codec: add SID preening functions for FR & EFR
Those network elements which receive a stream of codec frames that
may come from the uplink of GSM call A and which are responsible
for preparing the frame stream for the downlink of GSM call B
(OsmoMGW feeding TRAU-DL, or OsmoBTS receiving RTP and feeding DL
to its PHY) must be prepared for the possibility that their
incoming frame stream may contain corrupted SID frames, presumably
from bit errors on radio link A. Per the rules of section 6.1.1
of GSM 06.31 for FR and GSM 06.81 for EFR, SID frames with just one
errored bit are still to be accepted as valid, whereas frames with
more corrupted bits which are still recognizable as SID are classified
as invalid SID.
In the case of a TrFO call, the entity switching from leg A UL to
leg B DL is responsible for *not* transmitting invalid SID frames
on the destination leg (they should be treated like BFIs), and any
deemed-valid SID frames that are forwarded should be preened,
correcting that one bit error they may exhibit. The functions
added here provide that functionality.
Change-Id: Iec5c1f2619a82499f61cb3e5a7cd03ff0f020ad8
diff --git a/src/codec/gsm610.c b/src/codec/gsm610.c
index d29d9cb..4731e9e 100644
--- a/src/codec/gsm610.c
+++ b/src/codec/gsm610.c
@@ -389,3 +389,44 @@
else
return OSMO_GSM631_SID_CLASS_VALID;
}
+
+/*! Preen potentially-SID FR codec frame in RTP format, ensuring that it is
+ * either a speech frame or a valid SID, and if the latter, making it a
+ * perfect, error-free SID frame.
+ * \param[in] rtp_payload Buffer with RTP payload - must be writable!
+ * \returns true if the frame is good, false otherwise
+ */
+bool osmo_fr_sid_preen(uint8_t *rtp_payload)
+{
+ enum osmo_gsm631_sid_class sidc;
+ uint8_t *p, sub;
+
+ sidc = osmo_fr_sid_classify(rtp_payload);
+ switch (sidc) {
+ case OSMO_GSM631_SID_CLASS_SPEECH:
+ return true;
+ case OSMO_GSM631_SID_CLASS_INVALID:
+ return false;
+ case OSMO_GSM631_SID_CLASS_VALID:
+ /* "Rejuvenate" this SID frame, correcting any errors:
+ * zero out all bits that aren't LARc or Xmaxc, thereby
+ * clearing all SID code word bits and all unused/reserved
+ * bits. */
+ p = rtp_payload + 5; /* skip magic+LARc */
+ for (sub = 0; sub < 4; sub++) {
+ *p++ = 0;
+ *p++ &= 0x1F;
+ *p++ &= 0x80;
+ *p++ = 0;
+ *p++ = 0;
+ *p++ = 0;
+ *p++ = 0;
+ }
+ return true;
+ default:
+ /* There are only 3 possible SID classifications per GSM 06.31
+ * section 6.1.1, thus any other return value is a grave error
+ * in the code. */
+ OSMO_ASSERT(0);
+ }
+}