l1sap: proper rate adaptation for CSD (RFC4040 'clearmode')

Since 95407f3f osmo-bts-trx supports scheduling all CSD specific
channel rates, however the rate adaptation was missing.

On the radio interface we deal with CSD-modified V.110 frames, which
need to be converted to normal 80-bit V.110 frames (RA1'/RA1), which
in turn need to be batched and sent in RFC4040 "clearmode" 160 octet
RTP payloads (RA1/RA2 as per I.460).

Note that this patch comments out TCH/F14.4 in bts_supports_cm_data(),
so that all channel allocations for this mode would be NACKed.  The
reason for this is that the rate adaptation functions for TCH/F14.4
are different than the RA1'/RA1 and the RA1/RA2.

For more information, see:

* 3GPP TS 44.021, section 8 (functions RA1'/RA1)
* ITU-T I.460, section 1.1 "Rate adaption of 8, 16 and 32 kbit/s streams"

Change-Id: I5e3701ad52d5d428fd02caff037881045f2d0a02
Related: OS#1572

13 files changed, 475 insertions, 14 deletions

diff --git a/.gitignore b/.gitignore
index bf0f844c..34d55b8e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -55,6 +55,7 @@ src/osmo-bts-oc2g/misc/.dirstamp
 tests/atconfig
 tests/package.m4
 tests/amr/amr_test
+tests/csd/csd_test
 tests/agch/agch_test
 tests/paging/paging_test
 tests/cipher/cipher_test
diff --git a/configure.ac b/configure.ac
index 82074d46..b2b651ae 100644
--- a/configure.ac
+++ b/configure.ac
@@ -445,6 +445,7 @@ AC_OUTPUT(
     tests/power/Makefile
     tests/meas/Makefile
     tests/amr/Makefile
+    tests/csd/Makefile
     doc/Makefile
     doc/examples/Makefile
     doc/manuals/Makefile
diff --git a/include/osmo-bts/Makefile.am b/include/osmo-bts/Makefile.am
index 0b4058d2..cbd0fc37 100644
--- a/include/osmo-bts/Makefile.am
+++ b/include/osmo-bts/Makefile.am
@@ -24,6 +24,7 @@ noinst_HEADERS = \
 	tx_power.h \
 	control_if.h \
 	cbch.h \
+	csd_v110.h \
 	l1sap.h \
 	lchan.h \
 	power_control.h \
diff --git a/include/osmo-bts/csd_v110.h b/include/osmo-bts/csd_v110.h
new file mode 100644
index 00000000..f6be0ae2
--- /dev/null
+++ b/include/osmo-bts/csd_v110.h
@@ -0,0 +1,23 @@
+#pragma once
+
+/* RFC4040 "clearmode" RTP payload length */
+#define RFC4040_RTP_PLEN 160
+
+struct gsm_lchan;
+
+struct csd_v110_frame_desc {
+	uint16_t num_blocks;
+	uint16_t num_bits;
+};
+
+struct csd_v110_lchan_desc {
+	struct csd_v110_frame_desc fr;
+	struct csd_v110_frame_desc hr;
+};
+
+extern const struct csd_v110_lchan_desc csd_v110_lchan_desc[256];
+
+int csd_v110_rtp_encode(const struct gsm_lchan *lchan, uint8_t *rtp,
+			const uint8_t *data, size_t data_len);
+int csd_v110_rtp_decode(const struct gsm_lchan *lchan, uint8_t *data,
+			const uint8_t *rtp, size_t rtp_len);
diff --git a/src/common/Makefile.am b/src/common/Makefile.am
index 5cd7909e..d13415d1 100644
--- a/src/common/Makefile.am
