Abstract-- Wireless network devices are equipped with multiple access technologies like UMTS and Wireless LAN. The handover between the technologies has been widely studied in literature. Several of these research papers build their handover mechanisms on top of existing protocols like Mobile IP. The protocols however, operate on the network layer and only react on link layer changes. Consequently, a seamless vertical handover between Wireless LAN and UMTS cannot be provided. In this paper, we introduce a vertical handover protocol based on a tight-coupled network architecture. With this architecture, it is possible to perform the handover on the link layer and thus, reducing the handover delays. The protocol is implemented in a simulation environment and the results reveal that a seamless vertical handover can be performed, with blackout times of only a few milliseconds.