falconia has uploaded this change for review.
FR/HR/EFR: implement SID filter in TCH UL path
As a result of how FR/HR/EFR DTX interacts with block diagonal
interleaving, at the beginning and end of each DTX pause a
correctly functioning TCH receiver will always pick up an
artifact consisting of 4 received bursts (2 for TCH/HS)
and same number of omitted bursts. Standard channel decoding
of this Rx artifact will produce a "half-block" in which half
of the bits prior to convolutional decoding will come from
a SID repetition whose Tx was notionally suppressed, while
the other half will be garbage. As a result of convolutional
decoding, the result will often appear as valid SID per
classification rules - but passing it as such to the Rx DTX
handler is wrong. Classic E1 BTS and GSM MS implementations
include a kind of SID filter at this point, setting BFI on
these received half-blocks, so that the Rx DTX handler will
see an invalid SID condition. Invalid SID means that comfort
noise generation is to be continued, but no updated CN
parameters are available - which is the truth in half-block
Rx situations. Implement the same filter.
Additional background info can be found here:
https://osmocom.org/projects/retro-gsm/wiki/DTXu_half-blocks
Change-Id: I46c62312316b04567bcadf6050597673f071247d
---
M include/osmo-bts/lchan.h
M src/common/l1sap.c
2 files changed, 55 insertions(+), 8 deletions(-)
git pull ssh://gerrit.osmocom.org:29418/osmo-bts refs/changes/49/41049/1
diff --git a/include/osmo-bts/lchan.h b/include/osmo-bts/lchan.h
index 5c34015..70ca5b9 100644
--- a/include/osmo-bts/lchan.h
+++ b/include/osmo-bts/lchan.h
@@ -292,6 +292,8 @@
bool dl_sid_transmitted;
/* The current frame in the DL is taken up by FACCH */
bool dl_facch_stealing;
+ /* UL SID filter to catch DTXu half-blocks */
+ bool ul_sid_filter;
} dtx_fr_hr_efr;
uint8_t last_cmr;
uint32_t last_fn;
diff --git a/src/common/l1sap.c b/src/common/l1sap.c
index 8226648..e8453b8 100644
--- a/src/common/l1sap.c
+++ b/src/common/l1sap.c
@@ -2326,7 +2326,19 @@
{
enum osmo_gsm631_sid_class sidc;
- /* If we got no payload (BFI without data), there is nothing
+ /* No matter what else is happening, even if we are about to bail out
+ * early because we received FACCH and thus BFI-no-data, if we are
+ * at a mandatory-Tx position for SID, we need to clear UL SID filter
+ * state so that the next valid SID frame will be allowed through,
+ * whether it occurs right now or in a later frame position because
+ * of FACCH. See the note in GSM 06.31 section 5.1.2: if the
+ * SACCH-aligned SID update position is stolen by FACCH, the next
+ * frame position shall carry the SID update as soon as FACCH stealing
+ * is over. */
+ if (fr_hr_efr_sid_position(lchan, fn))
+ lchan->tch.dtx_fr_hr_efr.ul_sid_filter = false;
+
+ /* If we got no payload (BFI without data), there is nothing more
* for us to do here. */
if (msg->len == 0)
return;
@@ -2367,11 +2379,24 @@
* is an accepted SID frame in the definition of GSM 06.31
* and its HR & EFR counterparts.
*
- * 2) Next patch in the series will introduce logic that sets BFI
- * under certain conditions dependent on SID classification and
- * previous state, in order to suppress false indications of
- * "valid" SID to the Rx DTX handler on the RTP receiving end
- * during "half-block" conditions.
+ * 2) As a result of how FR/HR/EFR DTX interacts with block diagonal
+ * interleaving, at the beginning and end of each DTX pause a
+ * correctly functioning TCH receiver will always pick up an
+ * artifact consisting of 4 received bursts (2 for TCH/HS)
+ * and same number of omitted bursts. Standard channel decoding
+ * of this Rx artifact will produce a "half-block" in which half
+ * of the bits prior to convolutional decoding will come from
+ * a SID repetition whose Tx was notionally suppressed, while
+ * the other half will be garbage. As a result of convolutional
+ * decoding, the result will often appear as valid SID per
+ * classification rules - but passing it as such to the Rx DTX
+ * handler is wrong. Classic E1 BTS and GSM MS implementations
+ * include a kind of SID filter at this point, setting BFI on
+ * these received half-blocks, so that the Rx DTX handler will
+ * see an invalid SID condition. Invalid SID means that comfort
+ * noise generation is to be continued, but no updated CN
+ * parameters are available - which is the truth in half-block
+ * Rx situations. We implement the same filter.
*
* 3) For HR codec only, RTP output format functions need to know
* both BFI flag and SID classification.
@@ -2380,12 +2405,32 @@
case OSMO_GSM631_SID_CLASS_SPEECH:
/* Only a good speech frame, not an unusable frame,
* using GSM 06.31 definitions, marks exit from a DTX pause. */
- if (!tch_ul_msg_bfi(msg))
+ if (!tch_ul_msg_bfi(msg)) {
lchan_set_marker(false, lchan);
+ lchan->tch.dtx_fr_hr_efr.ul_sid_filter = false;
+ }
break;
- case OSMO_GSM631_SID_CLASS_INVALID:
case OSMO_GSM631_SID_CLASS_VALID:
+ /* Here comes the just-described SID filter. The logic is
+ * thus: a valid SID is allowed through to the Rx DTX handler
+ * if it follows speech (end of talkspurt), if it appears
+ * in the expected mandatory-Tx position for SID updates,
+ * or if it happens after that mandatory-Tx position due to
+ * FACCH stealing. Otherwise, valid SID is converted to
+ * invalid by setting BFI.
+ */
+ if (lchan->tch.dtx_fr_hr_efr.ul_sid_filter)
+ tch_ul_msg_bfi(msg) = true;
+ /* fall through */
+ case OSMO_GSM631_SID_CLASS_INVALID:
+ /* Whether we got valid or invalid SID, we know that the MS
+ * has entered or remains in a DTXu pause. (Invalid SID
+ * means that MS state is known to be DTXu, but no CN
+ * parameters are available.) Hence we indicate DTXu pause
+ * state for both RTP marker bit and SID filter mechanisms.
+ */
lchan_set_marker(true, lchan);
+ lchan->tch.dtx_fr_hr_efr.ul_sid_filter = true;
break;
default:
/* There are only 3 possible SID classifications per
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