Classification of VoIP approaches
From the telephone to the SIP

Today, there are two approaches to providing VoIP and, more generally, multimedia services over IP. The first approach, defined by the IETF, is based on the SIP protocol. The second approach, defined by lTU-T, draws heavily on ISDN, and is described in the H.323 (for terminals located in a corporate network) and H.324 (for terminals connected over a modem link, or 3G videotelephony) recommendations. SIP and H.323 use very different signaling protocols. However, they are based on the same standards for audio and video codecs, as well as for the protocol used to transport samples from the various codecs (RTP/RTCP protocol).