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The rapid growth and innovation of the various mobile communication technologies have caused a change in the paradigm of internet access.Wireless technologies such as WiMAX,Wi-Fi and UMTS/LTE networks have shown great potential in dominating the wireless access markets.The existence of various access technologies requires a means for seamless internetworking to provide anywhere,anytime services without interruption in the ongoing session,especially in multimedia applications with rigid Quality of Services(QoS) requirements.The IEEE 802.21 Media Independent Handover (MIH) working group was formed to develop a set of mechanisms under a standard framework with the capability to support migration of mobile users across heterogeneous networks.Therefore,the implementation of handover is extremely important in the heterogeneous network environment.In order to guarantee various QoS requirements during handover execution especially in multimedia applications,in this paper we propose a novel MIH-based capacity estimation algorithm to execute handover with QoS provision supporting both horizontal and vertical handovers across UMTS and WiMAX networks.Simulation shows that the proposed mechanism achieves lower call dropping rate (highest approximate 3％ )and higher system throughput (average 92％ ) than the basic handover method does.