aboutsummaryrefslogtreecommitdiffstats
path: root/src/osmo-bts-trx/sched_lchan_tchf.c
blob: 4bd07564257ae27edafb39e5cc591a9910c8c4ed (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
/*
 * (C) 2013 by Andreas Eversberg <jolly@eversberg.eu>
 * (C) 2015-2017 by Harald Welte <laforge@gnumonks.org>
 * Contributions by sysmocom - s.f.m.c. GmbH
 *
 * All Rights Reserved
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation; either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

#include <stdint.h>
#include <errno.h>

#include <osmocom/core/bits.h>
#include <osmocom/core/msgb.h>
#include <osmocom/core/utils.h>

#include <osmocom/gsm/gsm0502.h>

#include <osmocom/codec/codec.h>
#include <osmocom/codec/ecu.h>

#include <osmocom/coding/gsm0503_coding.h>
#include <osmocom/coding/gsm0503_amr_dtx.h>

#include <osmo-bts/bts.h>
#include <osmo-bts/l1sap.h>
#include <osmo-bts/logging.h>
#include <osmo-bts/scheduler.h>
#include <osmo-bts/scheduler_backend.h>
#include <osmo-bts/msg_utils.h>

#include <sched_utils.h>
#include <loops.h>

/*! \brief a single TCH/F burst was received by the PHY, process it */
int rx_tchf_fn(struct l1sched_ts *l1ts, const struct trx_ul_burst_ind *bi)
{
	struct l1sched_chan_state *chan_state = &l1ts->chan_state[bi->chan];
	struct gsm_lchan *lchan = chan_state->lchan;
	sbit_t *burst, **bursts_p = &chan_state->ul_bursts;
	uint8_t *mask = &chan_state->ul_mask;
	uint8_t rsl_cmode = chan_state->rsl_cmode;
	uint8_t tch_mode = chan_state->tch_mode;
	uint8_t tch_data[128]; /* just to be safe */
	enum sched_meas_avg_mode meas_avg_mode = SCHED_MEAS_AVG_M_OCTO;
	struct l1sched_meas_set meas_avg;
	int rc, amr = 0;
	int n_errors = 0;
	int n_bits_total = 0;
	bool bfi_flag = false;
	unsigned int fn_begin;
	uint16_t ber10k;
	uint8_t is_sub = 0;
	uint8_t ft;
	bool amr_is_cmr;

	/* If handover RACH detection is turned on, treat this burst as an Access Burst.
	 * Handle NOPE.ind as usually to ensure proper Uplink measurement reporting. */
	if (chan_state->ho_rach_detect == 1 && ~bi->flags & TRX_BI_F_NOPE_IND)
		return rx_rach_fn(l1ts, bi);

	LOGL1SB(DL1P, LOGL_DEBUG, l1ts, bi, "Received TCH/F, bid=%u\n", bi->bid);

	/* allocate burst memory, if not already */
	if (!*bursts_p) {
		*bursts_p = talloc_zero_size(tall_bts_ctx, 928);
		if (!*bursts_p)
			return -ENOMEM;
	}

	/* clear burst */
	if (bi->bid == 0) {
		memset(*bursts_p + 464, 0, 464);
		*mask = 0x0;
	}

	/* update mask */
	*mask |= (1 << bi->bid);

	/* store measurements */
	trx_sched_meas_push(chan_state, bi);

	/* copy burst to end of buffer of 8 bursts */
	burst = *bursts_p + bi->bid * 116 + 464;
	if (bi->burst_len > 0) {
		memcpy(burst, bi->burst + 3, 58);
		memcpy(burst + 58, bi->burst + 87, 58);
	} else
		memset(burst, 0, 116);

	/* wait until complete set of bursts */
	if (bi->bid != 3)
		return 0;

	/* check for complete set of bursts */
	if ((*mask & 0xf) != 0xf) {
		LOGL1SB(DL1P, LOGL_NOTICE, l1ts, bi,
			"Received incomplete frame (%u/%u)\n",
			bi->fn % l1ts->mf_period, l1ts->mf_period);
	}
	*mask = 0x0;

