E.4.2.6 Handling of H.245 Command message

29.1633GPPInterworking between the IP Multimedia (IM) Core Network (CN) subsystem and Circuit Switched (CS) networksTS

E.4.2.6.1 Overview

The MGCF shall support the End Session command message. The MGCF may support the Flow Control command message and the videoFastUpdatePicture message. All these H.245 messages are conveyed between the MGCF and the CS side by the IM-MGW, as described in clause E.4.2.3.

E.4.2.6.2 Flow control command

The flow control command is used to restrict the upper limit of bit rate of either a single logical channel or the whole multiplex stream. The MGCF may support the flow control command received from the CS side.

NOTE: It is very unlikely that the MGCF will receive a Flow control command, as the capability of controlling the video bit rate during a video call was neither implemented nor tested for Ues offering 3G-324M since R99, due to the lack of such a necessity in WCDMA where a fixed 64 kbps bearer is maintained to continuously transport 3G-324M PDUs.

Figure E.4.2.6.2.1: Mn procedure of Flow control command

In Signal 1, the MGCF receives the Flow Control Command from the CS side.

If the minimum bitrate of the current codec is larger than the bitrate requested by the H.245 Flow Control Command message, the MGCF indicates the IM-MGW to stop the transmission of the media stream over the logical channel (signal 2). Then the MGCF may select another codec that can satisfy the requested bitrate limit. In signal 4, the MGCF closes the old logical channel and opens a new logical channel with new codec to satisfy the bitrate limit in the CS side. In signal 6, the MGCF indicates the IM-MGW to modify the LCN, codec and stream mode of the multiplexing termination. In signal 8, the MGCF sends flow control indication message to CS side with the current maximum bitrate. If the MGCF chooses to change the CS-side codec, the MGCF may also adjust the codec at the IMS side accordingly. To do so, the MGCF may need to re-negotiate the codec at the IMS side using a SIP re-INVITE message (signals 9 and 10). In addition, the MGCF may modify the codec of IMS termination accordingly (signal 11).

The MGCF may also apply interworking procedure towards RTCP at the IMS side as described in clause E.4.2.8 when receiving a H.245 Flow Control Command.

E.4.2.6.3 End Session Command

The end session command is used to close the H.245 control channel after all the logical channels have been closed.

The MGCF may send an end session command to the CS side through the IM-MGW to release a call.

If the MGCF receives an end session command from the CS side, it shall release the call if the call is in the active state.

E.4.2.6.4 videoFastUpdatePicture

The MGCF may apply interworking procedure towards RTCP as described in clause E.4.2.8 when receiving a H.245 videoFastUpdatePicture message. The MGCF may also be triggered by the procedures in clause E.4.2.8 to send a H.245 videoFastUpdatePicture message.

When receiving frequent H.245 videoFastUpdatePicture messages for a SCUDIF call (see 3GPP TS 23.172 [121]), an MGCF should initiate a service-change from multimedia to speech and remove the IMS MTSI video component with SIP/SDP signalling. When receiving frequent H.245 videoFastUpdatePicture messages for a non-SCUDIF call, an MGCF should disable the CS video and remove the IMS MTSI video component with SIP/SDP signalling.

NOTE: In a congestion situation at the IMS side, frequent packet losses might occur and dropping the video component may serve as a counter-measure to reduce the packet loss frequency. However, CS terminals might not be able to initiate such a switch-over to voice, but might rather react with repeated H.245 videoFastUpdatePicture message.