6.2.3 Codec Modification/ Mid-Call Codec Negotiation after Inter-MSC Relocation

23.1533GPPOut of band transcoder controlRelease 17Stage 2TS

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

6.2.3.1 Codec Modification Initiated by the Far End Side

Modification of Available Codec List

If after inter-MSC SRNS relocation the anchor MSC (MSC-S-A) receives a "Modification of Available Codec List" procedure from the far end side as described in clause 5.8.2, i.e. the Available Codecs List (BICC) is reduced, the anchor MSC may terminate the procedure at that point or forward the "Modification of Available Codec List" procedure to the serving MSC (MSC-S-A’). I.e. for a modification of the Available Codec List (BICC) without modification of the Selected codec, no MAP signalling is used.

Modification of Selected Codec

If after inter-MSC SRNS relocation the anchor MSC (MSC-S-A) receives a "Modification of Selected Codec" procedure from the far end side as described in clause 5.8.1, and both the old and the new Selected Codec (BICC) are speech codecs, the anchor MSC may terminate the codec modification procedure, inserting a transcoder if required. Alternatively, the anchor MSC may forward the request to modify the codec to the serving MSC (MSC-S-A’), using the procedure described below.

NOTE: The anchor MSC may decide to forward the request to the serving MSC (MSC-S-A’), e.g. when the new Selected Codec (BICC) for the call leg between the anchor MSC and the far end side was included in the Iu-Available Codec List previously received from the serving MSC, and it is possible to (re-)establish TrFO end-to-end from the far end side up to the serving MSC.

If the anchor MSC (MSC-S-A) receives a "Modification of Selected Codec" procedure from the far end side as described in clause 5.8.1, and either the old or the new Selected Codec (BICC) is the multimedia dummy codec, i.e. the far end side requests a service change between speech and multimedia, and the Available Codecs List (BICC) previously negotiated between the anchor MSC and the serving MSC (MSC-S-A’) indicates that the service change is supported end-to-end, the anchor MSC shall forward the request to modify the radio access bearer to the serving MSC (MSC-S-A’) and then perform a codec modification procedure for the Nb/Nc interface towards the serving MSC (MSC-S-A’). If the service change cannot be performed successfully, the anchor MSC shall reject the request for codec modification towards the far end party.

An example call sequence for the modification of the selected speech codec is shown in figures 6.2/7 and 6.2/8. The configuration depicted in figure 6.2/4 applies.

Figure 6.2/7: Codec modification after Inter-MSC SRNS Relocation. Flow Chart Part 1.

Note: There can be interim network transit nodes between MSC-A and MSC-A’

Figure 6.2/8: Codec modification after Inter-MSC SRNS Relocation. Flow Chart Part 2.

If the anchor MSC (MSC-S-A) wants to forward the modification of the codec used towards the UE and the serving MSC (MSC-S-A’) from one speech codec to another speech codec within the Iu-Available Codecs List, it shall apply the following procedure:

The anchor MSC shall send a MAP Forward Access Signalling request (3) containing the new Iu-Selected Codec and the corresponding RAB Assign Modify Request message to the serving MSC (MSC-S-A’).

On reception of the MAP Forward Access Signalling request (3) the serving MSC (MSC-S-A’) shall configure the attached MGW-A’ for the new codec and trigger the "RAB Assign Modify" procedure (5-8) towards the RNC-A’. When the serving MSC receives the RAB Assign Modify Response message (8), it shall send a MAP Process Access Signalling Request (9) containing the RAB Assign Modify Response and the Iu-Selected codec to the anchor MSC (MSC-S-A).

When the anchor MSC (MSC-S-A) receives the MAP Process Access Signalling Request (9), it shall start the codec modification procedure (11-19) for the Nb/Nc interface towards the serving MSC (MSC-S-A’), as described in clause 5.8.1. If the anchor MSC needs to change also the Available Codecs List (BICC), it shall additionally initiate a procedure as described in clause 5.8.2.

When receiving the "Modify Codec" request (11), the serving MSC (MSC-S-A’) shall not reconfigure the RAN and shall configure the attached MGW-A’ to initate an Nb framing protocol initiation towards MGW-A.