	/* decode
	 * also shift buffer by 4 bursts for interleaving */
	switch ((rsl_cmode != RSL_CMOD_SPD_SPEECH) ? GSM48_CMODE_SPEECH_V1
								: tch_mode) {
	case GSM48_CMODE_SPEECH_V1: /* FR */
		rc = gsm0503_tch_fr_decode(tch_data, *bursts_p, 1, 0, &n_errors, &n_bits_total);
		if (rc >= 0)
			lchan_set_marker(osmo_fr_check_sid(tch_data, rc), lchan); /* DTXu */
		break;
	case GSM48_CMODE_SPEECH_EFR: /* EFR */
		rc = gsm0503_tch_fr_decode(tch_data, *bursts_p, 1, 1, &n_errors, &n_bits_total);
		break;
	case GSM48_CMODE_SPEECH_AMR: /* AMR */
		/* the first FN 0,8,17 defines that CMI is included in frame,
		 * the first FN 4,13,21 defines that CMR is included in frame.
		 * NOTE: A frame ends 7 FN after start.
		 */
		fn_begin = gsm0502_fn_remap(bi->fn, FN_REMAP_TCH_F);
		amr_is_cmr = !ul_amr_fn_is_cmi(fn_begin);

		/* The AFS_ONSET frame itself does not result into an RTP frame
		 * since it only contains a recognition pattern that marks the
		 * end of the DTX interval. To mark the end of the DTX interval
		 * in the RTP stream as well, the voice frame after the
		 * AFS_ONSET frame is used. */
		if (chan_state->amr_last_dtx == AFS_ONSET)
			lchan_set_marker(false, lchan);

		/* we store tch_data + 2 header bytes, the amr variable set to
		 * 2 will allow us to skip the first 2 bytes in case we did
		 * receive an FACCH frame instead of a voice frame (we do not
		 * know this before we actually decode the frame) */
		amr = 2;
		rc = gsm0503_tch_afs_decode_dtx(tch_data + amr, *bursts_p,
			amr_is_cmr, chan_state->codec, chan_state->codecs, &chan_state->ul_ft,
			&chan_state->ul_cmr, &n_errors, &n_bits_total, &chan_state->amr_last_dtx);

		/* Tag all frames that are not regular AMR voice frames as
		 * SUB-Frames */
		if (chan_state->amr_last_dtx != AMR_OTHER) {
			LOGL1SB(DL1P, LOGL_DEBUG, l1ts, bi, "Received AMR SID frame: %s\n",
			       gsm0503_amr_dtx_frame_name(chan_state->amr_last_dtx));
			is_sub = 1;
		}

		/* The occurrence of the following frames indicates that we
		 * are either at the beginning or in the middle of a talk
		 * spurt. We update the SID status accordingly, but we do
		 * not want the marker to be set, since this must only
		 * happen when the talk spurt is over (see above) */
		switch (chan_state->amr_last_dtx) {
		case AFS_SID_FIRST:
		case AFS_SID_UPDATE:
		case AFS_SID_UPDATE_CN:
			lchan_set_marker(true, lchan);
			lchan->rtp_tx_marker = false;
			break;
		}

		switch (chan_state->amr_last_dtx) {
		case AFS_SID_FIRST:
		case AFS_SID_UPDATE_CN:
			meas_avg_mode = SCHED_MEAS_AVG_M8_FIRST_QUAD;
			break;
		case AFS_SID_UPDATE:
		case AFS_ONSET:
			meas_avg_mode = SCHED_MEAS_AVG_M_QUAD;
			break;
		}

		if (rc)
			trx_loop_amr_input(chan_state, n_errors, n_bits_total);
		/* only good speech frames get rtp header */
		if (rc != GSM_MACBLOCK_LEN && rc >= 4) {
			if (chan_state->amr_last_dtx == AMR_OTHER) {
				ft = chan_state->codec[chan_state->ul_ft];
			} else {
				/* SID frames will always get Frame Type Index 8 (AMR_SID) */
				ft = AMR_SID;
			}
			rc = osmo_amr_rtp_enc(tch_data,
				chan_state->codec[chan_state->ul_cmr],
			        ft, AMR_GOOD);
		}

		break;
	default:
		LOGL1SB(DL1P, LOGL_ERROR, l1ts, bi,
			"TCH mode %u invalid, please fix!\n",
			tch_mode);
		return -EINVAL;
	}
	memcpy(*bursts_p, *bursts_p + 464, 464);

