Abstract :
Wi-Fi based hotspots offer mobile users broadband wireless Internet connectivity in public work spaces and corporate/ university campuses. Despite aggressive deployment of these hotspots in recent years, high-speed wireless Internet access remains restricted to a small number of geographical areas because of limited physical coverage of wireless LANs. On the other hand, despite their lower throughput, cellular networks have a significantly wider coverage and are thus much more available. Recognizing that 2.5G or 3G cellular networks can effectively complement wireless LANs, we set out to develop a vertical handoff system that allows mobile users to seamlessly fall back to such cellular networks as GPRS or 3G whenever wireless LAN connectivity is not available. The resulting handoff mechanism allows a network connection on a mobile node to operate over multiple wireless access networks in a way that is completely transparent to end user applications. In this paper, we present the design, implementation, and evaluation of a fully operational vertical handoff system, called OmniCon, which enables mobile nodes to automatically switch between wireless LAN and GPRS, based on wireless LAN availability, by introducing a simple extension to existing Mobile IP implementation. We discuss the design issues in the proposed vertical handoff system for heterogeneous networks, including connection setup problems due to network address translation, and the disparity in link characteristics between wireless LANs and GPRS. A detailed performance evaluation study of the OmniCon prototype demonstrates its ability to migrate active network connections between these two wireless technologies with low handoff latency and close to zero packet loss.
