The most known protocols used in H.323 are:
- RTP/RTCP (Real-Time Transport Protocol / Real-Time Transport Control Protocol): Internet-standard protocol for the transport of real-time data, including audio and video. RTP is used in virtually all voice-over-IP architectures, for videoconferencing, media-on-demand, and other applications. A thin protocol, it supports content identification, timing reconstruction, and detection of lost packets.
- RAS (Registration, Admission and Status): A protocol for Registration, Admission and Status. In an H.323 audio or video system, the RAS is a control channel over which H.225.0 signaling messages are sent.
- H225.0: Protocol used to describe call signaling, the media (audio and video), the stream packetization, media stream synchronization and control message formats
- H.245: Control protocol for multimedia communication, describes the messages and procedures used for opening and closing logical channels for audio, video and data, capability exchange, control and indications
It manages the following functionalities:
1, Interchange of capacities: Terminals define the codecs they have and they send it to the other end point.
2, Opening and closing logical channels: H.323 audio and video channels are point to point and unidirectional. Therefore they will have to create at least two of these channels. This is responsibility of H.245.
3, Flow Control when there is a problem.
4, A lot of different small functions.
- Q.931: A protocol for Call Signaling, consisting of Setup, Teardown and Disengage. Q.931 is included in the H.225.0 Recommendation
- RSVP (Resource ReSerVation Protocol): Protocol for reserving network resources to provide guaranteed application QoS (Quality of Service)
- T.120: Standard for data conferencing and conference control for interactive multimedia communication - multipoint & point-to-point.
The following codecs are recommended by the H.323 standard:
· G.711: ITU-TSS recommendation "Pulse code modulation (PCM) of voice frequencies". This audio standard is mandatory for all video conferencing systems. It requires a data rate of 56 or 64 kbit/s.
· H.261y H.263: Video codecs of H.323 standard. However, other ones can be used.