In this paper, we study a strategy of network architecture to the integration of Universal Mobile Telecommunications System (UMTS) and Wireless Local Area Network (WLAN) for the seamless vertical handover between these heterogeneous systems. For the internetworking strategy, we consider Hierarchical Mobile IPv6 (HMIP) networks based on Mobility Anchor Points (MAP) that the latency of the vertical handover can be decreased to provide fast handover to mobile nodes. Based on the integration strategy of HMIP, we also propose a vertical handover scheme which avoids the call blocking (new calls or hard handover calls). In integrated UMTS and WLAN networks, mobile nodes having dual system modes can use heterogeneous network services.