+++ b/src/common/Makefile.am
@@ -51,6 +51,7 @@ libbts_a_SOURCES = \
 	bts_ctrl_commands.c \
 	bts_ctrl_lookup.c \
 	bts_shutdown_fsm.c \
+	csd_v110.c \
 	l1sap.c \
 	cbch.c \
 	power_control.c \
diff --git a/src/common/bts.c b/src/common/bts.c
index 7257f37b..1f0040c4 100644
--- a/src/common/bts.c
+++ b/src/common/bts.c
@@ -875,7 +875,7 @@ static bool bts_supports_cm_data(const struct gsm_bts *bts,
 	switch (bts->variant) {
 	case BTS_OSMO_TRX:
 		switch (cm->chan_rate) {
-		case RSL_CMOD_CSD_T_14k4:
+		/* TODO: RSL_CMOD_CSD_T_14k4 */
 		case RSL_CMOD_CSD_T_9k6:
 			if (cm->chan_rt != RSL_CMOD_CRT_TCH_Bm)
 				return false; /* invalid */
diff --git a/src/common/csd_v110.c b/src/common/csd_v110.c
new file mode 100644
index 00000000..6800761e
--- /dev/null
+++ b/src/common/csd_v110.c
@@ -0,0 +1,157 @@
+/*
+ * (C) 2023 by sysmocom - s.f.m.c. GmbH <info@sysmocom.de>
+ * Author: Vadim Yanitskiy <vyanitskiy@sysmocom.de>
+ *
+ * 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/utils.h>
+#include <osmocom/gsm/gsm44021.h>
+#include <osmocom/gsm/gsm_utils.h>
+#include <osmocom/gsm/protocol/gsm_04_08.h>
+#include <osmocom/isdn/v110.h>
+
+#include <osmo-bts/csd_v110.h>
+#include <osmo-bts/lchan.h>
+
+/* key is enum gsm48_chan_mode, so assuming a value in range 0..255 */
+const struct csd_v110_lchan_desc csd_v110_lchan_desc[256] = {
+#if 0
+	[GSM48_CMODE_DATA_14k5] = {
+		/* TCH/F14.4: 290 bits every 20 ms (14.5 kbit/s) */
+		.fr = { .num_blocks = 1, .num_bits = 290 },
+	},
+#endif
+	[GSM48_CMODE_DATA_12k0] = {
+		/* TCH/F9.6: 4 * 60 bits every 20 ms (12.0 kbit/s) */
+		.fr = { .num_blocks = 4, .num_bits = 60 },
+	},
+	[GSM48_CMODE_DATA_6k0] = {
+		/* TCH/F4.8: 2 * 60 bits every 20 ms (6.0 kbit/s) */
+		.fr = { .num_blocks = 2, .num_bits = 60 },
+		/* TCH/H4.8: 4 * 60 bits every 40 ms (6.0 kbit/s) */
+		.hr = { .num_blocks = 4, .num_bits = 60 },
+	},
+	[GSM48_CMODE_DATA_3k6] = {
+		/* TCH/F2.4: 2 * 36 bits every 20 ms (3.6 kbit/s) */
+		.fr = { .num_blocks = 2, .num_bits = 36 },
+		/* TCH/H2.4: 4 * 36 bits every 40 ms (3.6 kbit/s) */
+		.hr = { .num_blocks = 4, .num_bits = 36 },
+	},
+};
+
+int csd_v110_rtp_encode(const struct gsm_lchan *lchan, uint8_t *rtp,
+			const uint8_t *data, size_t data_len)
+{
+	const struct csd_v110_frame_desc *desc;
+	ubit_t ra_bits[80 * 4];
+
+	OSMO_ASSERT(lchan->tch_mode < ARRAY_SIZE(csd_v110_lchan_desc));
+	if (lchan->type == GSM_LCHAN_TCH_F)
+		desc = &csd_v110_lchan_desc[lchan->tch_mode].fr;
+	else
+		desc = &csd_v110_lchan_desc[lchan->tch_mode].hr;
+	if (OSMO_UNLIKELY(desc->num_blocks == 0))
+		return -ENOTSUP;
+
+	/* handle empty/incomplete frames gracefully */
+	if (OSMO_UNLIKELY(data_len < (desc->num_blocks * desc->num_bits)))
+		return -ENODATA;
+
+	/* RA1'/RA1: convert to an intermediate data rate */
