Introduction

The IP Multimedia Subsystem (IMS) is a next-generation multimedia communication framework that encompasses mobile, fixed, packet-switching, and traditional circuitswitching communication systems. It has been proposed by the Third Generation Partnership Project (3GPP) and uses the Voice over Internet Protocol (VoIP) framework, especially the Session Initiation Protocol (SIP) standard. The 3GPP is a standards organization driven by wireless carriers and equipment manufacturers and aims at producing globally applicable specification and reports for Third Generation (3G) Mobile Systems based upon GSM (Global System for Mobile Communication). The 3GPP also is working with the Internet Engineering Task Force (IETF) on SIP-related standards. The goal of the IMS is to provide a wide spectrum of services with ease and consistency These services include videoconferencing, Push-to-Talk (PTT), Text-to-Speech (TTS), instant messaging (IM), content sharing, and multipart gaming. To achieve this goal, IMS uses an open standard IP protocol and extension of SIP.

SIP is standardized by the IETF as multimedia signaling protocol. Its architecture is highlighted by a User Agent (UA), which is a terminal, and a set of servers, including proxy server, registration server, redirection server, and so on. From its inception, SIP has been gaining momentum due to its open architecture and extensibility to mobile device and multimedia communication. Because SIP signaling protocol is based on Hypertext Transfer Protocol (HTTP) and ASCII-based encoding, it is easy to understand. SIP also sits side by side with a standard media protocol named Real-time Transport Protocol (RTP), also standardized by the IETF. RTP is a...