GSM 06.31, 06.41 and 06.81 are the respective DTX specs for FR, HR
and EFR. In each of these specs, section 5.1.2 specifies the
expected shape of radio downlink in the presence of SIDs: one SID
frame after each talkspurt (after speech frames), and one SID frame
in every SACCH-aligned position every 480 ms (every 240 ms for HR),
or if the actual SACCH-aligned position is taken up by FACCH,
then just one SID frame as soon as possible after that FACCH -
and no transmitted SID frames in other positions.
This just-referenced spec section was written with the assumption
that it will only be applied when DTXd is enabled - however, if
the RTP stream for call leg B DL comes from call leg A UL (TrFO),
then we are going to receive SID frames in the stream intended for
our DL even when DTXd is disabled or not supported altogether.
The easiest solution is to apply FR/HR/EFR DTXd logic whenever
the incoming RTP stream contains SID frames, irrespective of physical
DTXd enable/disable state. If we apply such "logical DTXd" when
physical DTXd is disabled, the BTS model PHY will end up transmitting
induced BFIs (dummy FACCH or inverted CRC3) in those frame positions
where the "logical DTXd" function says "please transmit nothing".
The point remains, however, that the prescribed SID shape on the
radio downlink (expected positions of SID frames) won't happen on its
own: in the case of TrFO, whichever SID frames are present will be
in wrong positions for leg B DL, and even in the case of transcoded
calls the responsibility for DL SID shaping cannot be placed on the
RTP stream source because that source won't know where SACCH alignment
will lie. Therefore, the necessary DL SID reshaping has to be done
in the RTP stream receiver in OsmoBTS.
Related: OS#5996
Change-Id: I924ab21952dcf8bb03ba7ccef790474bf66fc9e5
a84b9a0261