fix codecs in internal call bridge

This is a fixup for the patch
'3G: decapsulate IuUP to AMR at the MGW; allow 3G<-AMR->2G'
I386a6a426c318040b019ab5541689c67e94672a1

After above patch, osmo-msc intelligently decides which codecs to run on
which legs of the RTP streams. In the meantime, it seems the necessary
matching changes to call_leg_local_bridge() had been lost somehow.

Testing 3G to 3G voice now, I noticed that call_leg_local_bridge()
overwrites the intelligent choices made earlier.

The history of an MGW endpoint that should convert from IUFP to plain
AMR, extracted from a pcap, looks like this:

    <- CRCX None None
    -> CRCX-OK audio 4050 RTP/AVP 112 None
    <- MDCX audio 4056 RTP/AVP 112 AMR
    -> MDCX-OK audio 4050 RTP/AVP 112 AMR
    <- MDCX audio 4056 RTP/AVP 96 VND.3GPP.IUFP
    -> MDCX-OK audio 4050 RTP/AVP 96 VND.3GPP.IUFP

So after call_leg_local_bridge(), there is an extra MDCX + MDCX-OK that
switches the codec from 112 AMR back to 96 IUFP.

That is because call_leg_local_bridge() copies the *RAN* side's codec to
both CN sides, which used to be ok when RAN and CN codecs were always
identical.

Instead, adjust only the CN sides of the MGW endpoints, and adjust them
so that both CN sides are identical. osmo-mgw should then be able to
trivially translate the codecs appropriately.

Change-Id: I130bcd77ec57e332370c487a11b0b973b6e1089d

0 files changed, 0 insertions, 0 deletions