H.323 requires that endpoints negotiate compatible settings before audio, video, and/or data communication links can be established. H.245 uses control messages and commands that are exchanged during the call to inform and instruct. The implementation of H.245 control is mandatory in all endpoints. H.245 provides the following media control functionalities:
Capability Exchange
H.323 allows endpoints to have different receive and send capabilities. Each endpoint records its receiving and sending capabilities (media types, codecs, bit rates, etc.) in a message and sends it to the other endpoint(s).
Opening and Closing of Logical Channels
H.323 audio and video logical channels are unidirectional end-to-end links (or multipoint links in the case of multipoint conferencing). Data channels are bidirectional. A separate channel is needed for audio, video, and data communications. H.245 messages control the opening and closing of such channels. H.245 control messages use logical channel 0, which is always open.
Flow Control Messages
These messages provide feedback to the endpoints when communication problems are encountered.
Other Commands and Messages
Several other commands and messages may be used during a call, such as a command to set the codec at the receiving endpoint when the sending endpoint switches its codec. H.245 control messages may also be routed through a gatekeeper if one exists.
Figure 1 shows H.245 signaling. After establishing a control signaling link between two gateways, media bandwidth is negotiated. After the two terminals agree, with acknowledgments, an open link is established for the call.
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