Information Technology Reference
In-Depth Information
The H.324 terminals may be integrated into personal computers or imple-
mented in the stand-alone devices such as video telephones. Support for each
media type (voice, data, and video) is optional, but if supported, the ability
to use a specified common mode of operation is required, so that all ter-
minals supporting that media type can interwork. H.324 allows more than
one channel of each type to be in use. Other ITU-T recommendations in the
H.324 series include the H.223 multiplex, H.245 control, H.263 video codec,
and G.723.1 audio codec.
H.324 makes use of the logical channel signaling procedures of ITU-T
Recommendations H.245 in which the content of each logical channel is
described when the channel is opened. Procedures are provided for expres-
sion of receiver and transmitter capabilities, so transmissions are limited to
what receivers can decode, and so that receivers may request a particular
desired mode from transmitters.
H.324 terminals may be used in multipoint configurations through MCUs,
and may interwork with H.320 terminals on the Integrated Services Digital
Network (ISDN), as well as with terminals on wireless networks. Annex
A defines the data protocol stack for use with the H.324 control channel.
Annex B defines High-level Data Link Controller (HDLC) frame structure
transparency for asynchronous transmission. Annex C defines the use of
H.324 terminals in error-prone transmission environments.
3.2.1.1 Functional Elements Covered by 3G324M
The Video codec (H.263 or H.261) carries out redundancy reduction coding
and decoding for video streams.
The Audio codec (G.723.1) encodes the audio signal from the microphone
for transmission, and decodes the audio code that is output to the speaker.
Optional delay in the receiving audio path compensates for the video delay,
so as to maintain audio and video synchronization.
The Data protocols support data applications such as electronic white-
boards, still image transfer, file exchange, database access, audio graphics
conferencing, remote device control, network protocols, and so on. Other
applications and protocols may also be used via H.245 negotiation.
The Control protocol (H.245) provides end-to-end signaling for proper
operation of the H.324 terminal, and signals all other end-to-end system
functions including reversion to analog speech-only telephony mode. It pro-
vides for capability exchange, signaling of commands and indications, and
messages to open and fully describe the content of logical channels.
The Multiplex protocol (H.223) multiplexes transmitted video, audio, data,
and control streams into a single bit stream, and demultiplexes a received
bit stream into various multimedia streams. In addition, it performs logical
framing, sequence numbering, error detection, and error correction by
means of retransmission, as appropriate to each media type.
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