+	for (unsigned int i = 0; i < desc->num_blocks; i++) {
+		struct osmo_v110_decoded_frame df;
+
+		/* convert modified V.110 frames to normal V.110 frames */
+		if (desc->num_bits == 60)
+			osmo_csd_12k_6k_decode_frame(&df, &data[i * 60], 60);
+		else /* desc->num_bits == 36 */
+			osmo_csd_3k6_decode_frame(&df, &data[i * 36], 36);
+
+		/* FIXME: E1 .. E3 must be set by out-of-band knowledge! */
+		memset(&df.e_bits[0], 0, 3);
+
+		osmo_v110_encode_frame(&ra_bits[i * 80], 80, &df);
+	}
+
+	/* RA1/RA2: convert from an intermediate rate to 64 kbit/s */
+	if (desc->num_blocks == 4) {
+		/* 4 * 80 bits (16 kbit/s) => 2 bits per octet */
+		for (unsigned int i = 0, j = 0; i < RFC4040_RTP_PLEN; i++) {
+			rtp[i]  = (0xff >> 2);
+			rtp[i] |= (ra_bits[j++] << 7);
+			rtp[i] |= (ra_bits[j++] << 6);
+		}
+	} else {
+		/* 2 * 80 bits (8 kbit/s) => 1 bit per octet */
+		for (unsigned int i = 0; i < RFC4040_RTP_PLEN; i++) {
+			rtp[i]  = (0xff >> 1);
+			rtp[i] |= (ra_bits[i] << 7);
+		}
+	}
+
+	return RFC4040_RTP_PLEN;
+}
+
+int csd_v110_rtp_decode(const struct gsm_lchan *lchan, uint8_t *data,
+			const uint8_t *rtp, size_t rtp_len)
+{
+	const struct csd_v110_frame_desc *desc;
+	ubit_t ra_bits[80 * 4];
+
+	OSMO_ASSERT(lchan->tch_mode < ARRAY_SIZE(csd_v110_lchan_desc));
+	if (lchan->type == GSM_LCHAN_TCH_F)
+		desc = &csd_v110_lchan_desc[lchan->tch_mode].fr;
+	else
+		desc = &csd_v110_lchan_desc[lchan->tch_mode].hr;
+	if (OSMO_UNLIKELY(desc->num_blocks == 0))
+		return -ENOTSUP;
+
+	if (OSMO_UNLIKELY(rtp_len != RFC4040_RTP_PLEN))
+		return -EINVAL;
+
+	/* RA1/RA2: convert from 64 kbit/s to an intermediate rate */
+	if (desc->num_blocks == 4) {
+		/* 4 * 80 bits (16 kbit/s) => 2 bits per octet */
+		for (unsigned int i = 0, j = 0; i < RFC4040_RTP_PLEN; i++) {
+			ra_bits[j++] = (rtp[i] >> 7);
+			ra_bits[j++] = (rtp[i] >> 6) & 0x01;
+		}
+	} else {
+		/* 2 * 80 bits (8 kbit/s) => 1 bit per octet */
+		for (unsigned int i = 0; i < RFC4040_RTP_PLEN; i++)
+			ra_bits[i] = (rtp[i] >> 7);
+	}
+
+	/* RA1'/RA1: convert to an intermediate data rate */
+	for (unsigned int i = 0; i < desc->num_blocks; i++) {
+		struct osmo_v110_decoded_frame df;
+
+		/* convert modified V.110 frames to normal V.110 frames */
+		osmo_v110_decode_frame(&df, &ra_bits[i * 80], 80);
+		if (desc->num_bits == 60)
+			osmo_csd_12k_6k_encode_frame(&data[i * 60], 60, &df);
+		else /* desc->num_bits == 36 */
+			osmo_csd_3k6_encode_frame(&data[i * 36], 36, &df);
+	}
+
+	return desc->num_blocks * desc->num_bits;
+}
diff --git a/src/common/l1sap.c b/src/common/l1sap.c
index 61fb3ee7..13d271ea 100644
--- a/src/common/l1sap.c
+++ b/src/common/l1sap.c
@@ -58,6 +58,7 @@
 #include <osmo-bts/pcuif_proto.h>
 #include <osmo-bts/cbch.h>
 #include <osmo-bts/asci.h>
+#include <osmo-bts/csd_v110.h>
 
