E.4.2.7 Mn Signalling Interactions to support the Media Oriented Negotiation Acceleration (MONA)

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

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

E.4.2.7.1 Overview

Media Oriented Negotiation Acceleration (MONA), as specified in ITU-T H.324 [81] provides simplified procedures that allow for a faster call set-up of a H.324 Multimedia call than standard H.324 procedures, and also allow for a fallback to standard H.324 procedures if either party does not support the enhanced procedures.

The support of MONA is optional for an IM-MGW and MGCF supporting multimedia interworking, as no call failure but only a fallback to standard H.324 setup procedures will occur if the procedures are not supported.

MONA "preference message" signalling is used instead of H.324 Multiplexing level negotiation. Should standard H.324 Multiplexing level stuffing flags be received, a fallback to standard H.324 procedures is triggered. The sending of MONA preference messages is repeated by each MONA capable H.324 terminal until a reception is acknowledged by the peer. During this phase, two PDU types may optionally be attached by MONA terminals to these preference messages:

– Media Preconfigured Channel (MPC) PDUs: MONA defines a small number of preconfigured H.223 channels for the most widespread audio and video codecs (AMR, AMR-WB, H.264 MPEG4 and H.263). Media PDUs for these codecs may be attached to the MONA "preference message" during the call setup.

– Signalling Preconfigured Channel (SPC) PDUs: These PDUs are H.245 generic request messages with special parameters defined by MONA. These PDUs may also be attached to MONA preference messages.

According to MONA, each MONA capable terminal shall support at least one of these PDU types. The MONA capability of the IM-MGW can be audited by the MGCF.

The MONA preference message exchange in combination with attached MPC or SPC PDUs may result in the establishment of the desired media channels without further H.245 signalling. Otherwise, H.245 will be used after the MONA preference message exchange is acknowledged to negotiate media channels, but MONA defines some accelerated H.245 procedures (ACP) to speed up these H.245 procedures.

The design of Mn procedures to support MONA is guided by the following considerations:

– The H.245 handling should be performed in the MGCF to keep procedures aligned as far as possible with standard Mn procedures to support H.324 interworking.

– The MGCF should also control MONA preference message exchange procedures in order to maintain the agreed architectural work split between MGCF and IM-MGW in analogy to the H.245 handling.

– However, the IM-MGW needs to understand the MONA preference messages at least to a sufficient degree to de-encapsulate the possibly attached MPC and SBC PDUs.

– Furthermore, the frequent retransmissions of MONA preference messages required by MONA procedures are to be performed by the IM-MGW autonomously to avoid unnecessary load at the Mn interface and the MGCF.

– Furthermore, for resource reservation at the IM-MGW, it is assumed that the IM-MGW has knowledge about supported predefined Media Preconfigured Channel configurations as specified in clause K.9.2 of H.324 [81] in order to limit the amount of information transferred in the Mn interface when establishing MPC channels. The offered channel resources are reserved by the IM-MGW.

– In order to avoid increasing call establishment time when interworking with legacy terminals and at the same time avoid unnecessary load at the Mn interface, the MGCF may initiate both MONA and legacy H.245 procedures simultaneously in parallel. The MGCF shall in this case arm a legacy detection event with an embedded signal descriptor including the initial legacy H.245 message. The IM-MGW will only send the signal in case a fallback condition to legacy is detected.

E.4.2.7.2 Mn Interactions for MONA preference messages

NOTE 1: MONA preference messages are repeated several times. One repetition is shown for each such message, with the same signal number as the first message.

NOTE 2: The Context model in figure E.4.2.2.1 is assumed in this call flow.

