H.245 is a multimedia communication control signaling protocol in H.323, which is mainly used for end-to-end H.245 information between H.323 endpoint terminals. The H.245 control information is carried out on the H.245 control channel. The control channel is logical channel 0 and is permanently open, which is not the same as the media channel. The transportation information includes terminal exchange capabilities and information such as opening and closing logical channels. After the call signaling program is established, the H.245 call control protocol is used to resolve the call medium type problem, and the protocol needs to create a medium stream before calling creation, and managing the call after the call is created. . This step includes:
Master Determination: In order to solve the problem of resource conflicts when multiple terminals trigger similar events while trigger similar events, the master-party decision procedure is introduced. Using this procedure can determine one of the terminals in multiple communications as the main terminal master, and others are considered to be from the terminal SLAVE. Ability Exchange: Communication Both parties exchange their respective communication capabilities. Logical channel: Used to open or close a one-way or bidirectional logical channel. Mode Request: Once the capacity exchange is completed, a terminal is allowed to request a remote to select an operating mode in its transmission direction. Flow control: The recipient is fixed to the transmission bit rate on any logical channel. Communication mode: For multi-point console to select the general mode of operation with a multi-point meeting. Conference requests and responses: used to control multi-point meetings, such as password requests, conference enclosures control, etc. Determine the loopback delay: used to determine the loop delay between the two terminals. Video Quick Update: Request video frame structure update in the case of data loss. End Session: The terminal closes all logical channels, cancels the call and notifys the title call end.
Protocol structure
The H.245 information is included in the ASN.1 syntax. Multimedia system control information types include: request, response, command, and instruction information. The main H.245 information is as follows:
The information function Master-slave determination determines the main terminal and the secondary terminal. Possible reply is: Acknowledge, REJECT, Release (assuming time). Terminal Capability Set includes terminal transmission and receiving multimedia stream, which may reply: Acknowledge, Reject, Release. Open Logical Channel opens a logical channel for the transmission of audiovisual devices and data information, which may reply: Acknowledge, Reject, Confirm. Close Logical Channel closes the logical channel between the two ends, which may answer: Acknowledge. The Request Mode Receive Terminal uses this information to request a special transmission mode to the transport terminal. General mode includes: video mode, audio mode, data mode, and encryption mode. Possible reply is: Acknowledge, Reject, Release. Send Terminal Capability Set The remote terminal indicates its transmission and reception performance by sending one or more Terminal Capability sets commands. End session Command Indicates the end of the H.245 session. After the transfer ends, the terminal will no longer send any H.245 information.
