H.245
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H.245 is the Control Protocol for Multimedia Communication within H.323 and H.324. It provides:
- Master Slave Determination
Aids in the settlement of potential collision situations in several negotiation phases.
- Terminal Capability Exchange
Allows each H.323 Terminal to communicate its audio/video capabilities, audio and video codecs, to the other party.
- Logical Channel Signaling
Provides the means to open, to control and to close a logical channel. The most important purpose of an H.245 logical channel in the context of VoIP is the negotiation of an audio stream. That is, which codecs are to be used, towards which UDP port a media stream shall be sent, etc.
One major drawback within the initial version of H.323 was the requirement for lengthy four-way H.245 protocol handshake to open up the logical channels of a telephony session. Later versions of H.323 introduced the Fast Connect procedure, using the fastStart element of an H.225.0 message. Fast Connect brought the negotiation down to a two-way handshake. Another recommendation, H.460.6, Extended Fast Connect Feature, exists that defines a one-way handshake.
H.245 offers the possibility to be tunneled within H.225 call signaling messages. This eases firewall traversing.