H.323 uses codec
Codecs
H.323 utilizes both ITU-defined codecs and codecs defined outside the ITU. Codecs that are widely implemented by H.323 equipment include:Audio codecs: G.711, G.729 (including G.729a), G.723.1, G.726, G.722, G.728, Speex, AAC-LD
Text codecs: T.140
Video codecs: H.261, H.263, H.264, H.265All H.323 terminals providing video communications shall be capable of encoding and decoding video according to H.261 QCIF. All H.323 terminals shall have an audio codec and shall be capable of encoding and decoding speech according to ITU-T Rec. G.711. All terminals shall be capable of transmitting and receiving A-law and μ-law. Support for other audio and video codecs is optional.Capability negotiation
Of the functionality provided by H.245, capability negotiation is arguably the most important, as it enables devices to communicate without having prior knowledge of the capabilities of the remote entity. H.245 enables rich multimedia capabilities, including audio, video, text, and data communication. For transmission of audio, video, or text, H.323 devices utilize both ITU-defined codecs and codecs defined outside the ITU. Codecs that are widely implemented by H.323 equipment include:Video codecs: H.261, H.263, H.264
Audio codecs: G.711, G.729, G.729a, G.723.1, G.726
Text codecs: T.140H.245 also enables real-time data conferencing capability through protocols like T.120. T.120-based applications generally operate in parallel with the H.323 system, but are integrated to provide the user with a seamless multimedia experience. T.120 provides such capabilities as application sharing T.128, electronic whiteboard T.126, file transfer T.127, and text chat T.134 within the context of the conference.
When an H.323 device initiates communication with a remote H.323 device and when H.245 communication is established between the two entities, the Terminal Capability Set (TCS) message is the first message transmitted to the other side.Alternatives
IAX2 - Inter-Asterisk eXchange, a binary protocol, designed to reduce overhead especially in regard to voice streams.  Defined in RFC 5456.
The IETF produced a standard called the Session Initiation Protocol (SIP) that also enables voice and video communication over IP.
There are also other ITU-T recommendations used for videoconferencing and videophone services – H.320 (using ISDN) and H.324 (using regular analog phone lines and 3G mobile phones).
Jingle (Jabber/XMPP extension) also enables video and voice over IP.
Some providers (such as Skype) also use their own closed, proprietary formats.
Access Grid provides broadly similar functionality, with more emphasis on open-source and utilizing multicast.
EVO also provides relatively open functionality via Java, and includes H.323 support.
utilizes employs applies operates works with