laforge has submitted this change. (
https://gerrit.osmocom.org/c/libosmocore/+/33063 )
Change subject: coding: gsm0503_tch_{fr,hr}_encode(): add ability to emit BFI
......................................................................
coding: gsm0503_tch_{fr,hr}_encode(): add ability to emit BFI
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/HS where FACCH
displaces two speech frames rather than one. A more elegant
solution is to emit a speech frame with inverted CRC3, causing
the MS receiver to declare a BFI condition to its Rx DTX handler.
Setting all u(k) bits to 0 is one way to produce such an inverted-CRC
speech frame (normal TCH FR/HR CRC3 for an all-zeros frame would be
111), and this method is in fact what sysmoBTS PHY is observed to do.
Add the same ability to gsm0503_tch_{fr,hr}_encode() functions,
indicated by payload length of 0.
Change-Id: Iade3310e16b906efb6892d28f474a0d15204e861
---
M src/coding/gsm0503_coding.c
1 file changed, 43 insertions(+), 1 deletion(-)
Approvals:
Jenkins Builder: Verified
fixeria: 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 724bea2..80286f7 100644
--- a/src/coding/gsm0503_coding.c
+++ b/src/coding/gsm0503_coding.c
@@ -1941,6 +1941,20 @@
osmo_conv_encode(&gsm0503_tch_fr, conv, cB);
h = 0;
break;
+ case 0: /* no data, induce BFI in the receiver */
+ /* Do the same thing that sysmoBTS PHY does when fed a 0-length
+ * payload for DL: set all u(k) bits to 0, and do the same
+ * with all class 2 bits. This operation is NOT the same as
+ * an FR codec frame of all zero bits: with all-zeros d(k) input
+ * the CRC3 function will produce 111 output, whereas we
+ * transmit 000 in those parity bits too. The result will be
+ * an induced BFI (bad frame indication) condition in the
+ * receiver, for both TCH/FS and TCH/EFS decoders. */
+ memset(conv, 0, sizeof(conv));
+ memset(cB + 378, 0, 78);
+ osmo_conv_encode(&gsm0503_tch_fr, conv, cB);
+ h = 0;
+ break;
case GSM_MACBLOCK_LEN: /* FACCH */
_xcch_encode_cB(cB, tch_data);
h = 1;
@@ -2084,8 +2098,9 @@
tch_hr_b_to_d(d, b);
osmo_crc8gen_set_bits(&gsm0503_tch_fr_crc3, d + 73, 22, p);
tch_hr_reorder(conv, d, p);
- osmo_conv_encode(&gsm0503_tch_hr, conv, cB);
memcpy(cB + 211, d + 95, 17);
+hr_conv_coding:
+ osmo_conv_encode(&gsm0503_tch_hr, conv, cB);
h = 0;
gsm0503_tch_hr_interleave(cB, iB);
for (i = 0; i < 4; i++) {
@@ -2093,6 +2108,11 @@
&bursts[i * 116], &h, i >> 1);
}
break;
+ case 0: /* no data, induce BFI in the receiver */
+ /* see comments in gsm0503_tch_fr_encode() - same deal here */
+ memset(conv, 0, sizeof(conv));
+ memset(cB + 211, 0, 17);
+ goto hr_conv_coding;
case GSM_MACBLOCK_LEN: /* FACCH */
_xcch_encode_cB(cB, tch_data);
h = 1;
--
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Gerrit-Project: libosmocore
Gerrit-Branch: master
Gerrit-Change-Id: Iade3310e16b906efb6892d28f474a0d15204e861
Gerrit-Change-Number: 33063
Gerrit-PatchSet: 2
Gerrit-Owner: falconia <[email protected]>
Gerrit-Reviewer: Jenkins Builder
Gerrit-Reviewer: fixeria <[email protected]>
Gerrit-Reviewer: laforge <[email protected]>
Gerrit-MessageType: merged
