Integration of communication networks has been accelerated and is recognized as an important evolution plan of the existing networks such as WiBro (Wireless Broadband Internet) and WLAN (Wireless LAN). There are two ways to integrate these networks: loose coupling and tight coupling. The loose coupling approach provides separate data paths, while the tight coupling provides a full integration of different networks. The loose coupling is worse in seamless mobility, QoS (Quality of Service) provision, and network security. In order to lessen the problems of loose coupling and improve tight coupling method, we propose a new convergence network architecture that allows users to roam between cells in wireless overlay networks. Also, we propose a vertical handoff algorithm for WiBro and WLAN networks. Our algorithm reduces handoff oscillation (so-called Ping-Pong effect) in which a MN (Mobile Node) with speed moves back and forth in two networks in a short time. We consider the speed of MN and the distance between MN and AP (Access Point). When MN moves on the overlapped area of two networks, we calculate DT (Dwell Time) value to reduce unnecessary handoff. The simulation results show that the total throughput can be increased by using proper DTthreshold value.