E.4.2.8 Interworking between RTCP messages and H.245 messages

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

E.4.2.8.1 Overview

The following clauses describe the interworking procedures between the RTCP messages listed in table E.4.2.8.1.1, which are used to enhance the quality of the media distribution by MTSI terminals at the IMS side, and the corresponding H.245 messages in 3G-324M at the CS side listed in table E.4.2.8.1.1.

As the H.245 protocol is terminated at the MGCF and RTCP is received at the IM-MGW, Mn procedures to support the transfer of RTCP related information are defined in the following Clauses to support this interworking.

Table E.4.2.8.1.1: RTCP and H.245 messages that may be interworked

RTCP Messages

H.245 messages

Remarks

Picture Loss Indication (PLI) ([109])

VideoFastUpdatePicture

Temporary Maximum Media Stream Bit Rate Request (TMMBR) ([110])

Flow Control Command/ Flow Control Indication

This interworking procedure is not recommended.

The reception of TMMBR at the IMS side is expected to be more frequent than the reception of Flow Control Command at the CS side.

NOTE 1: 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.

NOTE 2: Terminals are expected to support receiving the FlowControlCommand, as this is a mandatory feature in clause 6.4.3 in H.324. But most terminals would not really follow the received FlowControlCommand, but rather ignore it. However, for older 3G-324M terminal lock-up or system crash was not uncommon during IOTs. Therefore safe handling of this command is not guaranteed absolutely

The interworking shown in table E.4.2.8.1.1 is only applicable if video is relayed without transcoding.

If transcoding is applied, the IM-MGW should adopt its transcoder accordingly if receiving the RTCP messages listed in table E.4.2.8.1.1, and the MGCF should configure the transcoder in the IM-MGW accordingly if it receives the H.245 messages in table E.4.2.8.1.1.

For an MGCF and IM-MGW that support video interworking as specified in the present Annex E, the support of the procedures in the present clause E.4.2.8.2 and E.4.2.8.3 is optional.

E.4.2.8.2 IM CN subsystem originated RTCP messages

The MGCF may configure the IM-MGW to detect specific received RTCP packets and forward information derived from it by providing a bit pattern(s) for bits 3-15 of incoming RTCP packets as filter criterion in an event descriptor of event "RTCP-interworking". Bits 3-7 in the RTCP packet represent the Feedback message type (FMT), and bits 8-15 in the RTCP packet represent the Payload type (PT) RTCP header fields, see IETF RFC 4585 [109]. For instance, to request that an AVPF Picture Loss Indication (PLI) packet is detected for interworking, the MGCF needs to configure a combination of PT=206 (Payload-specific FB message) and FMT=1 (see IETF RFC 4585 [109]). To request that a AVPF Temporary Maximum Media Bit-rate Request (TMMBR) packet is detected for interworking, the MGCF needs to configure a combination of PT=205 (transport layer feedback message) and FMT=3 (see IETF RFC 5104 [110]).

A IM-MGW configured in this way shall check after receiving an incoming RTCP Packet if it is of a desired type, as expressed by a bit pattern received from the MGCF. An incoming RTCP message can be of compound format and contain several RTCP packets. The IM-MGW shall then perform the check separately for each packet. If the IM-MGW determines that the received RTCP packet is of a desired type, the IM-MGW shall send information derived from the RTCP message as ObservedEventsDescriptor Parameter(s) of event "RTCP-interworking" in an H.248 "Notify" message to the MGCF.

The information derived from the RTCP message to be sent to the MGCF is listed in table E.4.2.8.2.1. Information elements in table E.4.2.8.2.1 shall only be sent if the corresponding RTCP message has been received.

Table E.4.2.8.2.1: RTCP information elements to be notified over Mn

RTCP message

Information elements transported over Mn interface

Remarks

Picture Loss Indication (PLI) ([109])

UpdatePicture

Temporary Maximum Media Stream Bit Rate Request (TMMBR) ([110])

MaxBitRate

This information indicates the maximum media stream bit rate as received within the TMMBR.

Figure E.4.2.8.2.2.1 shows examples of interactions between RTCP and H.245 for IM CN subsystem originated feedback on the quality of the media distribution.

Figure E.4.2.8.2.2.1: Interactions between RTCP and H.245 for IM CN subsystem originated RTCP feedback on the quality of the media distribution

In signal 1, the MGCF requests the IM-MGW to detect received RTCP feedback message from the IMS side and notify the feedback information elements to the MGCF when the interworking is required. To request the IM-MGW to detect and notify these feedback information elements, the MGCF shall send the "RTCP-interworking" H.248 event to the IM-MGW. The event may be indicated through an H.248 MOD command.

In signal 3, the IM-MGW receives a RTCP feedback message from the IMS side.

In signal5, the IM-MGW notifies the feedback information elements from RTCP message to the MGCF.

In signal7, the MGCF send the corresponding H.245 message to the CS side to request for the media adaption.

E.4.2.8.3 CS network originated H.245 messages

Figure E.4.2.8.3.2.1 shows examples of interactions between H.245 and RTCP for CS network originated feedback on the quality of the media distribution.

Figure E.4.2.8.3.2.1: Interactions between H.245 and RTCP for CS network originated H.245 feedback on the quality of the media distribution

In signal 1, the MGCF receives an H.245 feedback message from the CS side.

In signal3, the MGCF requests the IM_MGW to send an RTCP message provisioning the signal "H.245 interworking" with parameters identifying information to be transported within this RTCP message.

In signal5, the IM-MGW send the corresponding RTCP message to the IMS side to request for the media adaption.

To request the IM-MGW to send an RTCP message, the MGCF shall send the corresponding information elements listed in table E.4.2.8.3.2.1 as parameters of the "H.245-interworking signal" over the Mn interface.

Table E.4.2.8.3.2.1: Feedback information elements to be indicated

RTCP message

Information elements transported over Mn interface

Remarks

Picture Loss Indication (PLI) ([109])

UpdatePicture

Temporary Maximum Media Stream Bit Rate Request (TMMBR) ([110])

MaxBitRate

This information indicates the maximum media stream bit rate to be sent within the TMMBR.