laforge has submitted this change. (
https://gerrit.osmocom.org/c/libosmocore/+/28127 )
Change subject: coding: prevent marking FACCH frames as AMR's special DTX frames
......................................................................
coding: prevent marking FACCH frames as AMR's special DTX frames
Both gsm0503_tch_a[fh]s_decode_dtx() functions accept an optional
'dtx' pointer, which is used to indicate type of a received AMR
block to the caller in DTX mode of operation. If not NULL, it's
expected to be updated by gsm0503_detect_a[fh]s_dtx_frame() every
time one of the mentioned functions is called.
However, in case of FACCH both functions return early, so the value
of dtx remains unchanged and thus FACCH frames may be misinterpreted
as AMR's special DTX frames. This is rather critical during the DTX
silence periods, when all special DTX frames (e.g. SID_UPDATE) are
being treated as SUB frames. Each unsuccessful FACCH decoding
attempt will 'poison' SUB measurements, causing unexpected RxQual-
SUB values in the Uplink measurement reports.
Fix this by resetting *dtx to AMR_OTHER in the FACCH specific path.
Change-Id: I2e6f4b748c6445725211e264ab5f3f5a2712087a
Related: SYS#5853
---
M src/coding/gsm0503_coding.c
1 file changed, 13 insertions(+), 0 deletions(-)
Approvals:
Jenkins Builder: Verified
pespin: Looks good to me, but someone else must approve
laforge: Looks good to me, approved
diff --git a/src/coding/gsm0503_coding.c b/src/coding/gsm0503_coding.c
index 98439e3..e81bf02 100644
--- a/src/coding/gsm0503_coding.c
+++ b/src/coding/gsm0503_coding.c
@@ -2171,6 +2171,12 @@
gsm0503_tch_fr_deinterleave(cB, iB);
if (steal > 0) {
+ /* If not NULL, dtx indicates type of previously decoded
TCH/AFS frame.
+ * It's normally updated by gsm0503_detect_afs_dtx_frame(),
which is not
+ * reached in case of FACCH. Reset it here to avoid FACCH/F
frames being
+ * misinterpreted as AMR's special DTX frames. */
+ if (dtx != NULL)
+ *dtx = AMR_OTHER;
rv = _xcch_decode_cB(tch_data, cB, n_errors, n_bits_total);
if (rv) {
/* Error decoding FACCH frame */
@@ -2633,6 +2639,13 @@
/* if we found a stole FACCH, but only at correct alignment */
if (steal > 0) {
+ /* If not NULL, dtx indicates type of previously decoded
TCH/AHS frame.
+ * It's normally updated by gsm0503_detect_ahs_dtx_frame(),
which is not
+ * reached in case of FACCH. Reset it here to avoid FACCH/H
frames being
+ * misinterpreted as AMR's special DTX frames. */
+ if (dtx != NULL)
+ *dtx = AMR_OTHER;
+
for (i = 0; i < 6; i++) {
gsm0503_tch_burst_unmap(&iB[i * 114],
&bursts[i * 116], NULL, i >> 2);
--
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Gerrit-Project: libosmocore
Gerrit-Branch: master
Gerrit-Change-Id: I2e6f4b748c6445725211e264ab5f3f5a2712087a
Gerrit-Change-Number: 28127
Gerrit-PatchSet: 1
Gerrit-Owner: fixeria <vyanits...@sysmocom.de>
Gerrit-Reviewer: Jenkins Builder
Gerrit-Reviewer: laforge <lafo...@osmocom.org>
Gerrit-Reviewer: pespin <pes...@sysmocom.de>
Gerrit-MessageType: merged
