H.323 is a suite of protocols that can be used to initiate, control and
conduct audio-visual communication sessions over packet-switching networks
such as the Internet.
For example:
H.225.0 describes call signaling, the media (audio and video),
the stream packetization, media stream synchronization and control
message formats.
H.245 describes the messages and procedures used for opening and
closing logical channels for audio, video and data, capability exchange,
control and indications.
H.450 describes Supplementary Services
H.235 describes security in H.323
H.323 requires the presence of the capability to encode and decode video
compressed using the H.261 video compression standard and to encode
and decode audio via G.711 audio compression standard, but allows other
compression/decompression schemes (codecs), such as H.263 and G.723,
to be used as well.
(Note that H.261 will provide 352x288 resolution at best, and that most
H.323 interactions operate at this resolution.)
In addition, H.323 references (but does not require use of) the T.120 suite
of protocols that define capabilities that may be used to build
collaboration applications (control of multimember groups, simultaneous
data transfer among members, transmission of whiteboard information,
application sharing, etc.).
Finally, H.323 includes specs for
Multipoint Control Units (MCUs) for "centralized" multipoint
conferences (although H.323 supports the notion of "decentralized"
conferences that use the MCU only for call-control, while allowing
Multicast of media traffic),
Gatekeepers to control access to the various H.323 components and
overall bandwidth utilization within a network, and
Gateways between H.323 and ISDN conferencing based on the H.320
protocol suite.
