A heterogeneous wireless network is characterized by the presence of different wireless access technologies that coexist in an overlay fashion. These wireless access technologies usually differ in terms of their operating parameters. On the other hand, Mobile Stations (MSs) in a heterogeneous wireless network are equipped with multiple interfaces to access different types of services from these wireless access technologies. The ultimate goal of these heterogeneous wireless networks is to provide global connectivity with efficient ubiquitous computing to these MSs based on the Always Best Connected (ABC) principle. This is where the need for intelligent and efficient Vertical Handoffs (VHOs) between wireless technologies in a heterogeneous environment becomes apparent. This paper presents the design and implementation of a fuzzy multicriteria based Vertical Handoff Necessity Estimation (VHONE) scheme that determines the proper time for VHO, while considering the continuity and quality of the currently utilized service, and the end-users' satisfaction. 1. Introduction During recent years, the need for global connectivity to access different types of services at any place and any time has significantly increased. The latest innovations  in wireless access technology provide uninterrupted on-demand services, such as real-time multimedia, which are independent of device type, locations, and available networks ; the ultimate goal is to maintain a satisfactory end-user experience by providing the necessary quality and continuity of the currently utilized service in a cost-efficient manner. A heterogeneous wireless network comprises of different wireless networks including IEEE 802.15 based Wireless Personal Area Network (WPAN), IEEE 802.11 based Wireless Local Area Networks (WLAN), IEEE 802.16 based Wireless Metropolitan Area Network (WMAN), different types of cellular technologies such as Global System of Mobile Communication (GSM) and Universal Mobile Telecommunication System (UMTS), Vehicular Ad hoc Network (VANET), and satellite networks. To provide seamless mobility and convergence, integration of these diverse wireless networks is required. A multimodal Mobile Station (MS), in the presence of these integrated networks with overlapping coverage, can connect to any of these wireless access technologies. An MS, in a heterogeneous wireless network, switches its current Point of Attachment (PoA) to a new wireless network using a process called Vertical Handoff (VHO). These VHOs are required to maintain the continuity and quality of the current session
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