Comparison of SIP and H.323 Protocols

This paper compares two most important protocols for signaling in voice over IP (VoIP) networks of today, namely SIP and H.323. Several aspects of protocol functionality are covered. We have tried to focus on those aspects that are most important for successful operation of VoIP networks, ranging from characteristics of syntax to functions of infrastructure elements and support for third party call control and other communications applications. Security features and some implementation issues are analyzed as well. Conclusion is that although H.323 appeared earlier, SIP is better adapted to the Internet environment which is the most important reason for its success. Also greater standardization activity related to applications of SIP (e.g. presence services, instant messaging) made it more attractive to developers.