Protocol

There are two major protocols being used for VoIP. Both protocols define ways for devices to connect to each other using VoIP. Also, they include specifications for audio codecs. A codec, which stands for coder-decoder, converts an audio signal into a compressed digital form for transmission and back into an uncompressed audio signal for replay. The first protocol is H.323, a standard created by the International Telecommunications Union (ITU). H.323 is a comprehensive and very complex protocol. It provides specifications for real-time, interactive videoconferencing, data sharing and audio applications such as IP telephony. Actually a suite of protocols, H.323 incorporates many individual protocols that have been developed for specific applications.H.323 Protocol SuiteVideo Audio Data TransportH.261H.263 G.711G.722G.723.1G.728G.729 T.122T.124T.125T.126T.127 H.225H.235H.245H.450.1H.450.2 H.450.3RTPX.224.0As you can see, full implementation of H.323 requires a lot of overhead. This page provides detailed information about the entire H.323 suite of protocols and how they relate to the OSI Reference Model. An alternative to H.323 emerged with the development of Session Initiation Protocol (SIP) under the auspices of the Internet Engineering Task Force (IETF). SIP is a much more streamlined protocol, developed specifically for IP telephony. Smaller and more efficient than H.323, SIP takes advantage of existing protocols to handle certain parts of the process. For example, Media Gateway Control Protocol (MGCP) is used by SIP to establish a gateway connecting to the PSTN system. You can learn more about the architecture of SIP on this page.