6.11 Inter-MSC Handover during TrFO

23.1533GPPOut of band transcoder controlRelease 17Stage 2TS

6.11.1 Inter-MSC Handover

If MSC-A supports GERAN AoIP-mode then it may insert AoIP-Supported Codecs List (Anchor) (MAP) in the MAP Prepare Handover Request message to allow MSC-A’ to use this information if target radio access is GERAN AoIP-mode, or during a subsequent intra-MSC handover to GERAN AoIP-mode. The detailed procedure is described in clause 6.14.1.

In order to enable the use of Tandem free and Transcoder free operation after inter-MSC handover, the procedures specified in 3GPP TS 23.205 [8] and 3GPP TS 23.009 [11] for "Inter-MSC Handover" shall be followed. For the handling of the codec lists and selected codecs the following rules apply:

The Prepare Handover request message shall include the Iu-Supported Codecs List (MAP).

NOTE 1: The codec types listed in the Channel Type Information Element included in the BSSMAP HANDOVER REQUEST message in the MAP Prepare Handover request are the full list of codec types supported by the UE (as recognized by the Anchor MSC-A); this list is not affected by the Available Codecs List (BICC).

If the serving radio access is UTRAN or GERAN Iu-mode, the Prepare Handover request message shall contain the Iu-Currently Used Codec (MAP). Otherwise, if the serving radio access is A/Gb mode, the currently used codec is indicated by the Speech Version (Chosen) in the BSSMAP Handover Request message included in the Prepare Handover request message.

If the target radio access is UTRAN or GERAN Iu-mode, the Prepare Handover response message shall contain the Iu-Selected Codec (MAP) and the Iu-Available Codecs List (MAP). Otherwise, if the target radio access is A/Gb mode, the selected codec is indicated by the Speech Version (Chosen) in the BSSMAP Handover Request Ack message included in the Prepare Handover response message.

If the target radio access is UTRAN or GERAN Iu-mode, then for a speech bearer, the MSC-A server shall perform a call set-up with codec negotiation towards the MSC-A’ server, using a Supported Codecs List (BICC) as specified in clause 6.2.2. When MSC-A’ receives a Supported Codecs List (BICC) with the IAM message, it shall follow the procedures specified in clause 6.2.2.

If the target radio access is GERAN A/Gb mode, then for a speech bearer the anchor MSC shall perform a call set-up with codec negotiation towards the target MSC, using a Supported Codec List (BICC) ordered as:

a) optionally, the GSM codec indicated by the serving MSC during the MAP E-interface signalling as Speech Version (Chosen), if it is different from list item b and if supported on Nb bearer;

NOTE 2: This codec can be included as the preferred codec, if MSC-A knows by means of configuration information that all nodes of the network support TrFO/TFO interworking and TFO, including codec mismatch resolution. If this codec does not match the Selected Codec (BICC), but is eventually selected for the network side bearer to the target MSC (MSC-A’), then either MSC-A must transcode at the anchor MGW or MSC-A will need to trigger a codec modification to the far end. Given that the anchor MSC cannot trigger all these changes at once, it must first establish the network side bearer – i.e. insert a transcoder at the anchor MGW and then trigger the codec modification to the far end as a second step.

b) the Selected codec (BICC), previously selected for the leg towards the far end party;

NOTE 3: This codec is preferred, if the anchor MSC-A does not know by means of configuration information if all nodes in the network support OoBTC, or TrFO/TFO interworking and TFO, including codec mismatch resolution. This is also the best codec to be selected if the goal is to avoid additional transcoding in MSC-A or a codec modification from MSC-A towards the far end party.

c) the default PCM codec;

d) optionally, further GSM codecs that are supported by its associated MGW, e.g. the GSM codec indicated by the serving MSC during the MAP E-interface signalling as Speech Version (Chosen), if it is not already included according to list item a or b and if supported on Nb bearer; and

e) optionally, for subsequent inter-RAT intra MSC-A’ handover further codecs from the Available Codec List (BICC) that are applicable to other radio access types.

For UDI/RDI multimedia calls with fallback and service change according to 3GPP TS 23.172 [17], the Supported Codecs List (BICC) shall contain the multimedia dummy codec and the Available Codecs List (BICC) can contain this codec (see [17], clause 4.3.7). If the MSC-A server wants to establish a bearer for the multimedia dummy codec, it shall include this codec as the preferred codec.