 /* determine the CCCH block number based on the frame number */
 unsigned int l1sap_fn2ccch_block(uint32_t fn)
@@ -1829,9 +1830,27 @@ static int l1sap_ph_data_ind(struct gsm_bts_trx *trx,
 	return 1;
 }
 
+static void send_ul_rtp_packet_data(struct gsm_lchan *lchan, uint32_t fn,
+				    const uint8_t *data, uint16_t data_len)
+{
+	uint8_t rtp_pl[RFC4040_RTP_PLEN];
+	int rc;
+
+	rc = csd_v110_rtp_encode(lchan, &rtp_pl[0], data, data_len);
+	if (rc < 0)
+		return;
+
+	osmo_rtp_send_frame_ext(lchan->abis_ip.rtp_socket,
+				&rtp_pl[0], sizeof(rtp_pl),
+				fn_ms_adj(fn, lchan),
+				lchan->rtp_tx_marker);
+	/* Only clear the marker bit once we have sent a RTP packet with it */
+	lchan->rtp_tx_marker = false;
+}
+
 /* a helper function for the logic in l1sap_tch_ind() */
-static void send_ul_rtp_packet(struct gsm_lchan *lchan, uint32_t fn,
-			       const uint8_t *rtp_pl, uint16_t rtp_pl_len)
+static void send_ul_rtp_packet_speech(struct gsm_lchan *lchan, uint32_t fn,
+				      const uint8_t *rtp_pl, uint16_t rtp_pl_len)
 {
 	if (lchan->abis_ip.osmux.use) {
 		lchan_osmux_send_frame(lchan, rtp_pl, rtp_pl_len,
@@ -1859,7 +1878,7 @@ static void send_rtp_rfc5993(struct gsm_lchan *lchan, uint32_t fn,
 	else
 		toc = 0x00;
 	msgb_push_u8(msg, toc);
-	send_ul_rtp_packet(lchan, fn, msg->data, msg->len);
+	send_ul_rtp_packet_speech(lchan, fn, msg->data, msg->len);
 }
 
 /* A helper function for l1sap_tch_ind(): handling BFI
@@ -1884,7 +1903,7 @@ static void tch_ul_bfi_handler(struct gsm_lchan *lchan,
 		/* did it actually give us some output? */
 		if (rc > 0) {
 			/* yes, send it out in RTP */
-			send_ul_rtp_packet(lchan, fn, ecu_out, rc);
+			send_ul_rtp_packet_speech(lchan, fn, ecu_out, rc);
 			return;
 		}
 	}
@@ -1892,7 +1911,7 @@ static void tch_ul_bfi_handler(struct gsm_lchan *lchan,
 	/* Are we in rtp continuous-streaming special mode? If so, send out
 	 * a BFI packet as zero-length RTP payload. */
 	if (lchan->ts->trx->bts->rtp_nogaps_mode) {
-		send_ul_rtp_packet(lchan, fn, NULL, 0);
+		send_ul_rtp_packet_speech(lchan, fn, NULL, 0);
 		return;
 	}
 
