E.4.2.4 Call establishment procedure

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

The following information shall be provided from the MGCF towards the IM-MGW:

– Properties to start H.223 Negotiation.

– Request for events for Notification of H.245 message received by the IM-MGW.

– Signals to provide H.245 messages that the IM-MGW shall send towards the CS side.

– Properties to provide incoming and outgoing H.223 multiplex table.

– Properties to provide H.223 logical channel parameters.

– Property to provide remote H.223 capabilities.

The Highest Multiplexing Level shall be predefined in the IM-MGW.

NOTE 1: All H.245 messages (from Signal 9 to Signal 18) are transported through the IM-MGW between the MGCF and the CS side, using the procedures described in clause E.4.2.3

NOTE 2: Signals 21 and 22 are omitted if the same codec information has already been provisioned in signal 3.

Figure E.4.2.4.1: Mn signalling interactions for H.245 termination at the MGCF

The MGCF shall request terminations towards the CS network (Signal 1 and 2) and towards the IMS (Signal 3 and 4). For the terminations towards the IMS, the MGCF provides an estimate about the applicable codecs in the required information elements "Local IMS Resources" (for both "Reserve IMS Connection Point" procedure and "Reserve IMS Connection Point and Configure Remote Resources" procedure) and possibly "Remote IMS Resources" (only for "Reserve IMS Connection Point and Configure Remote Resources" procedure).

The MGCF shall request that the H.223 stream is (de-)multiplexed at the MUX termination T2, and that the H.245 control in H.223 Logical channel 0 is separated (signal 6). Furthermore, the MGCF shall request that the H.223 negotiation is started, and shall request to be notified about all H.245 messages received by the IM-MGW.

The IM-MGW shall start the H.223 Multiplexing Level Negotiation after receipt of the corresponding request from the MGCF and CS bearer establishment (Signal 8).

Upon reception of a H.245 Terminal Capability Set message (Signal 9), the MGCF sends a H.245 Acknowledgment message (Signal 10).

The MGCF shall know the H.324 related capabilities of the IM-MGW before starting the H.245 capability negotiation with the CS side, e.g. through configuration. The H.245 Terminal Capability Set message send by the MGCF (Signal 11) should include the codecs which are supported by both the IMS side and the IM-MGW, and the codecs which could be transcoded by the IM-MGW from the codecs supported by the IMS side.

The MGCF may defer sending the Terminal Capability Set message (Signal 11) for some time to wait for codec information from the CS peer’s Terminal Capability Set message and perform a possible IMS-side codec re-negotiation. To avoid blocking situations, the MGCF shall not defer sending the signal for an excessive period of time.

To avoid the CS side selecting the codecs that need to be transcoded at the IM-MGW, the MGCF should aim to be the leader in the procedures in H.245 [82] Annex C.2 (Signals 9 to 12). The MGCF shall set the Terminal Type parameter as a number larger than 128 in the message in H.245 [82] Annex B.1.1. The procedure in H.245 [82] Annex C.2 could be combined with the messages used for the H.245 capability exchange.

The codecs contained both in the sent and received terminal capability set message may be selected at the CS side. The final decision of the selected codecs at the CS side is taken with the H.245 open logical channel procedure (Signals 13 to 16).

After the completion of the H.245 multiplex table exchange procedure (Signals 17 to 20), the MGCF shall configure the multiplexing termination T2 by indicating to the IM-MGW the contents of the incoming and outgoing multiplex tables (Signal 21).

If codec information needs to be changed compared to what has been provisioned in signal 3, the MGCF shall also configure T3 with the appropriate video and/or speech codec(s) (signal 23).

The call is in the active state.