23.1533GPPOut of band transcoder controlRelease 17Stage 2TS
Inter-system handover procedures are described at call control level in  and details for bearer independent architecture is described in . For TrFO connected UMTS call, when a handover occurs to GSM radio access, by definition the A-interface to the BSC shall be default PCM. Prior to receipt of Handover Detect the Anchor MGW has one leg (A-interface) stream mode as default PCM and two terminations with compressed voice codec properties. It is recommended that after the Handover Complete procedure, the network property is maintained as compressed. Thus the Anchor MGW inserts a "TFO Partner" transcoder. Thus no modification to the compressed bearer to 64k PCM is required. TFO procedures may then ensure that speech quality is maintained by avoiding transcoding.
In the Intra-MSC case shown in Figure 6.5/1 the MSC controlling the handover has both codec lists for each radio access. The codec negotiation for the UMTS call was performed end to end with UMTS list. If this negotiation resulted in a codec being selected that is also included in the GSM list then at handover the MSC shall indicate this codec as the current speech version to the BSC and TFO can be achieved. If the selected codec is not supported for the GSM radio access but the GSM list contains a codec that is also in the Available Codecs list then the MSC has the option to perform codec modification to ensure TFO can be achieved. The MSC may also perform codec list modification by sending forward the GSM list to update nodes in the network of the change to the Available Codecs List.
Figure 6.5/1: UMTS to GSM Inter-System Handover
If the Inter-system handover is an inter-MSC handover then the Anchor MSC sends the current speech version and the supported speech versions in the Prepare Handover Request message to the MSC-B. If the anchor MSC supports GERAN AoIP mode then the Prepare Handover Request message may contain AoIP-Supported Codecs List (Anchor) (MAP), see Clause 6.14.1.
If the current speech version (codec selected for UMTS call) is not included in the GSM list then the MSC-A shall indicate a preferred codec in the current speech version parameter. The speech version for the GSM access that is finally selected by the MSC-B’s BSS, is returned to MSC-A in the Prepare Handover Response message. If the following are satisfied:
– target MSC and BSC support GERAN AoIP mode;
– AoIP-Supported Codecs List (Anchor) (MAP) was received in MAP Prepare Handover Request; and
– target MSC supports AoIP codec selection based on received AoIP-Supported Codecs List (Anchor) (MAP),
then the Prepare Handover Response message shall contain the AoIP-Selected Codec (Target) (MAP) and AoIP-Available Codecs List (MAP).
The MSC-A can then decide if codec modification or codec re-negotiation shall be performed as described for the intra-MSC case. For further details on the inter-MSC signalling see clause 6.11. The connections are shown in Figure 6.5/2.
Figure 6.5/2: Inter-MSC, UMTS to GSM handover, with TDM transport between anchor MSC (MSC‑A) and target MSC (MSC-B)