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. It fills the gap between the two most successful communication paradigms, cellular and Internet technology. Do you ever imagine that you can surf the Web, play an online game or join a videoconference no matter where you are using your 3G handheld device? This is the vision for IMS; to provide cellular access to all the services that the Internet provides.