Figure E.4.2.7.2.1: Mn signalling interactions for MONA preference messages

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 (signal 6). Furthermore, the MGCF shall request that MONA preferences negotiation is started, and shall provision the MONA preferences to be indicated by the IM-MGW. The MGCF shall encode the MONA preferences as described in clause K.6 of ITU-T H.324 [81]. The MGCF shall take the H.324 related capabilities of the IM-MGW into account in the MONA preferences. The MGCF can know these capabilities by configuration. The IM-MGW will only support symmetric codec usage. If several codec alternatives are offered for MPC, it is the responsibility of the MGCF to ensure that symmetric codecs are established by not selecting transmit codec until the receive channel has been opened by MPC media. The MGCF shall also request to be notified about the reception of the remote MONA preferences and about the completion of the MONA preference exchange, or an H.245 message on the H.223 control channel. The MGCF may also initiate standard H.245 signalling in parallel in order to minimize the time for a legacy interworking fallback. This is done by arming a legacy detection event including an embedded signal descriptor. The embedded signal is the initial H.245 message out signal (including H.245 TCS+MSD) to send in case fallback to legacy interworking is detected. The IM-MGW will only send the embedded signal in case it detects H.223 related indications of a legacy interworking as specified in clause K.7.1.2 in H.324 [81]. Upon receiving the legacy detected event, the MGCF continues with standard H.245 call setup procedures waiting for the reception of a remote H.245 TCS as well as acknowledgements on the sent H.245 TCS+MSD. If the MGCF indicates the capability to receive SPC PDUs within the MONA preferences, it shall also request to be notified about incoming SPC PDUs, as detailed in clause E.4.2.7.4. If the MGCF indicates the capability to receive any MPC PDUs within the MONA preferences, it shall also request to be notified about incoming MPC PDUs, as detailed in clause E.4.2.7.3.

The IM-MGW shall start sending MONA preference messages after receipt of the corresponding request from the MGCF and CS bearer establishment (Signal 8). The IM-MGW shall repeat sending those messages and increment the acknowledgment bits of sent MONA preference messages when receiving incoming MONA preference messages according to MONA procedures (signals 8, 10, 14).

After sending at least 10 MONA preference messages, while the IM-MGW continues to send and receive MONA preference messages, it shall attach MPC or SPC PDUs if requested to do so by the MGCF as described in clauses E.4.2.7.3 and E.4.2.7.4, respectively. If the IM-MGW receives preference messages with an attachment, it shall inspect the first octet of that attachment that will contain a MUX code according to table K.15 of ITU-T H.324 [81] that identifies the attached PDU as either a MPC PDU of one of the predefined channels or a SPC PDU. The IM-MGW shall handle the attached MPC or SPC PDUs as described in clause E.4.2.7.3 and E.4.2.7.4, respectively.

After sending at least 10 MONA preference messages, the IM-MGW should insert stuffing flags indicating the multiplexing level received from the MGCF between MONA preference messages as described in clause K.7.1.1 of ITU-T H.324 [81].

The IM-MGW shall notify the MGCF when receiving the first incoming MONA preference message (Signals 11 and 12) and forward the received information. Subsequent incoming MONA preference message will be identical apart from possible increments in the acknowledgement bits. The IM-MGW shall not notify the MGCF about these messages. Upon reception of the notification of a MONA preference message, the MGCF shall compare the received MONA preferences message with the preferences message it sent and react as described in clause 7.1 of ITU-T H.324 [81].

When receiving an incoming MONA preference message with acknowledgment bits 10 or receiving the first non-empty H.223 MUX PDU, the IM-MGW shall notify the MGCF about the completion of the MONA preference exchange procedure (signals 16 and 17). The notification shall be only triggered by the one of the two events which occurs first. If the IM-MGW is not configured to send and detect SPCs, the IM-MGW shall then stop sending MONA preference messages and, if a MONA preference message with acknowledgment bits 10 has been received, it shall also stop receiving MONA preference messages. If the IM-MGW is configured to send and detect SPCs, the IM-MGW shall continue sending and receiving MONA Preference messages encapsulating the SPC/MOS messages until the MGCF configures the IM-MGW stop sending and detecting SPCs.

NOTE: In the unlikely case that a first non-empty H.223 MUX-PDU is received before the first MONA Preference Message has been received, the IM-MGW will continue the detection of incoming MONA Preference Messages and will apply the Notify-MONA-preference-Reception-procedure after the Notify-MONA-preference-completed procedure.

