In the past few years, the telecommunications field experienced a rapid evolution on two main grounds: Multiple mobile network interface terminals, and mobile radio technologies. The needs of the users in terms of flow and mobility are in constant increase, making it necessary to develop a new generation of networks. The latter would be a heterogeneous wireless environment in which different access networks must coexist to reach a compromise between mobility, transparence and performance. In the current paper, we focus on increasing the performances of UMTS and WIMAX heterogeneous networks by the integration of the concept of Multihoming. With this aim in mind, we paid close attention to the selection of the transport protocol to be used, as well as to the interconnection models of these two networks.