	/* average measurements of the last N (depends on mode) bursts */
	trx_sched_meas_avg(chan_state, &meas_avg, meas_avg_mode);

	/* Check if the frame is bad */
	if (rc < 0) {
		LOGL1SB(DL1P, LOGL_NOTICE, l1ts, bi, "Received bad data (%u/%u)\n",
			bi->fn % l1ts->mf_period, l1ts->mf_period);
		bfi_flag = true;
	} else if (rc < 4) {
		LOGL1SB(DL1P, LOGL_NOTICE, l1ts, bi,
			"Received bad data (%u/%u) with invalid codec mode %d\n",
			bi->fn % l1ts->mf_period, l1ts->mf_period, rc);
		bfi_flag = true;
	}

	if (rc != GSM_MACBLOCK_LEN && lchan->ecu_state)
		osmo_ecu_frame_in(lchan->ecu_state, bfi_flag, tch_data, rc);

	ber10k = compute_ber10k(n_bits_total, n_errors);
	if (bfi_flag)
		goto bfi;

	/* FACCH */
	if (rc == GSM_MACBLOCK_LEN) {
		fn_begin = gsm0502_fn_remap(bi->fn, FN_REMAP_FACCH_F);
		_sched_compose_ph_data_ind(l1ts, fn_begin, bi->chan,
			tch_data + amr, GSM_MACBLOCK_LEN,
			meas_avg.rssi, meas_avg.toa256,
			meas_avg.ci_cb, ber10k,
			PRES_INFO_UNKNOWN);

		/* If we are in SPEECH mode we will generate a fake (BFI) TCH
		 * indication as well. This indication is needed by the higher
		 * layers, however we already have reported the measurement
		 * result for the current block together with the FACCH.
		 * To avoid reporting the same measurement result again with
		 * the fake (BFI) TCH indication we set meas_avg.rssi to zero.
		 * Doing so tells l1sap.c to ignore the measurement result. */
		meas_avg.rssi = 0;

bfi:
		if (rsl_cmode == RSL_CMOD_SPD_SPEECH) {
			/* indicate bad frame */
			if (lchan->tch.dtx.ul_sid) {
				/* DTXu: pause in progress. Push empty payload to upper layers */
				rc = 0;
				goto compose_l1sap;
			}

			/* If there is an ECU active on this channel, use its output */
			if (lchan->ecu_state) {
				rc = osmo_ecu_frame_out(lchan->ecu_state, tch_data);
				if (rc >= 0) /* Otherwise we send a BFI */
					goto compose_l1sap;
			}

			switch (tch_mode) {
			case GSM48_CMODE_SPEECH_V1: /* FR */
				memset(tch_data, 0, GSM_FR_BYTES);
				tch_data[0] = 0xd0;
				rc = GSM_FR_BYTES;
				break;
			case GSM48_CMODE_SPEECH_EFR: /* EFR */
				memset(tch_data, 0, GSM_EFR_BYTES);
				tch_data[0] = 0xc0;
				rc = GSM_EFR_BYTES;
				break;
			case GSM48_CMODE_SPEECH_AMR: /* AMR */
				rc = osmo_amr_rtp_enc(tch_data,
					chan_state->codec[chan_state->ul_cmr],
					chan_state->codec[chan_state->ul_ft],
					AMR_BAD);
				if (rc < 2) {
					LOGL1SB(DL1P, LOGL_ERROR, l1ts, bi,
					       "Failed to encode AMR_BAD frame (rc=%d), "
					       "not sending BFI\n", rc);
					return -EINVAL;
				}
				memset(tch_data + 2, 0, rc - 2);
				break;
			default:
				LOGL1SB(DL1P, LOGL_ERROR, l1ts, bi,
					"TCH mode %u invalid, please fix!\n", tch_mode);
				return -EINVAL;
			}
		}
	}

	if (rsl_cmode != RSL_CMOD_SPD_SPEECH)
		return 0;

