Introduction

Growing consumer demand for access to communication services anywhere and anytime is driving an accelerated technological development towards the integration of various wireless access technologies. Such integration combines islands of access networks into a seamless system, referred to as Fourth Generation (4G) wireless systems. 4G wireless systems will provide significantly higher data rates, offer a variety of services and applications previously not possible due to bandwidth limitations, and allow global roaming among a diverse range of mobile access networks.

In a typical 4G networking scenario, handsets or Mobile Terminals (MTs) with multiple interfaces will be able to choose the most appropriate access link among the available alternatives. These access links include IEEE 802.11 Wireless Local Area Network (WLAN) access, IEEE 802.16 Worldwide Interoperability for Microwave Access (WiMAX) , satellite systems and Bluetooth, in addition to the traditional cellular telephony networks. For a satisfactory user experience, MTs must be able to seamlessly transfer to the “best” access link among all available candidates with no perceivable interruption to any ongoing voice or video conversation. Such ability to handover between heterogeneous networks is referred to as vertical hand overs . As an important step towards achieving this objective, the emerging IEEE 802.21 standard creates a framework to support protocols for enabling seamless vertical handovers. IEEE 802.21 provides only the overall framework, leaving the implementation of the actual algorithms to the engineers designing the system. Therefore, it is essential to develop efficient vertical handover decision (VHD) algorithms to ensure the success of this new framework.