After receiving the notification about the completion of the MONA preference exchange procedure, and a completion of the possible subsequent accelerated H.245 setup procedures, the MGCF shall configure the multiplexing termination T2 by indicating to the IM-MGW the contents of the incoming and outgoing multiplex tables (Signal 19), and may modify the selected codecs at both the MUX and the IMS side (signal 21).

E.4.2.7.3 Mn Interactions for MONA MPCs

NOTE 1: MONA preference messages are repeated several times. One repetition is shown for each such message, with the same signal number as the first message.

NOTE 2: The Context model in figure E.4.2.2.1 is assumed in this call flow.

Figure E.4.2.7.3.1: Mn interactions for MONA MPCs

If the MGCF indicates the ability to receive any predefined MPCs channel types in the MONA preferences messages, the MGCF shall request the IM-MGW to report the channel type of received MPC PDUs (Signal 1 in figure E.4.2.7.3.1, Event "MPC Reception").

If the MGCF intends to use MPCs for sending media during the MONA setup, the MGCF shall request the IM-MGW to send available media encoded according to the media predefined channel types defined by MONA (signal 1 in figure E.4.2.7.3.1, Signal "MPC MUX Code" ) while the MONA preference exchange described in clause E.4.2.7.2 is ongoing. The MGCF should select channel types for codecs which are supported by both the IMS side and the IM-MGW, and/or for codecs which could be transcoded by the IM-MGW from the codecs supported by the IMS side. The MGCF shall only include one channel type per each desired media type (audio, video) in the "MPC MUX Code" signal. The MGCF may also configure the MGW to receive these channels at the same time by supplying the Incoming Multiplex Table IE in the Add Multiplex Termination Procedure.

Upon reception of this request, the IM-MGW shall forward any media received from the IMS side in MPC PDUs of the corresponding predefined channel type attached to MONA preference messages, transcoding the media if required (Signal 4).

According to the procedures in clause E.4.2.7.2, the IM-MGW will notify the MGCF (signal 6) when receiving the first incoming MONA preference message (Signals 5). The MGCF shall then analyse the Media Preconfigured Channel Receive bits within the MONA preference message and configure the MGW accordingly:

– If MPC(s) that the MGCF has previously configured within the "MPC MUX Code" signal are supported according to these bits, the MGCF shall configure the IM-MGW to send these MPC using the predefined channels (signal 10) by supplying the corresponding Outgoing Multiplex Table in the Configure Multiplex Termination procedure (signal 8).

– If some of the MPC(s) that the MGCF has previously configured within the "MPC MUX Code" signal are not supported according to these bits, the MGCF shall configure the IM-MGW to terminate the media on those MPC(s) by modifying the MPC Mux Code signal using the Configure Multiplex Termination procedure. In addition, the MGCF may configure the IM-MGW to send media on other supported MPC(s) by supplying the corresponding Outgoing Multiplex Table in the Configure Multiplex Termination procedure (not shown in figure E.4.2.7.3.1).

– If all of the MPC(s) that the MGCF has previously configured within the "MPC MUX Code" signal are not supported according to these bits, the MGCF shall configure the IM-MGW to terminate MPC operations in those unsupported channels. The MGCF shall either remove the MPC Mux Code signal using the Stop MPC procedure or configure the IM-MGW to send media on other supported MPC(s) by supplying the corresponding Outgoing Multiplex Table in the Configure Multiplex Termination procedure (not shown in figure E.4.2.7.3.1).

When the MGCF provides an Outgoing Multiplex Table, the IM-MGW shall terminate sending any MPC PDUs as attachments to MONA preference message.

NOTE: Any previously provisioned "MPC MUX Code" signal becomes irrelevant.

When being notified about the receipt of the first incoming MONA preference message, the MGCF may also analyse the Media Preconfigured Channel Receive bits within the MONA preference message and configure the MGW with a suitable Incoming Multiplex Table using the Configure Multiplex Termination procedure (not shown in figure E.4.2.7.3.1).