	/* TCH or BFI */
compose_l1sap:
	fn_begin = gsm0502_fn_remap(bi->fn, FN_REMAP_TCH_F);
	return _sched_compose_tch_ind(l1ts, fn_begin, bi->chan, tch_data, rc,
				      /* FIXME: what should we use for BFI here? */
				      bfi_flag ? bi->toa256 : meas_avg.toa256, ber10k,
				      bfi_flag ? bi->rssi : meas_avg.rssi, is_sub);
}

/* common section for generation of TCH bursts (TCH/H and TCH/F).
 * FIXME: this function is over-complicated, refactor / get rid of it. */
void tx_tch_common(struct l1sched_ts *l1ts, struct trx_dl_burst_req *br,
		   struct msgb **_msg_tch, struct msgb **_msg_facch)
{
	struct msgb *msg1, *msg2, *msg_tch = NULL, *msg_facch = NULL;
	struct l1sched_chan_state *chan_state = &l1ts->chan_state[br->chan];
	uint8_t rsl_cmode = chan_state->rsl_cmode;
	uint8_t tch_mode = chan_state->tch_mode;
	struct osmo_phsap_prim *l1sap;

	/* handle loss detection of received TCH frames */
	if (rsl_cmode == RSL_CMOD_SPD_SPEECH
	    && ++(chan_state->lost_frames) > 5) {
		uint8_t tch_data[GSM_FR_BYTES];
		int len;

		LOGL1SB(DL1P, LOGL_NOTICE, l1ts, br, "Missing TCH bursts detected, sending BFI\n");

		/* indicate bad frame */
		switch (tch_mode) {
		case GSM48_CMODE_SPEECH_V1: /* FR / HR */
			if (br->chan != TRXC_TCHF) { /* HR */
				tch_data[0] = 0x70; /* F = 0, FT = 111 */
				memset(tch_data + 1, 0, 14);
				len = 15;
				break;
			}
			memset(tch_data, 0, GSM_FR_BYTES);
			len = GSM_FR_BYTES;
			break;
		case GSM48_CMODE_SPEECH_EFR: /* EFR */
			if (br->chan != TRXC_TCHF)
				goto inval_mode1;
			memset(tch_data, 0, GSM_EFR_BYTES);
			len = GSM_EFR_BYTES;
			break;
		case GSM48_CMODE_SPEECH_AMR: /* AMR */
			len = osmo_amr_rtp_enc(tch_data,
				chan_state->codec[chan_state->dl_cmr],
				chan_state->codec[chan_state->dl_ft], AMR_BAD);
			if (len < 2) {
				LOGL1SB(DL1P, LOGL_ERROR, l1ts, br,
					"Failed to encode AMR_BAD frame (rc=%d), "
					"not sending BFI\n", len);
				return;
			}
			memset(tch_data + 2, 0, len - 2);
			break;
		default:
inval_mode1:
			LOGL1SB(DL1P, LOGL_ERROR, l1ts, br, "TCH mode invalid, please fix!\n");
			len = 0;
		}

		if (len) {
			/* Note: RSSI/ToA256 is set to 0 to indicate to the higher
			 * layers that this is a faked tch_ind */
			_sched_compose_tch_ind(l1ts, br->fn, br->chan,
					       tch_data, len, 0, 10000, 0, 0);
		}
	}