If the target radio access is GERAN A/Gb mode and MSC-A’ receives a Supported Codec List (BICC) with the IAM message, MSC-A’ shall select a codec from this list, taking the Speech Version (Chosen) and the priorities expressed by the order of the codecs in the Supported Codec List (BICC) into account.

NOTE 4: Usually, selection of the preferred codec of the Supported Codec List (BICC) will result in the best speech quality or best transcoder location or both; however, e.g. if MSC-A’ knows by means of configuration information that OoBTC is not supported on some links in the network, and some nodes in the network are not supporting the creation of a "structured" Supported Codec List (BICC) as specified in clause 9.7.2, MSC-A’ can select another codec, e.g. the codec corresponding to the Speech Version (Chosen) or the default PCM codec.

NOTE 5: If the procedure specified in this clause is followed for AoIP (e.g. if the AoIP MAP codec negotiation is not supported by MSC-A or MSC-A’), the selection of the Speech Version (Chosen) when not the Selected Codec (BICC) will result in transcoding in the anchor MSC or codec modification to the far end. If the MSC-A’ selects the preferred codec in the Supported Codec List (BICC) above and this codec is not equal to the Speech Version (Chosen), this will result in transcoding in the non-anchor MGW.

If the Selected Codec (BICC) received by MSC-A in response to the BICC codec negotiation towards the target MSC (MSC-A’) is different from the current Selected Codec (BICC) used towards the far end, the MSC-A shall insert a transcoder in MGW-A.

6.11.2 Codec Modification/Mid-Call Codec Negotiation after Inter-MSC Handover

6.11.2.1 Codec Modification/Mid-Call Codec Negotiation Initiated by the Far End Side

If the serving radio access after inter-MSC handover is GERAN A/Gb mode, and the anchor MSC (MSC-S-A) receives a "Modification of Selected Codec" procedure or a "Mid-Call Codec Negotiation" procedure from the far end side the MAP signalling between the anchor MSC (MSC-S-A) and the serving MSC (MSC-S-A’) shall be performed only, if the old or the new Selected Codec (BICC) is the multimedia dummy codec.

If both the old and the new Selected Codec (BICC) are speech codecs, the anchor MSC may terminate the codec modification or mid-call negotiation procedure, inserting a transcoder if required. Alternatively, the anchor MSC may forward the request to the serving MSC (MSC-S-A’), using the procedures as described in clause 5.8.

NOTE: The anchor MSC may decide to forward the request to the serving MSC (MSC-S-A’), e.g. when it is possible to (re-)establish Tandem free and Transcoder free operation end-to-end from the far end side up to the serving TRAU.

If 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 or mid-call negotiation for the Nb/Nc interface towards the serving MSC (MSC-S-A’), using the procedures as described in clause 5.8. If the service change between speech and multimedia cannot be performed successfully, the anchor MSC shall reject the request for codec modification or mid-call negotiation towards the far end party.

6.11.2.2 TFO Codec Mismatch Resolution in the Serving MSC

If the serving radio access after inter-MSC handover is GERAN A/Gb mode, and TrFO has been established between the anchor MSC and the serving MSC, the serving MSC may detect a TFO codec mismatch between the Selected Codec (BICC) used on the TrFO link and the GSM speech codec chosen by the serving BSC.

If the serving MSC supports the codec mismatch resolution procedure (see 3GPP TS 28.062 [10], clause 6.3) and wants to change the Selected Codec (BICC) due to this procedure, the serving MSC shall initiate a codec modification or mid-call codec negotiation procedure towards the anchor MSC as described in clauses 5.8.1, 5.8.2 and 5.8.3.

In the event of a collision of codec modification/mid-call codec negotiation procedures initiated by the anchor MSC and the serving MSC, the procedures described in Q.1902.4, clause 10.4.7.5 [6] shall apply, with the following modification of the first sentence in clause 10.4.7.5 [6], list item 2:

Codec modification/mid-call codec negotiation requests initiated in the direction towards the serving MSC shall take precedence over codec modification/mid-call codec negotiation requests initiated in the direction towards the anchor MSC.

6.11.2.3 Modification Procedure after Codec Change in the Serving MSC

The procedures as specified in clause 6.2.3.3 apply.