If the anchor MSC (MSC-S-A) needs to perform a service change from multimedia to speech, it shall send a MAP Forward Access Signalling request (3) containing the Iu-Supported Codecs List and the corresponding RAB Assign Modify Request message to the serving MSC (MSC-S-A’). After successful modification of the RAB, on reception of the MAP Process Access Signalling Request (9) the anchor MSC (MSC-S-A) shall start the codec modification procedure (11-19) for the Nb/Nc interface towards the serving MSC (MSC-S-A’), as described in clause 5.8.1.

If the anchor MSC (MSC-S-A) needs to perform a service change from speech to multimedia, it shall send a MAP Forward Access Signalling request (3) containing the corresponding RAB Assign Modify Request message for a data bearer, but no Iu-Selected Codec to the serving MSC (MSC-S-A’). After successful modification of the RAB, on reception of the MAP Process Access Signalling Request (9) the anchor MSC (MSC-S-A) shall start the codec modification procedure (11-19) for the Nb/Nc interface towards the serving MSC (MSC-S-A’), as described in clause 5.8.1.

Unsuccessful Codec Modification in the Serving MSC

After receipt of MAP Forward Access Signalling request (3), if the modification to the new Iu-Selected codec is not possible, e.g. because necessary resources are temporarily unavailable in the serving cell or in MGW-A’, or the RAN does not support the "RAB Assign Modify" procedure, the serving MSC (MSC-S-A’) shall keep the old codec and the corresponding RAB configuration and shall send a MAP Process Access Signalling request, containing a RAB Assign Modify Response ("failed to modify") message, to the anchor MSC (MSC-S-A). If the anchor MSC detects that the RAB modification failed, it shall abort the codec modification procedure towards the serving MSC and shall complete the codec modification procedure towards the far end side.

Unsuccessful BICC Codec Modification between Anchor MSC and Serving MSC

After receipt of a MAP Process Access Signalling Request, containing a RAB Assign Modify Response ("success") message, if the subsequent BICC codec modification procedure between anchor MSC and serving MSC fails due to a MGW rejecting a request to reserve the resources or a server rejecting the request to modify the codec, the anchor MSC shall change the codec used at the Iu interface back by sending a MAP Forward Access Signalling request containing the previous Iu-Selected Codec to the serving MSC (MSC-S-A’). After receipt of the confirmation that the previous codec has been restored at the Iu interface, the anchor MSC shall complete the codec modification procedure towards the far end side.

6.2.3.2 Mid-Call Codec Negotiation Initiated by the Far End Side

If after inter-MSC SRNS relocation the anchor MSC receives a "Mid-call Codec Negotiation" procedure from the far end side as described in clause 5.8.3, and both the old and the new Selected Codec (BICC) are speech codecs, the anchor MSC may terminate the mid-call codec negotiation procedure, inserting a transcoder if required. Alternatively, if the new Selected Codec (BICC) was included in the last Iu-Available Codec List sent by the serving MSC (MSC-S-A’) the anchor MSC may forward the request to the serving MSC (MSC-S-A’), using the procedure described below.

If the anchor MSC (MSC-S-A) receives a "Mid-call Codec Negotiation" procedure from the far end side as described in clause 5.8.3, and either the old or the new Selected Codec (BICC) is the multimedia dummy codec, i.e. the far end side requests a service change between speech and multimedia, and the Available Codecs List (BICC) previously negotiated between the anchor MSC and the serving MSC (MSC-S-A’) indicates that the service change is supported end-to-end, the anchor MSC shall forward the request to modify the radio access bearer to the serving MSC (MSC-S-A’) and then perform a mid-call codec negotiation procedure for the Nb/Nc interface towards the serving MSC (MSC-S-A’). If the service change between speech and multimedia cannot be performed successfully, the anchor MSC shall reject the request for mid-call codec negotiation towards the far end party.

An example call sequence for the mid-call codec negotiation of speech codecs is shown in figures 6.2/9 and 6.2/10. The configuration depicted in figure 6.2/4 applies.

Figure 6.2/9: Mid-call codec negotiation after Inter-MSC SRNS Relocation. Flow Chart Part 1.

Note: There can be interim network transit nodes between MSC-A and MSC-A’

Figure 6.2/10: Mid-call codec negotation after Inter-MSC SRNS Relocation. Flow Chart Part 2.