If the IM-MGW receives the first MONA preference message with attached MPC PDU of a given predefined channel type (signal 12), or if the IM-MGW receives the first non-empty H.223 MUX PDU of a given predefined channel type which has been configured with the Incoming Multiplex Table, and the MGCF has requested a notification about such an event, the IM-MGW shall notify the MGCF about the received channel type (signal 13). The IM-MGW shall not notify the MGCF about subsequent receptions of MPC PDUs of the same channel type.

Upon reception of such a notification, if the IM-MGW supports the indicated channel type and has not yet been configured to receive media of that channel type, and if the MGCF has previously indicated the capability to receive MPCs of that channel type within MONA preference messages, the MGCF shall configure the IM-MGW to receive media of that channel type and forward them to the IMS side by supplying the Incoming Multiplex Table IE in the Configure Multiplex Termination procedure (Signal 15).

If the MGCF determines (e.g. after receipt of signal 5 or signal 12) that some desired media channels can not be established using the MPC procedures, the MGCF should use accelerated H.245 procedures as defined by MONA to set up media channels. Corresponding H.245 messages shall be transported transparently between the IM-MGW and the MGCF using the "Signal H.245 message" and "Notify H.245 message" procedures.

The MGCF can receive a forwarded MONA preference message from the IM-MGW when the MGCF has already configured the IM-MGW to send media on an MPC. If the processing of this message in the MGCF leads to the conclusion that MPC procedures need to be aborted because SPC preferred (SPP) is determined, the MGCF shall configure the IM-MGW to terminate MPC operations by removing the MPC related event MPC-reception and MPC Mux Code signal using the Stop MPC procedure.

E.4.2.7.4 Mn Interactions for MONA SPCs

E.4.2.7.4.1 General

H.245 PDUs for SPC shall be transported between the IM-MGW and MGCF over the Mn interface using H.248 Events (from the IM‑MGW towards the MGCF) and H.248 Signals (from the MGCF towards the IM-MGW). The Events/Signals shall contain the following information:

– H.245 message (binary).

The related procedures are distinct from the procedures in clause E.4.2.3, since the PDUs are received or sent by the IM-MGW using the SPC, i.e. as attachment to MONA preference messages.

If the MGCF supports SPCs, it shall comply with the SPC procedures in clause K.8 of ITU-T H.324 [81]. The repetition of sending the same SPCs will be handled by the IM-MGW. When the MGCF receives an acknowledgement from the CS side it shall request the IM-MGW to stop the repetition sending of the SPCs,

Within the sent SPC PDUs, the MGCF 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.

E.4.2.7.4.2 Transport from MGCF to IM-MGW

NOTE 1: MONA preference messages are repeated several times. One repetition is shown for each such message, with the same signal number as the first message.

NOTE 2: The Context model in figure E.4.2.2.1 is assumed in this call flow.

Figure E.4.2.7.4.2.1: Mn interactions for sending MONA SPCs

In Signal 1, the MGCF requests the IM-MGW to send an H.245 message to the CS side. To request the IM-MGW to send a H.245 message to the CS side, the MGCF shall sent an H.248 signal to the IM-MGW with the complete H.245 message content.

Upon reception of this signal, the IM-MGW shall send the encapsulated H.245 message within the H.248 signal, as attachment to a MONA preference message as described in clause K.9.4 of ITU-T H.324 [81] (signal 3). It should repeat sending this H.245 message as attachment to subsequent MONA preference messages.

E.4.2.7.4.3 Transport from IM-MGW to MGCF

NOTE 1: MONA preference messages are repeated several times. One repetition is shown for each such message, with the same signal number as the first message.

NOTE 2: The Context model in figure E.4.2.2.1 is assumed in this call flow.

Figure E.4.2.7.4.3.1: Mn interactions for receiving MONA SPCs

In signal 1, the MGCF requests the IM-MGW to detect received H.245 message from the CS side in SPC PDUs attached to MONA preference messages and forward them to the MGCF. To request the IM-MGW to detect and forward these H.245 message, the MGCF shall send a suitable H.248 event to the IM-MGW. The event may be indicated through an H.248 ADD command.

