6.14.1 Inter-MSC Handover in GERAN AoIP mode

23.1533GPPOut of band transcoder controlRelease 17Stage 2TS

The following figures are describing inter-MSC handover in GERAN AoIP mode.

Figure 6.14/1: Configuration during inter-MSC Handover in GERAN AoIP mode

Figure 6.14/1 shows the configuration during inter-MSC Handover in GERAN AoIP mode. After setting up the new AoIP interface (towards BSC-A’) until releasing the old one, the original TrFO relation (A⇔B) and the target TrFO relation (A’⇔B) exist in parallel. Within the respective contexts interworking between T4 and T5 at MGW-A’ and T1, T2 and T3 at MGW-A are necessary:

T3 (MGW-A) shall perform initialisation towards MGW-A’.

Figure 6.14/2: Inter-MSC Handover in GERAN AoIP mode and TrFO. Flow chart part 1

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

Figure 6.14/3: Inter-MSC Handover in GERAN AoIP mode and TrFO. Flow chart part 2

MAP signalling for handover and codec negotiation

The MSC-A server may include an AoIP-Supported Codecs List (Anchor) (MAP) in the MAP Prepare Handover request if MSC-A supports GERAN AoIP mode. Only GSM codecs supported both by the UE and by the anchor MSC may be included in this list. The codec capabilities of the serving radio access, i.e. the radio access used prior to the inter-MSC handover or relocation, need not be taken into account.

The anchor MSC-A may decide to include in the AoIP-Supported Codecs List (Anchor) (MAP) only codecs that are also included in the Available Codecs List (BICC) or that are TrFO compatible to codecs in this list.

NOTE 1: The codec types listed in the Channel Type Information Element are the full list of codec types supported by the UE (as recognised by the Anchor MSC); this list is not affected by the Available Codecs List (BICC) or any resultant AoIP-Supported Codecs List (Anchor) (MAP) IE.

NOTE 2: If the Available Codecs List (BICC) is kept up-to-date by the far end side by means of modifications of the Available Codecs List (BICC), and the target BSC selects one of the codecs from this list, it may be possible to achieve TrFO after the inter-MSC handover by performing a codec modification to the far end instead of a mid-call codec negotiation.

When selecting the order of priority for the codecs within the AoIP-Supported Codecs List (Anchor) (MAP), MSC-A shall take the Selected Codec (BICC) and the Available Codecs List (BICC) negotiated with the far end party into account.

– If it is desired to achieve immediate TrFO upon completion of the inter-MSC handover, then the MSC-A server shall include the codec type and configuration of the Selected Codec (BICC) negotiated with the far end or a TrFO compatible codec type and configuration as preferred codec in the AoIP-Supported Codecs List (Anchor) (MAP).

– If the anchor MSC-A prefers a codec different from the Selected Codec (BICC), then the MSC-A server shall include this other codec as the preferred codec in the AoIP-Supported-Codecs List (Anchor) (MAP). If the preferred codec is selected by the target BSC, transcoding will be required after the inter-MSC handover. The location of the transcoder will be determined during the network side codec negotiation.

NOTE 3: The anchor MSC-A can select the second alternative e.g. if a subsequent codec modification or mid-call codec negotiation towards the far end is intended.

If the target MSC-A’ receives the AoIP-Supported Codecs List (Anchor) (MAP) and supports AoIP codec selection based on this list, it shall remove from the AoIP-Supported Codecs List (Anchor) (MAP) and from the BSSMAP Channel Type IE any codecs not supported by its own MGW-A’ or by local policy. MSC-A’ shall include the remaining codecs in the MSC Preferred Codec List (BSSMAP).

MSC-A’ shall ensure consistency between the contents of the MSC Preferred Codec List (BSSMAP) and the BSSMAP Channel Type IE sent to the target BSS (see 3GPP TS 48.008 [15], clause 3.2.1.8). To this purpose MSC-A’ may add codecs to the MSC Preferred Codec List (BSSMAP), if the codec type is also included in the Channel Type IE. For the added codecs MSC-A’ shall select a codec configuration supported by MGW-A’ and according to preferences configured locally in the target MSC-A’. MSC-A’ shall remove any codec types from the BSSMAP Channel Type IE that are not included in the resultant MSC Preferred Codec List (BSSMAP).

If MSC-A does not include an AoIP-Supported Codecs List (Anchor) (MAP) in the MAP Prepare Handover request then MSC-A’ shall proceed with the handover as described in clause 6.11.1.

MSC-A’ shall include in the MAP Prepare Handover response the AoIP-Selected codec (Target) (MAP) and the AoIP-Available Codecs List (MAP), i.e. the BSS Supported Codecs list returned by BSS-A’, if:

– the target access is GERAN AoIP mode;

– the target MSC received the AoIP-Supported Codecs List (Anchor) (MAP) in MAP Prepare Handover Request; and

– the target MSC supports AoIP codec selection based on the received AoIP-Supported Codecs List (Anchor) (MAP).

If MSC-A’ does not include the AoIP-Selected Codec (Target) (MAP) and the AoIP-Available Codecs List (MAP) IEs in the MAP Prepare Handover response, both MSC-A and MSC-A’ shall proceed as described in clause 6.11.1.

Network side bearer establishment and codec handling

The handling of the bearer establishment between MSC-A and MSC-A’ shall be performed as for a normal call with OoBTC. 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 Codec List (BICC) containing:

a) optionally, the AoIP-Selected Codec (Target) (MAP) (negotiated with MSC-A’ during the MAP E-interface signalling), if it is not already included according to list item b;

NOTE 3: 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 4: 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 codecs from the AoIP-Supported Codecs List (Anchor) (MAP) that are applicable to the target radio access, e.g. the AoIP-Selected Codec (Target) (MAP), if it is not already included according to list item a or b; 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 MSC-A’ receives a Supported Codec List (BICC) with the IAM message, MSC-A’ shall select a codec from this list, taking the AoIP-Selected codec (Target) (MAP) and the priorities expressed by the order of the codecs in the Supported Codec List (BICC) into account.

NOTE 5: 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 AoIP-Selected codec (Target) (MAP) or the default PCM codec.

If MSC-A’ selects a codec different from the AoIP-Selected codec (Target) (MAP) or if MSC-A’ receives an IAM message without a Supported Codec List (BICC), MSC-A’ shall insert a transcoder in MGW-A’.

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.

Subsequent inter MSC handover to a third MSC

The anchor MSC (MSC-A) server may include AoIP-Supported Codecs List (Anchor) (MAP) in the MAP Prepare Handover request towards the third MSC.