Dual-Mode handsets containing both WLAN and cellular interfaces are becoming increasingly common. When moving between WLAN and cellular radio coverage,the switching of an active session from one interface to another is referred to as Vertical Handover (VHO). To provide successful VHO the WLAN link has to be maintained until the cellular connection is established. Our previous measurements of the WLAN coverage inside a building shows that this is very difficult given the time constraints in a loosely coupled cellular/WLAN architecture. In this paper we extend our previous work and propose an explicit trigger node to assist early detection of a VHO when transitioning out of a building, which will give the handset enough time to connect to the cellular network before loosing its WLAN link. A trigger node is a very simple IEEE 802.11 device that merely transmits beacons and is placed at the building exits. It provides no coverage extension or relaying and does not communicate with any other entity. We also provide some guidelines for the deployment of an indoor WLAN so that conventional signal threshold based HO triggering can be used with acceptable success probability.
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