Cellular Network Infrastructure » Core Network (CN) » OsmoMSC

In a voice Call, it appears that trans->bearer_cap (with e.g. the list of supported codecs) is only initialized when receiving CC SETUP, CC CALL CONFIRMED, CC MODIFY and CC COMPLETE.

There appear to be no MNCC messages by which an external MNCC handler can have any influence on the contents of trans->bearer_cap.

However, trans->bearer_cap is exactly what is used to determine the voice codec when encoding the BSSMAP ASSIGNMENT towards the BSC.

From my point of view, the external MNCC handler should be able to specify the permitted codecs at any time, for example in a MO call in (at least) the following primitives:

• MNCC_CALL_PROC_REQ
• MNCC_SETUP_COMPL_REQ

The normal operation would be for the MNCC handler to intersect the permitted codecs as received from the MNCC_SETUP_IND (i.e. decoded from CC SETUP) with whatever it is capable of handling on the external PBX side, or with local policy. The returned set of codecs then is passed back via MNCC, trans->bearer_cap updated and finally the related BSSMAP ASSIGNMENT codec list computed.