7 is the latest approved patch-set.
No files were changed between the latest approved patch-set and the submitted one.


Transmit invalid AMR speech blocks instead of dummy FACCH

Every BTS needs to have some graceful handling for the scenario
where it is time to send out a speech frame on TCH DL, but there is
no frame to be sent. One possible solution is to transmit dummy
FACCH, but this option is unattractive for TCH/AHS where FACCH
displaces two speech frames rather than one. A more elegant solution
is to emit a speech frame that is bad, causing the MS receiver to
declare a BFI condition to trigger substitution and muting procedure.
A bad frame is generated by gsm0503_tch_{afs,ahs}_encode() by setting
the payload length to 0.

Depends: libosmocore.git I82ce2adf995a4b42d1f378c5819f88d773b9104a
Related: OS#6049
Change-Id: I056f379715c91ad968f198e112d363a9009dc1c3
---
M src/osmo-bts-trx/sched_lchan_tchf.c
M src/osmo-bts-trx/sched_lchan_tchh.c
2 files changed, 57 insertions(+), 8 deletions(-)

diff --git a/src/osmo-bts-trx/sched_lchan_tchf.c b/src/osmo-bts-trx/sched_lchan_tchf.c
index 0138a2a..b621b23 100644
--- a/src/osmo-bts-trx/sched_lchan_tchf.c
+++ b/src/osmo-bts-trx/sched_lchan_tchf.c
@@ -538,15 +538,14 @@
 		/* - If the channel mode is TCH/FS or TCH/EFS, transmit a dummy
 		 *   speech block with inverted CRC3, designed to induce a BFI
 		 *   condition in the MS receiver.
+		 * - If the channel mode is TCH/AFS, transmit a dummy speech
+		 *   block with inverted CRC6, designed to induce a BFI
+		 *   condition in the MS receiver.
 		 * - If the channel mode is one of the CSD modes, transmit an
 		 *   idle frame as described in 3GPP TS 44.021, sections 8.1.6
 		 *   and 10.2.3 (all data, status and E-bits set to binary '1').
 		 * - In all other channel modes, transmit dummy FACCH
 		 *   like we always did before.
-		 *
-		 * FIXME: someone who knows AMR needs to look at this problem
-		 * and decide what is the correct BTS Tx behavior for frame
-		 * gaps in TCH/AFS.  See OS#6049.
 		 */
 		switch (tch_mode) {
 		case GSM48_CMODE_DATA_12k0:
@@ -562,6 +561,21 @@
 			/* fall-through */
 		case GSM48_CMODE_SIGN:
 		default:
+			if (tch_mode == GSM48_CMODE_SPEECH_AMR) {
+				/* the first FN 4,13,21 defines that CMI is included in frame,
+				 * the first FN 0,8,17 defines that CMR is included in frame.
+				 */
+				rc = gsm0503_tch_afs_encode(BUFPOS(bursts_p, 0),
+							    NULL, 0,
+							    !sched_tchf_dl_amr_cmi_map[br->fn % 26],
+							    chan_state->codec,
+							    chan_state->codecs,
+							    chan_state->dl_ft,
+							    chan_state->dl_cmr);
+				if (rc == 0)
+					goto send_burst;
+			}
+
 			/* TODO: use randomized padding */
 			msg_facch = tch_dummy_msgb(GSM_MACBLOCK_LEN, GSM_MACBLOCK_PADDING);
 			/* dummy LAPDm func=UI frame */
diff --git a/src/osmo-bts-trx/sched_lchan_tchh.c b/src/osmo-bts-trx/sched_lchan_tchh.c
index e0d7aca..05e1af4 100644
--- a/src/osmo-bts-trx/sched_lchan_tchh.c
+++ b/src/osmo-bts-trx/sched_lchan_tchh.c
@@ -453,15 +453,14 @@
 		/* - If the channel mode is TCH/HS, transmit a dummy speech block
 		 *   with inverted CRC3, designed to induce a BFI condition in
 		 *   the MS receiver.
+		 * - If the channel mode is TCH/AHS, transmit a dummy speech
+		 *   block with inverted CRC6, designed to induce a BFI
+		 *   condition in the MS receiver.
 		 * - If the channel mode is one of the CSD modes, transmit an
 		 *   idle frame as described in 3GPP TS 44.021, sections 8.1.6
 		 *   and 10.2.3 (all data, status and E-bits set to binary '1').
 		 * - In all other channel modes, transmit dummy FACCH
 		 *   like we always did before.
-		 *
-		 * FIXME: someone who knows AMR needs to look at this problem
-		 * and decide what is the correct BTS Tx behavior for frame
-		 * gaps in TCH/AHS.  See OS#6049.
 		 */
 		switch (tch_mode) {
 		case GSM48_CMODE_DATA_6k0:
@@ -474,6 +473,21 @@
 			/* fall-through */
 		case GSM48_CMODE_SIGN:
 		default:
+			if (tch_mode == GSM48_CMODE_SPEECH_AMR) {
+				/* the first FN 4,13,21 or 5,14,22 defines that CMI is included
+				 * in frame, the first FN 0,8,17 or 1,9,18 defines that CMR is
+				 * included in frame. */
+				rc = gsm0503_tch_ahs_encode(BUFPOS(bursts_p, 0),
+							    NULL, 0,
+							    !sched_tchh_dl_amr_cmi_map[br->fn % 26],
+							    chan_state->codec,
+							    chan_state->codecs,
+							    chan_state->dl_ft,
+							    chan_state->dl_cmr);
+				if (rc == 0)
+					goto send_burst;
+			}
+
 			/* FACCH/H can only be scheduled at specific TDMA offset */
 			if (!sched_tchh_dl_facch_map[br->fn % 26]) {
 				/* FACCH/H is not allowed, send half-filled bursts with even numbered

