical to the 802.11 MAC operation, is the handoff function which occurs when a mobile node moves its association from one access point to another. In this paper, we present an empirical study of this handoff process at the link layer, with a detailed breakup of the latency into various components. In particular, we show that a MAC layer function - probe is the primary contributor to the overall handoff latency. In our study, we observe that the latency is significant enough to affect the quality of service for many applications (or network connections). Further we find a large variation in the latency with from one handoff to another and also among APs and STAs used from different vendors. In this study, we account for this variation and also draw the guidelines for future handoff schemes.01/2003;