abstract
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Wireless handsets are becoming increasingly multi-modal, supporting both cellular and wireless LAN air interfaces. When such a handset roams from one network to another during an active voice call, the call must be re-routed into the new network. The process of re-routing calls in this manner is referred to as a vertical handover.
Achieving seamless WLAN-to-cellular handover can be very difficult due to the fact that WLAN coverage can often be lost long before a cellular call leg replacement can be established. A possible worse-case scenario occurs when mobile users walk from indoor building WLAN coverage to outdoors during voice connections. In this thesis, results from a measurement-based study of dual-mode handover are given. The presented results come from extensive IEEE 802.11 measurements that were made on the McMaster University campus. These results give important insights into the difficulty of this problem, and relate to various system parameters such as WiFi deployment type and link loss threshold. The results show that it is almost impossible to successfully complete the handover unless the WLAN deployment has been carefully engineered. In WLAN deployments which would ordinarily restrict the use of lower data rates, the results also suggest that a "limited data rate use" (LDU) algorithm can greatly improve the probability of seamless handover. This can be done without adversely affecting the capacity of the WLAN network.
Vertical handover can be performed using enterprise voice gateways which support basic conference bridging. This feature can be used to achieve soft handover, since a new leg of the call can be established to the conference bridge while the existing media stream path is active. Unfortunately this requires that all intra-enterprise calls be routed through the gateway when the call is established. In this work we consider a SIP based architecture to perform conferenced dual-mode handover and propose a much more scalable mechanism for short-delay environments, whereby active calls are handed off into the conference bridge prior to the initiation of the vertical handover. Results are presented which are taken from a dual-mode handset testbed, from analytic models, and from simulations which characterize the scalability of the proposed mechanism.