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The next generation of mobile communications, broadly referred to as 4G, is based on a heterogeneous infrastructure comprising different wireless (and wired) access systems in a complementary manner. 4G mobile users enjoys seamless mobility and ubiquitous access to applications in an always best connected (ABC) mode that employs the most efficient combination of available access systems. The ongoing commercialization of 3G cellular mobile networks and their upcoming enhancement with WLAN radio access provides a wireless platform suitable for the introduction of "ABC" capabilities. We analyze the implications of the "ABC" vision in a UMTS/WLAN network context, and reveal important issues that arise. Further on, we identify major requirements, point out the limitations of current UTMS/WLAN standards from an ABC viewpoint, and discuss key enabling technologies and research efforts. We formulate a generic application model for an ABC capability in the interworked UMTS/WLAN architecture and analyze its complexity proving that, in principle, being always best connected translates to a family of NP-hard problems. To complement our analysis, we present an object-oriented design of a real-time UML model for an ABC mobile system. Finally, we summarize the advantages of our ABC model and provide directions for future work.