H.245, a control signaling protocol in the H.323 multimedia communication architecture, is for of the exchange of end-to-end H.245 messages between communicating H.323 endpoints/terminals. The H.245 control messages are carried over H.245 control channels. The H.245 control channel is the logical channel 0 and is permanently open, unlike the media channels. The messages carried include messages to exchange capabilities of terminals and to open and close logical channels.
After a connection has been set up via the call signaling procedure, the H.245 call control protocol is used to resolve the call media type and establish the media flow, before the call can be established. It also manages the call after it has been established. The steps involved are:
- Master-slave determination process. It is used to determine the master of the call and is useful for avoiding conflicts during call control operations.
- Capability exchange procedure. Each endpoint notifies the other what kind of information it is capable of receiving and transmitting through the receive and transmit capabilities.
- Logical channel procedures. Used for opening and closing logical channels, which are multiplexed paths between the endpoints used for data transfer.
- Request mode command. Using this command, at any point during the conference, the receiving endpoint can request for a change in mode of the transmitted information provided the mode is in the transmit capability of the transmitter.
- Control flow command. This can be used by the receiver to fix an upper limit for the transmitter bit rate on any logical channel.
- Communication mode messages. Used by the multipoint controller for selecting a common mode of operation in a multipoint conference.
- Conference request and response messages. Used for controlling a multipoint conference, eg. password requests, conference chair control.
- Round trip delay commands. Used for measuring the round-trip delay between two endpoints on the control channel.
- Video fast update command. Used for requesting updates for video frames, in case of data loss.
- End session command. After this command the endpoints close all logical channels, drop the call and inform the gatekeeper about the end of the call.
Protocol Structure
H.245 messages are in ASN.1 syntax. MultimediaSystemControlMessage types can be defined as request, response, command and indication messages. Key H.245 messages are as follows:
| Message | Function |
| Master-Slave Determination | Determines which terminal is the master and which is the slave. Possible replies: Acknowledge, Reject, Release (in case of a time out). |
| Terminal Capability Set | Contains information about a terminal"s capability to transmit and receive multimedia treams. Possible replies: Acknowledge, Reject, Release. |
| Open Logical Channel | Opens a logical channel for transport of audiovisual and data information. Possible replies: Acknowledge, Reject, Confirm. |
| Close Logical Channel | Closes a logical channel between two endpoints. Possible replies: Acknowledge |
| Request Mode | Used by a receive terminal to request particular modes of transmission from a transmit terminal. General mode types include VideoMode, AudioMode, DataMode and Encryption Mode. Possible replies: Acknowledge, Reject, Release. |
| Send Terminal Capability Set | Commands the far-end terminal to indicate its transmit and receive capabilities by sending one or more Terminal Capability Sets. |
| End Session Command | Indicates the end of the H.245 session. After transmission, the terminal will not send any more H.245 messages. |
Related protocols: RTSP, SIP, RTP, RTCP, SDP, Megaco/H.248, Q.931, H.323, H.225
Sponsor Source: H.245 is an ITU-T (http://www.itu.int/ITU-T/ ) standard.
Reference:
http://www.javvin.com/protocol/H245v9.pdf: Control Protocol for Multimedia Communication (version 9)
http://www.h323forum.org/papers/: H.323 papers and documents