Moving toward end-to-end support for handoffs across heterogeneous telephony systems on dual-mode mobile devices

More and more handsets manufactured today are being equipped with WLAN modules, allowing users to access VoIP service whenever they are within the coverage of WLAN service area. One of the greatest challenges for such dual-mode (say GSM and WiFi) handsets to support ubiquitous communication is the ability to maintain session continuity so a phone call established through one mode can be transferred to the other mode without user intervention. While related work has investigated the problem of supporting vertical handoffs for data communications across heterogeneous wireless data networks (e.g. GPRS and WiFi), these approaches cannot be directly used for vertical handoffs across heterogeneous telephony systems that involve both circuit-switched and packet-switched systems. In this paper, we investigate the problem of supporting vertical handoffs for voice communications across circuit-switched and packet-switched telephony systems on dual-mode mobile devices. We first contend that an end-to-end solution has advantages over infrastructure-based solutions due to the unique characteristics of the problem considered. We then propose an end-to-end approach that builds atop existing cellular network and VoIP infrastructure without introducing new entities for supporting vertical handoffs on dual-mode mobile devices. The proposed approach is based on the Session Initiation Protocol (SIP) and requires modifications only on end devices. We conduct testbed experiments using a GSM–WiFi dual-mode PDA and a SIP soft phone, and evaluate the quality of speech on the dual-mode PDA when the call is migrated from WiFi to GSM. Evaluation results show that the proposed approach allows soft handoffs and is a promising solution for seamless vertical handoffs on dual-mode mobile devices. 2007 Elsevier B.V. All rights reserved.