Flow Control Mapping for Interworking Function (IWF) Video

H.245 is a protocol for the transmission of call management and control signals in networks using H.323 equipment. The H.245 specification is used in audio, video, and data transmissions, as well as in voice over IP (VoIP). H.245 messages are sent over special channels called H.245 control channels.

H.245 signaling is used to manage and control call setup and connection. Functions of H.245 include determining which endpoint is to be the master and which is to be the slave during the call, opening and closing of multiplexed data-transfer paths between the endpoints, establishing an upper limit to the data transfer speed on each logical channel, information exchanges between endpoints concerning the types of data each endpoint can send and receive, requests by the receiving endpoint for changes in the mode of the data sent by the transmitting endpoint, and requests by either endpoint to end the call.

In the H.245 standard, the FlowControlCommand message is used to specify the upper limit of bit rate of either a single logical channel or the whole multiplex. The following is an excerpt from the H.245 standard.

Command Message: Flow Control (from H.245 standard)

===================================================
FlowControlCommand ::= SEQUENCE
{
		scope CHOICE
	{
	logicalChannelNumber LogicalChannelNumber,
	resourceID INTEGER (0..65535),
	wholeMultiplex NULL
	},
	restriction CHOICE
	{
	maximumBitRate INTEGER (0..16777215), -- units 100 bit/s
	noRestriction NULL
	},
	...
}
===================================================

A terminal may send this command to restrict the bit rate that the far-end terminal sends. A receiving terminal must comply with this command.

In an H.323 environment, the Oracle® Enterprise Session Border Controller previously used the FlowControlCommand to map to SIP using either the Real-Time Control Protocol (RTCP) feedback function, or the SIP signaling path (for example, the INFO method).

The Oracle® Enterprise Session Border Controller now supports the SIP counter part of the H.245 FlowControlCommand using the SIP signaling path with the INFO method. The Oracle® Enterprise Session Border Controller sends the SIP INFO message with "change_bitrate" rate parameter that has the value 100* maxBitRate from the corresponding H.245 FlowControlCommnad message. For example, in the following messages, the incoming H.323 message with the H.245 FlowControlCommand, is converted into the outgoing SIP INFO message with the message body.

Incoming H.323 Message with H.245 FlowControlCommand:

H.245
PDU Type: command (2)
        command: flowControlCommand (4)
            flowControlCommand
                scope: logicalChannelNumber (0)
                    logicalChannelNumber: 102
                restriction: maximumBitRate (0)
                    maximumBitRate: 4480

Outgoing SIP INFO Message:

Message Body
        eXtensible Markup Language
            <?xml
                version="1.0"
                encoding="utf-8"
                ?>
            <media_control>
                <vc_primitive>
                    <to_encoder>
                        <change_bitrate>
                            4480000
                            </change_bitrate>
                        </to_encoder>
                    </vc_primitive>
                </media_control>