Handoff Trigger Nodes for Hybrid IEEE 802.11 WLAN/Cellular Networks Pejman Khadivi Department of Electrical and Computer Engineering Isfahan University of Technology Isfahan, IRAN Terence D. Todd and Dongmei Zhao Department of Electrical and Computer Engineering McMaster University Hamilton, Ontario, CANADA Abstract Future mobile handsets will often be multi-mode, con- taining 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 out- side of WLAN radio coverage. Unfortunately, the transi- tion from WLAN hotspot to cellular coverage is often very abrupt and leads to unacceptable call dropping rates. In this paper we propose and investigate the use of explicit WLAN/cellular handoff triggering. A simple WiFi handoff trigger node (HTN) can be installed in the WLAN/cellular 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 collabora- tion between the cellular and WLAN hotspot providers. Re- sults are presented which show that the call dropping prob- ability can be dramatically reduced by the use of a handoff trigger node. 1. Introduction In the past decade there has been a huge proliferation of wireless local area networks (WLANs) based on the IEEE 802.11 WLAN standard [1]. As the cost of WiFi components decreases, wireless manufacturers will increas- ingly incorporate IEEE 802.11 into cellular handsets and multi-mode devices. Seamless real-time voice operation is a highly desirable capability for these types of handsets. This work was supported by a grant from the Natural Sciences and En- gineering Research Council of Canada (NSERC). The authors may be contacted at todd@mcmaster.ca. AP 2 AP 3 AP 1 BS A Figure 1. Deployment Example with WiFi/Cellular Transition Area An example of a hybrid system is when a WLAN is used to cover a hotspot area inside a cellular coverage area [2]. A WLAN can provide high bandwidth but with a much shorter transmission range. There are two different inter-working solutions for WLAN/cellular interoperation [3]. Tight cou- pling corresponds to the integration of the WLAN hotspot into the cellular infrastructure. This is typically done by having the WLAN operate as a slave to the cellular sys- tem, and the WiFi hotspot is viewed as a separate cellular coverage cell [4][5]. Tightly coupled systems make more sense when the two systems are under the same administra- tive control. In a loosely coupled architecture, there is no such close relationship between the two networks [4]. This is a less complex solution and allows many network opera- tors and service providers to operate in the same market, ei- Proceedings of the First International Conference on Quality of Service in Heterogeneous Wired/Wireless Networks (QSHINE’04) 0-7695-2233-5/04 $20.00 © 2004 IEEE