H.245
H.245 is an ITU-T recommendation and describes the control protocol for multimedia communication within the H.323 protocol family. H.245 describes how terminal messages are structured and what their meaning is. It also describes the procedures for using the messages at the beginning of or during communication. The messages include a. Reception and transmission functions and preferred methods of the recipient terminal. Signaling procedures are specified to ensure reliable audio, video and data communication.
Functions
The following main functions are provided in H.245:
- Master Slave Determination: Helps to resolve potential collision situations during various negotiation phases.
- Terminal Capability Exchange: Allows every H.323 terminal to communicate its audio / video capabilities, audio and video codecs to the other communication party.
- Logical Channel Signaling: Signaling of logical channels. Provides the means to open, control and close logical channels. In the context of IP telephony , the most important purpose of a logical channel is to negotiate the audio stream ; H. determine which codecs are used, for which UDP - Port a media stream to be transmitted and other parameters.
A disadvantage within the first version of H.323 was the need for a four-way protocol handshake to open a logical channel for a telephony session. Later H.323 versions introduced the Fast Connect procedure using the fastStart element of an H.225.0 message. Recommendation H.460.6 Extended Fast Connect feature defines a procedure that works without handshaking between the end devices.
H.245 offers the possibility of being tunneled within H.225 call signaling messages. This method makes it easier to penetrate firewalls.
Individual evidence
- ↑ H.245: Control protocol for multimedia communication. May 2011.