Feature #4717
opensupport Handover with changing codec
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Description
Analogous to #4716 in OsmoBSC, support a Handover where the codec changes in OsmoMSC.
osmo-msc will mid-call receive a Handover Performed message that indicates a different codec.
osmo-msc should then:
- make sure the BSS-facing MGW endpoint conn includes that new codec.
- since osmo-mgw does not transcode, also make sure the PLMN facing MGW endpoint conn includes that new codec.
- communicate over MNCC that the codec has changed.
If the available codecs are included in the SDP as secondary payload types right from the start,
then the incoming RTP packet may simply change their payload type number on the fly.
However, the PBX should be instructed to also send back RTP in the new codec type (typically by allowing only one codec in the SDP).
This is also related to unmerged patch https://gerrit.osmocom.org/c/osmo-msc/+/15953 that implements full SDP between MSC and PBX.
Note that mismatching codec types are not possible for internal MNCC in osmo-msc,
so handover with codec change requires an external PBX that is able to transcode.
(Or alternatively implement transcoding in osmo-mgw...)
Related issues