	/* get frame and unlink from queue */
	msg1 = _sched_dequeue_prim(l1ts, br);
	msg2 = _sched_dequeue_prim(l1ts, br);
	if (msg1) {
		l1sap = msgb_l1sap_prim(msg1);
		if (l1sap->oph.primitive == PRIM_TCH) {
			msg_tch = msg1;
			if (msg2) {
				l1sap = msgb_l1sap_prim(msg2);
				if (l1sap->oph.primitive == PRIM_TCH) {
					LOGL1SB(DL1P, LOGL_FATAL, l1ts, br, "TCH twice, please FIX!\n");
					msgb_free(msg2);
				} else
					msg_facch = msg2;
			}
		} else {
			msg_facch = msg1;
			if (msg2) {
				l1sap = msgb_l1sap_prim(msg2);
				if (l1sap->oph.primitive != PRIM_TCH) {
					LOGL1SB(DL1P, LOGL_FATAL, l1ts, br, "FACCH twice, please FIX!\n");
					msgb_free(msg2);
				} else
					msg_tch = msg2;
			}
		}
	} else if (msg2) {
		l1sap = msgb_l1sap_prim(msg2);
		if (l1sap->oph.primitive == PRIM_TCH)
			msg_tch = msg2;
		else
			msg_facch = msg2;
	}

	/* check validity of message */
	if (msg_facch && msgb_l2len(msg_facch) != GSM_MACBLOCK_LEN) {
		LOGL1SB(DL1P, LOGL_FATAL, l1ts, br, "Prim has odd len=%u != %u\n",
			msgb_l2len(msg_facch), GSM_MACBLOCK_LEN);
		/* free message */
		msgb_free(msg_facch);
		msg_facch = NULL;
	}

	/* check validity of message, get AMR ft and cmr */
	if (!msg_facch && msg_tch) {
		int len;
		uint8_t cmr_codec;
		int cmr, ft, i;
		enum osmo_amr_type ft_codec;
		enum osmo_amr_quality bfi;
		int8_t sti, cmi;
		bool amr_is_cmr = !dl_amr_fn_is_cmi(br->fn);

		if (rsl_cmode != RSL_CMOD_SPD_SPEECH) {
			LOGL1SB(DL1P, LOGL_NOTICE, l1ts, br, "Dropping speech frame, "
				"because we are not in speech mode\n");
			goto free_bad_msg;
		}

		switch (tch_mode) {
		case GSM48_CMODE_SPEECH_V1: /* FR / HR */
			if (br->chan != TRXC_TCHF) /* HR */
				len = 15;
			else
				len = GSM_FR_BYTES;
			break;
		case GSM48_CMODE_SPEECH_EFR: /* EFR */
			if (br->chan != TRXC_TCHF)
				goto inval_mode2;
			len = GSM_EFR_BYTES;
			break;
		case GSM48_CMODE_SPEECH_AMR: /* AMR */
			len = osmo_amr_rtp_dec(msg_tch->l2h, msgb_l2len(msg_tch),
					       &cmr_codec, &cmi, &ft_codec,
					       &bfi, &sti);
			cmr = -1;
			ft = -1;
			for (i = 0; i < chan_state->codecs; i++) {
				if (chan_state->codec[i] == cmr_codec)
					cmr = i;
				if (chan_state->codec[i] == ft_codec)
					ft = i;
			}
			if (cmr >= 0) { /* new request */
				chan_state->dl_cmr = cmr;
				/* disable AMR loop */
				trx_loop_amr_set(chan_state, 0);
			} else {
				/* enable AMR loop */
				trx_loop_amr_set(chan_state, 1);
			}
			if (ft < 0) {
				LOGL1SB(DL1P, LOGL_ERROR, l1ts, br,
					"Codec (FT = %d) of RTP frame not in list\n", ft_codec);
				goto free_bad_msg;
			}
			if (amr_is_cmr && chan_state->dl_ft != ft) {
				LOGL1SB(DL1P, LOGL_NOTICE, l1ts, br, "Codec (FT = %d) "
					" of RTP cannot be changed now, but in next frame\n", ft_codec);
				goto free_bad_msg;
			}
			chan_state->dl_ft = ft;
			if (bfi == AMR_BAD) {
				LOGL1SB(DL1P, LOGL_NOTICE, l1ts, br, "Transmitting 'bad AMR frame'\n");
				goto free_bad_msg;
			}
			break;
		default:
inval_mode2:
			LOGL1SB(DL1P, LOGL_ERROR, l1ts, br, "TCH mode invalid, please fix!\n");
			goto free_bad_msg;
		}
		if (len < 0) {
			LOGL1SB(DL1P, LOGL_ERROR, l1ts, br, "Cannot send invalid AMR payload\n");
			goto free_bad_msg;
		}
		if (msgb_l2len(msg_tch) != len) {
			LOGL1SB(DL1P, LOGL_ERROR, l1ts, br, "Cannot send payload with "
				"invalid length! (expecting %d, received %d)\n",
				len, msgb_l2len(msg_tch));
free_bad_msg:
			/* free message */
			msgb_free(msg_tch);
			msg_tch = NULL;
			goto send_frame;
		}
	}

