IP Multimedia Subsystem (IMS) is a set of specifications that describes the Next Generation Networking (NGN) architecture for implementing IP based telephony and multimedia services. IMS defines a complete architecture and framework that enables the convergence of voice, video, data and mobile network technology over an IP-based infrastructure. IMS is based on the specification of Session Initiation Protocol (SIP) as standardized by Internet Engineering Task Force (IETF). IMS can be considered as architecture for the convergence of data, speech, fixed and mobile. A heterogeneous network model based on the IMS is presented, this model integrates the Worldwide Interoperability for Microwave Access (WiMAX), New Generation Network (NGN) technologies and provides guaranteed QoS. This paper present the complete signaling flow concerning the authorization, registration, session set up, as well as, an analytic model for cost analysis of the proposed architecture is analyzed.