The vision of next generation wireless network (NGWN) is to integrate
different wireless access technologies, each with its own characteristics, into
a common IP-based core network to provide mobile user with service continuity
and seamless roaming. One of the major issues for the converged heterogeneous
networks is providing a seamless vertical handover (VHO) with QoS support. In
this paper we have reviewed the various interworking architectures and handover
scenarios between UMTS and WiMAX.