send_frame:
	*_msg_tch = msg_tch;
	*_msg_facch = msg_facch;
}

/* obtain a to-be-transmitted TCH/F (Full Traffic Channel) burst */
int tx_tchf_fn(struct l1sched_ts *l1ts, struct trx_dl_burst_req *br)
{
	struct msgb *msg_tch = NULL, *msg_facch = NULL;
	struct l1sched_chan_state *chan_state = &l1ts->chan_state[br->chan];
	uint8_t tch_mode = chan_state->tch_mode;
	ubit_t *burst, **bursts_p = &chan_state->dl_bursts;

	/* send burst, if we already got a frame */
	if (br->bid > 0) {
		if (!*bursts_p)
			return 0;
		goto send_burst;
	}

	tx_tch_common(l1ts, br, &msg_tch, &msg_facch);

	/* BURST BYPASS */

	/* allocate burst memory, if not already,
	 * otherwise shift buffer by 4 bursts for interleaving */
	if (!*bursts_p) {
		*bursts_p = talloc_zero_size(tall_bts_ctx, 928);
		if (!*bursts_p)
			return -ENOMEM;
	} else {
		memcpy(*bursts_p, *bursts_p + 464, 464);
		memset(*bursts_p + 464, 0, 464);
	}

	/* no message at all, send a dummy L2 frame on FACCH */
	if (!msg_tch && !msg_facch) {
		static const uint8_t dummy[GSM_MACBLOCK_LEN] = {
			0x03, 0x03, 0x01, /* TODO: use randomized padding */
			0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b,
			0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b,
		};

		LOGL1SB(DL1P, LOGL_INFO, l1ts, br, "No TCH or FACCH prim for transmit.\n");
		gsm0503_tch_fr_encode(*bursts_p, dummy, sizeof(dummy), 1);
		goto send_burst;
	}

	/* encode bursts (prioritize FACCH) */
	if (msg_facch)
		gsm0503_tch_fr_encode(*bursts_p, msg_facch->l2h, msgb_l2len(msg_facch),
			1);
	else 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.
		 */
		gsm0503_tch_afs_encode(*bursts_p, msg_tch->l2h + 2,
			msgb_l2len(msg_tch) - 2, !dl_amr_fn_is_cmi(br->fn),
			chan_state->codec, chan_state->codecs,
			chan_state->dl_ft,
			chan_state->dl_cmr);
	else
		gsm0503_tch_fr_encode(*bursts_p, msg_tch->l2h, msgb_l2len(msg_tch), 1);

	/* free message */
	if (msg_tch)
		msgb_free(msg_tch);
	if (msg_facch)
		msgb_free(msg_facch);

send_burst:
	/* compose burst */
	burst = *bursts_p + br->bid * 116;
	memcpy(br->burst + 3, burst, 58);
	memcpy(br->burst + 61, TRX_GMSK_NB_TSC(br), 26);
	memcpy(br->burst + 87, burst + 58, 58);

	br->burst_len = GSM_BURST_LEN;

	LOGL1SB(DL1P, LOGL_DEBUG, l1ts, br, "Transmitting burst=%u.\n", br->bid);

	return 0;
}