Handoff Trigger Nodes for Hybrid IEEE 802.11 WLAN/Cellular Networks††This work was supported by a grant from the Natural Sciences and Engineering Research Council of Canada (NSERC). The authors may be contacted at todd@mcmaster.ca.

Future mobile handsets will often be multi-mode, containing both wireless LAN (WLAN) and cellular air in-terfaces. Vertical handoffs will commonly be used to pass voice calls to a cellular network when the user roams outside of WLAN radio coverage. Unfortunately, the transition from WLAN hotspot to cellular coverage is often very abrupt and leads to unacceptable call dropping rates. In this paper we propose andigoi investigate the use of explicit WLANlcellular handoff triggering. A simple WiFi handoff trigger node (HTN) can be installed in the WLANlcellular transition region, and generates link layer triggers which cause the initiation of the vertical handoff process. A key function provided by the HTN is to significantly reduce the call dropping rate even when there is very little collaboration between the cellular and WLAN hotspot providers. Results are presented which show that the call dropping probability can be dramatically reduced by the use of a handoff trigger node.