Abstract

The dawn of the next generation wireless networks will be characterized by the ability to integrate currently available heterogeneous networks. Network integration will not only bring about the realization of ubiquitous connectivity (anytime, anywhere), it will also achieve the dream of always best connected (ABC). This is because these heterogeneous networks are complementary, for example, the universal mobile telecommunications system (UMTS) networks have wide coverage area but with low data rate. On the other hand, wide local area network (WLAN) with high data rate suffers from limited coverage area. Again, worldwide interoperability for microwave access (WiMAX) networks has emerged with a different set of complementary features. Only through the technology of network integration will a user derive the best that these networks have to offer at anywhere and at anytime. The glue for the integration of these networks is the process called vertical handoff (VHO). VHO is the seamless transfer of an ongoing user session between different heterogeneous radio access technologies. The success of any VHO process depends largely on the ability to make accurate and precise decisions about available wireless networks for connection. A considerable number of works have appeared in literature, each proposing techniques for VHO decision. These works have considered different number of decision policies under different scenarios. They used different approaches to demonstrate their solutions. A number of them made many useful contributions; however, there remain some limitations. This survey paper seeks to provide a comprehensive exposition into the various decision making policies used in the VHO decision phase.