In signal 3, the IM-MGW receives an H.245 message from the CS side attached as SPC PDU to a MONA preference message. Upon reception of such an H.245 message from the CS side, the IM-MGW may check, based on bitwise comparison of the previously received H.245 message, if it has already forwarded the same H.245 message to the MGCF, in which case the IM-MGW may choose not to forward the same H.245 message to the MGCF. Otherwise the IM-MGW shall forward the H.245 message to the MGCF within an H.248 Notify command (signal 4).

NOTE: According to H.324 [81] a MOS request Ack message shall be sent to every received MOS request. If the IM-MGW chooses, based on the bitwise comparison, not to forward the received H.245 message to the MGCF, no MOS request Ack message will be generated. However, the MGCF will request the IM-MGW to automatically retransmit the MOS request Ack message generated by the MGCF itself.

If the IM-MGW does not support forwarding SPC PDUs or has not been requested by the MGCF to forward these PDUs, it shall discard received SPC PDUs.

E.4.2.7.4.4 Termination of SPC procedure

NOTE 2: The Context model in figure E.4.2.2.1 is assumed in this call flow.

Figure E.4.2.7.4.4.1: Mn interactions for terminating SPC procedure

If the processing of an incoming MONA Preference Message in the MGCF leads to the conclusion that SPC procedures need to be aborted as a result of the test of the SPC bits, the MGCF shall configure the IM-MGW to terminate SPC operations by removing the SPC related event SPC-reception and SPCout signal using the Stop SPC procedure. In this case, H.245 signalling may be started simultaneously.

If the MGCF determines the completion of the SPC procedures according to clause K.8 of ITU-T H.324 [81] (based on incoming signal 1 in figure E.4.2.7.4.4.1), the MGCF shall configure the IM-MGW to stop sending and detecting SPCs (signal 4 in figure E.4.2.7.4.4.1).

E.4.2.7.5 Mn Interactions for fallback from MONA procedures to standard H.324 setup

NOTE 1: MONA preference messages are repeated several times. One repetition is shown for each such message, with the same signal number as the first message.

NOTE 2: The Context model in figure E.4.2.2.1 is assumed in this call flow.

Figure E.4.2.7.5.1: Mn signalling interactions for fallback from MONA procedures to standard H.324 setup

When the MGCF requests that the MONA preferences negotiation is started, the MGCF may also initiate standard H.245 signalling towards the IM-MGW that shall only be sent in case the IM-MGW detects legacy interworking. The MGCF arms an event to detect legacy interworking with an embedded signal descriptor including an H.245 message out signal. The embedded H.245 signal is the initial H.245 TCS+MSD signal to send in case fallback to legacy interworking is detected. The MGCF shall also request to be notified about a H.245 message on the H.223 control channel. The MGCF shall also provision a multiplexing level which will be advertised by the IM-MGW (Signal 6).

If the IM-MGW detects a legacy interworking condition (signal 9), it shall stop sending MONA preference messages, including those MONA messages used to encapsulate MPC or SPC. The IM-MGW shall engage in normal H.324 multiplexing level negotiations. In case the MGCF armed a legacy detection event, the embedded H.245 signal shall be sent by the IM-MGW (signal 10). The legacy detection event is sent to the MGCF (signal 11).

The MGCF shall upon detection of legacy interworking stop MONA procedures and continue with standard H.245 call set up procedures, as depicted in figure E.4.2.4.1 starting with step 9.

If the IM-MGW receives a normal H.245 message (not depicted), it shall also forward this message to the MGCF. If the MGCF receives such a H.245 message during the MONA call setup, and this H.245 message is a normal Terminal Capability Set message, the MGCF shall also stop MONA procedures and continue with standard H.245 call set up procedures, as depicted in figure E.4.2.4.1 starting with step 9.

To stop MONA related procedures at the IM-MGW, the MGCF shall remove the MONA related signals that were active (MONA Preferences, SPCout, MPC Mux Code; note that MONA Preference and SPCout causes the MGW to send the MONA Preference message or the contained H.245 message several times) and events (MonaPreferenceReception, MonaPreferenceCompleted, SPC-reception and MPC-reception, using the Stop MONA Negotiation procedure.