Currently two standards exist for signaling and control of voice over IP calls, namely ITU-T Recommendation H.323 and the IETF Session Initiation Protocol (SIP). Although there are a significant number of similarities between these two protocols, they behave significantly different when providing different higher level services. On one hand, SIP provides flexibility with broader scope, offering functions specifically designed to enable easy extensions of new technologies. On the other hand, H.323 stands still with more established standard providing better framework and interworking. This paper describes some noteworthy differences between them and presents some solutions to make these two giant players co-existing.
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