Tremendous innovations and inventions in wireless network technology has led to the development of Next Generation Wireless Network. The primary aim of Next Generation Wireless Network is to conceal heterogeneities and to achieve convergence of diverse networks to provide seamless mobility. In a heterogeneous environment, numerous technologies co-exist with their unique characteristics. A mobile node equipped with multiple network interfaces will be able to access heterogeneous wireless access network. This textbook takes a unified view of the fundamentals of Handoff and provides an optimized Vertical Handoff Decision scheme, which minimizes the probability of handoff failure, unnecessary handoff, and decision delay whilst maintaining user satisfaction at high levels. The scheme consists of three closely integrated modules: Handoff Necessity Estimation Module (HNEM), Handoff Triggering Condition Estimation Module (HTCM), and Handoff Target Network Selection & Execution Module (HTSM). This book is especially useful to professionals in Telecommunication and will also be of great interest to practicing engineers, & research scholars.