falconia has uploaded this change for review. ( 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(-) git pull ssh://gerrit.osmocom.org:29418/libosmocore refs/changes/63/33063/1 diff --git a/src/coding/gsm0503_coding.c b/src/coding/gsm0503_coding.c index 724bea2..c3b7bbc 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. */ + memset(conv, 0, 185); + 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, 98); + memset(cB + 211, 0, 17); + goto hr_conv_coding; case GSM_MACBLOCK_LEN: /* FACCH */ _xcch_encode_cB(cB, tch_data); h = 1; -- To view, visit https://gerrit.osmocom.org/c/libosmocore/+/33063 To unsubscribe, or for help writing mail filters, visit https://gerrit.osmocom.org/settings Gerrit-Project: libosmocore Gerrit-Branch: master Gerrit-Change-Id: Iade3310e16b906efb6892d28f474a0d15204e861 Gerrit-Change-Number: 33063 Gerrit-PatchSet: 1 Gerrit-Owner: falconia &lt;falcon(a)freecalypso.org&gt; Gerrit-MessageType: newchange