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Third generation (3G) cellular networks and IEEE 802.11 wireless LANs (WLAN) are commonly considered to be complementary technologies. By internetworking these systems, one can offer subscribers ubiquitous connectivity as well as high-speed wireless access in high user density locations known as "hotspots". The key issue involved in achieving these integration benefits is the development of internetworking architectures of WLAN and 3G technologies that can provide seamless roaming services between these two distinct radio access networks. One feasible solution is to employ mobile devices with dual network interfaces and the means to switch data transmission between these interfaces depending upon the availability of the access network. We discuss the design and implementation of such a dual-mode mobile terminal with two radio-access interfaces and a dynamic interface switching algorithm …