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Next generation wireless communications will integrate multiple wireless access networks, including WLAN and WWAN. Vertical handoff decision plays an important role in heterogeneous networks. In this paper, we propose a vertical handoff decision algorithm (VHDA) to provide ubiquitous access at any time. We consider received signal strength (RSS), available network bandwidth (B), monetary cost (C) and user preference (PF) as the vertical handoff decision criteria. Handoffs are divided into two classes: handoffs initiated by mobile nodes (MNs) and handoffs initiated by attachment points (e.g., base stations or access points). Our proposed algorithm consists of two parts: fuzzy logic based VHDA and cost function based VHDA. For the handoffs initiated by MNs, fuzzy logic based VHDA is used to evaluate the performance of the network and make handoff decisions to choose the most appropriate network. For the handoffs initiated by attachment points, cost function based VHDA is utilized to select the MNs with lower cost values. Then the selected MNs are handed over to relieve heavy load of the current network. A performance evaluation using the integration of WLAN and TD-SCDMA as an example shows that fuzzy logic based VHDA is able to make accurate handoff decisions, reduce unnecessary handoffs and eliminate the ping-pong effect. Cost function based VHDA can make selections quickly, balance the network resources and decrease the probability of call blocking and dropping.