@@ -1948,11 +1967,21 @@ static int l1sap_tch_ind(struct gsm_bts_trx *trx, struct osmo_phsap_prim *l1sap,
 		if (lchan->ecu_state)
 			osmo_ecu_frame_in(lchan->ecu_state, false, msg->data, msg->len);
 		/* hand msg to RTP code for transmission */
-		if (bts->emit_hr_rfc5993 && lchan->type == GSM_LCHAN_TCH_H &&
-		    lchan->tch_mode == GSM48_CMODE_SPEECH_V1)
-			send_rtp_rfc5993(lchan, fn, msg);
-		else
-			send_ul_rtp_packet(lchan, fn, msg->data, msg->len);
+		switch (lchan->rsl_cmode) {
+		case RSL_CMOD_SPD_SPEECH:
+			if (bts->emit_hr_rfc5993 && lchan->type == GSM_LCHAN_TCH_H &&
+			    lchan->tch_mode == GSM48_CMODE_SPEECH_V1)
+				send_rtp_rfc5993(lchan, fn, msg);
+			else
+				send_ul_rtp_packet_speech(lchan, fn, msg->data, msg->len);
+			break;
+		case RSL_CMOD_SPD_DATA:
+			send_ul_rtp_packet_data(lchan, fn, msg->data, msg->len);
+			break;
+		case RSL_CMOD_SPD_SIGN:
+		default: /* shall not happen */
+			OSMO_ASSERT(0);
+		}
 		/* if loopback is enabled, also queue received RTP data */
 		if (lchan->loopback) {
 			/* add new frame to queue, make sure the queue doesn't get too long */
@@ -2247,10 +2276,23 @@ void l1sap_rtp_rx_cb(struct osmo_rtp_socket *rs, const uint8_t *rtp_pl,
 		OSMO_ASSERT(0);
 	}
 
-	msg = l1sap_msgb_alloc(rtp_pl_len);
+	msg = l1sap_msgb_alloc(512);
 	if (!msg)
 		return;
-	memcpy(msgb_put(msg, rtp_pl_len), rtp_pl, rtp_pl_len);
+
+	if (lchan->rsl_cmode == RSL_CMOD_SPD_DATA) {
+		int rc = csd_v110_rtp_decode(lchan, msg->tail,
+					     rtp_pl, rtp_pl_len);
+		if (rc > 0) {
+			msgb_put(msg, rc);
+		} else {
+			msgb_free(msg);
+			return;
+		}
+	} else {
+		memcpy(msgb_put(msg, rtp_pl_len), rtp_pl, rtp_pl_len);
+	}
+
 	msgb_pull(msg, sizeof(struct osmo_phsap_prim));
 
 	/* Store RTP header Marker bit in control buffer */
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 100c5d48..8d175be8 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -1,4 +1,4 @@
-SUBDIRS = paging cipher agch misc handover tx_power power meas ta_control amr
+SUBDIRS = paging cipher agch misc handover tx_power power meas ta_control amr csd
 