If the anchor MSC (MSC-S-A) wants to forward the (re-)negotiation of the selected codec and the Available Codecs List (BICC) towards the UE and the serving MSC (MSC-S-A’), it shall apply the following procedure:

The anchor MSC shall send a MAP Forward Access Signalling request (2) containing the new Iu-Supported Codecs List and the corresponding RAB Assign Modify Request to the current serving MSC (MSC-S-A’). When selecting the order of priority for the codecs within the new Iu-Supported Codecs List, the anchor MSC shall take the new Available Codecs List (BICC) negotiated with the far end party into account.

On reception of the MAP Forward Access Signalling request (2) the serving MSC (MSC-S-A’) shall select a codec from the Iu-Supported Codecs List, configure the attached MGW-A’ for the new codec, and trigger the "RAB Assign Modify" procedure (4-7) towards the RNC-A’. For details concerning the handling of the RAB Assign Modify Request message by MSC-S-A’ see 3GPP TS 23.009 [11], clause 13.4.1. When the serving MSC receives the RAB Assign Modify Response message (7), it shall send a MAP Process Access Signalling Request (10) containing the RAB Assign Modify Response, the Iu-Selected codec, and the Iu-Available Codecs List to the anchor MSC (MSC-S-A).

When the anchor MSC (MSC-S-A) receives the MAP Process Access Signalling Request (8), it shall start the mid-call codec negotiation procedure (9-25) for the Nb/Nc interface towards the serving MSC (MSC-S-A’), as described in clause 5.8.

When receiving the "Mid-call codec negotiation" procedure (9), the serving MSC (MSC-S-A’) shall not reconfigure the RAN and shall configure the attached MGW-A’ to wait for an Nb framing protocol initiation from MGW-A.

Unsuccessful Codec Modification in the Serving MSC

After receipt of MAP Forward Access Signalling request (3), if the modification to the new Iu-Selected codec is not possible, e.g. because necessary resources are temporarily unavailable in the serving cell or in MGW-A’, or the RAN does not support the "RAB Assign Modify" procedure, the serving MSC (MSC-S-A’) shall keep the old codec and the corresponding RAB configuration and shall send a MAP Process Access Signalling request, containing a RAB Assign Modify Response ("failed to modify") message, to the anchor MSC (MSC-S-A). If the anchor MSC detects that the RAB modification failed, it shall abort the mid-call codec negotiation procedure towards the serving MSC and complete the mid-call codec negotiation procedure towards the far end side.

Unsuccessful BICC Mid-call Codec Negotiation between Anchor MSC and Serving MSC

If after a successful modification of the Iu-Selected Codec (MAP) the subsequent BICC codec mid-call codec negotiation procedure between anchor MSC and serving MSC fails due to a MGW rejecting a request to reserve the resources or a server rejecting the request to (re-)negotiate the codecs, the anchor MSC shall change the codec used at the Iu interface back by sending a MAP Forward Access Signalling request containing the previous Iu-Selected Codec to the serving MSC (MSC-S-A’). After receipt of the confirmation that the previous codec has been restored at the Iu interface, the anchor MSC shall complete the mid-call codec negotiation procedure towards the far end side.

6.2.3.3 Modification Procedure after Codec Change in the Serving MSC

According to 3GPP TS 23.009 [11], clause 4.4.1, the serving MSC (MSC-S-A’) will inform the anchor MSC (MSC-S-A) when the Iu-Selected codec was changed during a subsequent intra-MSC handover/relocation by sending a MAP Process Access Signalling request. If the Iu-Available Codecs List was changed during the handover/relocation, the serving MSC shall send a MAP Process Access Signalling request including the new Iu-Available Codecs List.

On reception of the MAP Process Access Signalling request the anchor MSC may initiate one of the modification procedures as described in clauses 5.8.1, 5.8.2, and 5.8.3 towards the serving MSC and/or towards the far end side. I.e. towards the serving MSC no MAP signalling is used. Besides, towards the serving MSC (MSC-A’) the procedures described in clauses 5.8.1, 5.8.2, and 5.8.3 are applicable with the modification that the serving MSC shall not modify the radio access bearer.