 if ENABLE_SYSMOBTS
 SUBDIRS += sysmobts
diff --git a/tests/csd/Makefile.am b/tests/csd/Makefile.am
new file mode 100644
index 00000000..2041dd7d
--- /dev/null
+++ b/tests/csd/Makefile.am
@@ -0,0 +1,25 @@
+AM_CPPFLAGS = $(all_includes) -I$(top_srcdir)/include
+AM_CFLAGS = \
+	-Wall \
+	$(LIBOSMOCORE_CFLAGS) \
+	$(LIBOSMOGSM_CFLAGS) \
+	$(LIBOSMOCODEC_CFLAGS) \
+	$(LIBOSMOABIS_CFLAGS) \
+	$(LIBOSMOTRAU_CFLAGS) \
+	$(LIBOSMONETIF_CFLAGS) \
+	$(NULL)
+AM_LDFLAGS = -no-install
+LDADD = \
+	$(LIBOSMOCORE_LIBS) \
+	$(LIBOSMOGSM_LIBS) \
+	$(LIBOSMOCODEC_LIBS) \
+	$(LIBOSMOTRAU_LIBS) \
+	$(LIBOSMOABIS_LIBS) \
+	$(LIBOSMONETIF_LIBS) \
+	$(NULL)
+
+check_PROGRAMS = csd_test
+EXTRA_DIST = csd_test.err
+
+csd_test_SOURCES = csd_test.c
+csd_test_LDADD = $(top_builddir)/src/common/libbts.a $(LDADD)
diff --git a/tests/csd/csd_test.c b/tests/csd/csd_test.c
new file mode 100644
index 00000000..16995b1b
--- /dev/null
+++ b/tests/csd/csd_test.c
@@ -0,0 +1,138 @@
+/*
+ * (C) 2023 by sysmocom - s.f.m.c. GmbH <info@sysmocom.de>
+ * Author: Vadim Yanitskiy <vyanitskiy@sysmocom.de>
+ *
+ * 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/utils.h>
+#include <osmocom/gsm/gsm_utils.h>
+#include <osmocom/gsm/protocol/gsm_04_08.h>
+#include <osmocom/isdn/v110.h>
+
+#include <osmo-bts/csd_v110.h>
+#include <osmo-bts/lchan.h>
+
+#define BBUF_MAX 290
+
+struct test_case {
+	const char *name;
+	enum gsm_chan_t lchan_type;
+	enum gsm48_chan_mode tch_mode;
+};
+
+static const struct test_case tests[] = {
+	{
+		.name = "TCH/F14.4",
+		.lchan_type = GSM_LCHAN_TCH_F,
+		.tch_mode = GSM48_CMODE_DATA_14k5,
+	},
+	{
+		.name = "TCH/F9.6",
+		.lchan_type = GSM_LCHAN_TCH_F,
+		.tch_mode = GSM48_CMODE_DATA_12k0,
+	},
+	{
+		.name = "TCH/F4.8",
+		.lchan_type = GSM_LCHAN_TCH_F,
+		.tch_mode = GSM48_CMODE_DATA_6k0,
+	},
+	{
+		.name = "TCH/H4.8",
+		.lchan_type = GSM_LCHAN_TCH_H,
+		.tch_mode = GSM48_CMODE_DATA_6k0,
+	},
+	{
+		.name = "TCH/F2.4",
+		.lchan_type = GSM_LCHAN_TCH_F,
+		.tch_mode = GSM48_CMODE_DATA_3k6,
+	},
+	{
+		.name = "TCH/H2.4",
+		.lchan_type = GSM_LCHAN_TCH_H,
+		.tch_mode = GSM48_CMODE_DATA_3k6,
+	},
+};
+
+static void exec_test_case(const struct test_case *tc)
+{
+	const struct csd_v110_frame_desc *desc;
+	uint8_t rtp[RFC4040_RTP_PLEN] = { 0 };
+	ubit_t data_enc[BBUF_MAX];
+	ubit_t data_dec[BBUF_MAX];
+	int rc;
+
+	/* obtain a V.110 frame description for the given channel type/rate */
+	OSMO_ASSERT(tc->tch_mode < ARRAY_SIZE(csd_v110_lchan_desc));
+	if (tc->lchan_type == GSM_LCHAN_TCH_F)
+		desc = &csd_v110_lchan_desc[tc->tch_mode].fr;
+	else
+		desc = &csd_v110_lchan_desc[tc->tch_mode].hr;
+
+	/* total number of bits carried by a radio interface block */
+	const unsigned int bit_num = desc->num_bits * desc->num_blocks;
+	if (bit_num == 0) {
+		fprintf(stderr, "[i] Skipping '%s' (not implemented)\n", tc->name);
+		return;
+	}
+
+	fprintf(stderr, "[i] Testing '%s' (bitnum=%u)\n", tc->name, bit_num);
+
+	struct gsm_lchan lchan = {
+		.type = tc->lchan_type,
+		.tch_mode = tc->tch_mode,
+	};
+
+	/* populate the data_enc[] buffer with some bits */
+	OSMO_ASSERT(bit_num <= BBUF_MAX);
+	for (unsigned int i = 0; i < bit_num; i++)
+		data_enc[i] = i & 0x01;
+
+	/* encode an RTP frame and print it */
+	rc = csd_v110_rtp_encode(&lchan, &rtp[0], &data_enc[0], bit_num);
+	fprintf(stderr, "[i] csd_v110_rtp_encode() returns %d\n", rc);
+	if (rc != RFC4040_RTP_PLEN)
+		return;
+	/* print the encoded RTP frame (16 bytes per row) */
+	for (unsigned int i = 0; i < sizeof(rtp) / 16; i++)
+		fprintf(stderr, "    %s\n", osmo_hexdump(&rtp[i * 16], 16));
+
+	/* decode the encoded RTP frame */
+	rc = csd_v110_rtp_decode(&lchan, &data_dec[0], &rtp[0], sizeof(rtp));
+	fprintf(stderr, "[i] csd_v110_rtp_decode() returns %d\n", rc);
+	if (rc != bit_num)
+		return;
+	/* compare data_dec[] vs data_enc[] */
+	for (unsigned int i = 0; i < bit_num; i++) {
+		if (data_dec[i] == data_enc[i])
+			continue;
+		fprintf(stderr, "[!] Data mismatch @ %03u: D%u vs E%u\n",
+			i, data_dec[i], data_enc[i]);
+	}
+}
+
+int main(int argc, char **argv)
+{
+	for (unsigned int i = 0; i < ARRAY_SIZE(tests); i++)
+		exec_test_case(&tests[i]);
+
+	return 0;
+}
diff --git a/tests/csd/csd_test.err b/tests/csd/csd_test.err
new file mode 100644
index 00000000..bc9747cc
--- /dev/null
+++ b/tests/csd/csd_test.err
@@ -0,0 +1,66 @@
+[i] Skipping 'TCH/F14.4' (not implemented)
+[i] Testing 'TCH/F9.6' (bitnum=240)
+[i] csd_v110_rtp_encode() returns 160
+    3f 3f 3f 3f bf bf bf bf ff 7f 7f 7f bf bf bf bf 
+    ff 7f 7f 7f bf 3f 7f 7f bf bf bf bf ff 7f 7f 7f 
+    bf bf bf bf ff 7f 7f 7f 3f 3f 3f 3f bf bf bf bf 
+    ff 7f 7f 7f bf bf bf bf ff 7f 7f 7f bf 3f 7f 7f 
+    bf bf bf bf ff 7f 7f 7f bf bf bf bf ff 7f 7f 7f 
+    3f 3f 3f 3f bf bf bf bf ff 7f 7f 7f bf bf bf bf 
+    ff 7f 7f 7f bf 3f 7f 7f bf bf bf bf ff 7f 7f 7f 
+    bf bf bf bf ff 7f 7f 7f 3f 3f 3f 3f bf bf bf bf 
+    ff 7f 7f 7f bf bf bf bf ff 7f 7f 7f bf 3f 7f 7f 
+    bf bf bf bf ff 7f 7f 7f bf bf bf bf ff 7f 7f 7f 
+[i] csd_v110_rtp_decode() returns 240
+[i] Testing 'TCH/F4.8' (bitnum=120)
+[i] csd_v110_rtp_encode() returns 160
+    7f 7f 7f 7f 7f 7f 7f 7f ff 7f ff 7f ff 7f ff 7f 
+    ff ff 7f ff 7f ff 7f ff ff 7f ff 7f ff 7f ff 7f 
+    ff ff 7f ff 7f ff 7f ff ff 7f 7f 7f 7f ff 7f ff 
+    ff 7f ff 7f ff 7f ff 7f ff ff 7f ff 7f ff 7f ff 
+    ff 7f ff 7f ff 7f ff 7f ff ff 7f ff 7f ff 7f ff 
+    7f 7f 7f 7f 7f 7f 7f 7f ff 7f ff 7f ff 7f ff 7f 
+    ff ff 7f ff 7f ff 7f ff ff 7f ff 7f ff 7f ff 7f 
+    ff ff 7f ff 7f ff 7f ff ff 7f 7f 7f 7f ff 7f ff 
+    ff 7f ff 7f ff 7f ff 7f ff ff 7f ff 7f ff 7f ff 
+    ff 7f ff 7f ff 7f ff 7f ff ff 7f ff 7f ff 7f ff 
+[i] csd_v110_rtp_decode() returns 120
+[i] Testing 'TCH/H4.8' (bitnum=240)
+[i] csd_v110_rtp_encode() returns 160
+    3f 3f 3f 3f bf bf bf bf ff 7f 7f 7f bf bf bf bf 
+    ff 7f 7f 7f bf 3f 7f 7f bf bf bf bf ff 7f 7f 7f 
+    bf bf bf bf ff 7f 7f 7f 3f 3f 3f 3f bf bf bf bf 
+    ff 7f 7f 7f bf bf bf bf ff 7f 7f 7f bf 3f 7f 7f 
+    bf bf bf bf ff 7f 7f 7f bf bf bf bf ff 7f 7f 7f 
+    3f 3f 3f 3f bf bf bf bf ff 7f 7f 7f bf bf bf bf 
+    ff 7f 7f 7f bf 3f 7f 7f bf bf bf bf ff 7f 7f 7f 
+    bf bf bf bf ff 7f 7f 7f 3f 3f 3f 3f bf bf bf bf 
+    ff 7f 7f 7f bf bf bf bf ff 7f 7f 7f bf 3f 7f 7f 
+    bf bf bf bf ff 7f 7f 7f bf bf bf bf ff 7f 7f 7f 
+[i] csd_v110_rtp_decode() returns 240
+[i] Testing 'TCH/F2.4' (bitnum=72)
+[i] csd_v110_rtp_encode() returns 160
+    7f 7f 7f 7f 7f 7f 7f 7f ff 7f 7f ff ff 7f 7f ff 
+    ff 7f 7f ff ff 7f 7f ff ff 7f 7f ff ff 7f 7f ff 
+    ff 7f 7f ff ff 7f 7f ff ff 7f 7f 7f 7f ff 7f ff 
+    ff 7f 7f ff ff 7f 7f ff ff 7f 7f ff ff 7f 7f ff 
+    ff 7f 7f ff ff 7f 7f ff ff 7f 7f ff ff 7f 7f ff 
+    7f 7f 7f 7f 7f 7f 7f 7f ff 7f 7f ff ff 7f 7f ff 
+    ff 7f 7f ff ff 7f 7f ff ff 7f 7f ff ff 7f 7f ff 
+    ff 7f 7f ff ff 7f 7f ff ff 7f 7f 7f 7f ff 7f ff 
+    ff 7f 7f ff ff 7f 7f ff ff 7f 7f ff ff 7f 7f ff 
+    ff 7f 7f ff ff 7f 7f ff ff 7f 7f ff ff 7f 7f ff 
+[i] csd_v110_rtp_decode() returns 72
+[i] Testing 'TCH/H2.4' (bitnum=144)
+[i] csd_v110_rtp_encode() returns 160
+    3f 3f 3f 3f bf 7f bf 7f bf 7f bf 7f bf 7f bf 7f 
+    bf 7f bf 7f bf 3f 7f 7f bf 7f bf 7f bf 7f bf 7f 
+    bf 7f bf 7f bf 7f bf 7f 3f 3f 3f 3f bf 7f bf 7f 
+    bf 7f bf 7f bf 7f bf 7f bf 7f bf 7f bf 3f 7f 7f 
+    bf 7f bf 7f bf 7f bf 7f bf 7f bf 7f bf 7f bf 7f 
+    3f 3f 3f 3f bf 7f bf 7f bf 7f bf 7f bf 7f bf 7f 
+    bf 7f bf 7f bf 3f 7f 7f bf 7f bf 7f bf 7f bf 7f 
+    bf 7f bf 7f bf 7f bf 7f 3f 3f 3f 3f bf 7f bf 7f 
+    bf 7f bf 7f bf 7f bf 7f bf 7f bf 7f bf 3f 7f 7f 
+    bf 7f bf 7f bf 7f bf 7f bf 7f bf 7f bf 7f bf 7f 
+[i] csd_v110_rtp_decode() returns 144
diff --git a/tests/testsuite.at b/tests/testsuite.at
index 7e6ec4b8..a7012b18 100644
--- a/tests/testsuite.at
+++ b/tests/testsuite.at
@@ -70,3 +70,9 @@ cat $abs_srcdir/amr/amr_test.ok > expout
 cat $abs_srcdir/amr/amr_test.err > experr
 AT_CHECK([$abs_top_builddir/tests/amr/amr_test], [], [expout], [experr])
 AT_CLEANUP
+
+AT_SETUP([csd])
+AT_KEYWORDS([csd])
+cat $abs_srcdir/csd/csd_test.err > experr
+AT_CHECK([$abs_top_builddir/tests/csd/csd_test], [], [ignore], [experr])
+AT_CLEANUP