diff --git a/src/trau/trau_sync.c b/src/trau/trau_sync.c
new file mode 100644
index 0000000..c4baad6
--- /dev/null
+++ b/src/trau/trau_sync.c
@@ -0,0 +1,522 @@
+/* GSM A-bis TRAU frame synchronization as per TS 48.060 / 48.061 */
+
+/* (C) 2020 by Harald Welte <laforge@gnumonks.org>
+ * All Rights Reserved
+ *
+ * SPDX-License-Identifier: GPL-2.0+
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 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 General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#include <stdint.h>
+
+#include <osmocom/core/msgb.h>
+#include <osmocom/core/bits.h>
+#include <osmocom/core/fsm.h>
+
+#include "ubit_buf.h"
+#include <osmocom/trau/trau_sync.h>
+
+#define S(x)	(1 << (x))
+
+#define MAX_TRAU_BYTES 	40
+
+#define T_SYNC		1
+
+struct sync_pattern {
+	/* provided by user */
+	const char *name;				/*!< human-readable name */
+	const uint8_t byte_pattern[MAX_TRAU_BYTES];	/*!< bytes to match against */
+	const uint8_t byte_mask[MAX_TRAU_BYTES];	/*!< mask applied before matching */
+	uint8_t byte_len;				/*!< length of mask in bytes */
+
+	/* generated by code */
+	ubit_t ubit_pattern[MAX_TRAU_BYTES*8];		/*!< bits to match against */
+	ubit_t ubit_mask[MAX_TRAU_BYTES*8];		/*!< mask applied before matching */
+	uint8_t bitcount;				/*!< number of high bits in mask */
+};
+
+static struct sync_pattern sync_patterns[] = {
+	[OSMO_TRAU_SYNCP_16_FR_EFR] = {
+		/* TS 08.60 Section 4.8.1 */
+		.name = "FR/EFR",
+		.byte_pattern = {
+			0x00, 0x00, 0x80, 0x00, 0x80, 0x00, 0x80, 0x00,
+			0x80, 0x00, 0x80, 0x00, 0x80, 0x00, 0x80, 0x00,
+			0x80, 0x00, 0x80, 0x00, 0x80, 0x00, 0x80, 0x00,
+			0x80, 0x00, 0x80, 0x00, 0x80, 0x00, 0x80, 0x00,
+			0x80, 0x00, 0x80, 0x00, 0x80, 0x00, 0x80, 0x00,
+		},
+		.byte_mask = {
+			0xff, 0xff, 0x80, 0x00, 0x80, 0x00, 0x80, 0x00,
+			0x80, 0x00, 0x80, 0x00, 0x80, 0x00, 0x80, 0x00,
+			0x80, 0x00, 0x80, 0x00, 0x80, 0x00, 0x80, 0x00,
+			0x80, 0x00, 0x80, 0x00, 0x80, 0x00, 0x80, 0x00,
+			0x80, 0x00, 0x80, 0x00, 0x80, 0x00, 0x80, 0x00,
+		},
+		.byte_len = 40,
+	},
+	[OSMO_TRAU_SYNCP_8_HR] = {
+		/* TS 08.61 Section 6.8.2.1.1 */
+		.name = "HR8",
+		.byte_pattern = {
+			0x00, 0x80, 0x40, 0x80,
+			0x80, 0x80, 0x80, 0x80,
+			0x80, 0x80, 0x80, 0x80,
+			0x80, 0x80, 0x80, 0x80,
+			0x80, 0x80, 0x80, 0x80,
+		},
+		.byte_mask = {
+			0xff, 0x80, 0xC0, 0x80,
+			0x80, 0x80, 0x80, 0x80,
+			0x80, 0x80, 0x80, 0x80,
+			0x80, 0x80, 0x80, 0x80,
+			0x80, 0x80, 0x80, 0x80,
+		},
+		.byte_len = 20,
+	},
+	[OSMO_TRAU_SYNCP_8_AMR_LOW] = {
+		/* TS 08.61 Section 6.8.2.1.2 */
+		/* The frame synchronisation for No_Speech frames and the speech frames of the three lower codec modes */
+		.name = "AMR8_LOW",
+		.byte_pattern = {
+			0x00, 0x80, 0x80, 0x40,
+			0x80, 0x80, 0x80, 0x80,
+			0x80, 0x80, 0x80, 0x80,
+			0x80, 0x80, 0x80, 0x80,
+			0x80, 0x80, 0x80, 0x80,
+		},
+		.byte_mask = {
+			0xff, 0x80, 0x80, 0xC0,
+			0x80, 0x80, 0x80, 0x80,
+			0x80, 0x80, 0x80, 0x80,
+			0x80, 0x80, 0x80, 0x80,
+			0x80, 0x80, 0x80, 0x80,
+		},
+		.byte_len = 20,
+	},
+	[OSMO_TRAU_SYNCP_8_AMR_6K7] = {
+		/* The frame synchronisation for the speech frames for codec mode 6,70 kBit/s */
+		.name = "AMR8_67",
+		.byte_pattern = {
+			0x00, 0x80, 0x80, 0x80,
+			0x80, 0x00, 0x80, 0x00,
+			0x80, 0x00, 0x80, 0x00,
+			0x80, 0x00, 0x80, 0x00,
+			0x80, 0x00, 0x80, 0x00,
+		},
+		.byte_mask = {
+			0xff, 0x80, 0x80, 0x80,
+			0x80, 0x80, 0x80, 0x00,
+			0x80, 0x00, 0x80, 0x00,
+			0x80, 0x00, 0x80, 0x00,
+			0x80, 0x00, 0x80, 0x00,
+		},
+		.byte_len = 20
+	},
+	[OSMO_TRAU_SYNCP_8_AMR_7K4] = {
+		/* The frame synchronisation for the speech frames for codec mode 7,40 kBit/s */
+		.name = "AMR8_74",
+		.byte_pattern = {
+			0x20, 0x00, 0x80, 0x00,
+			0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00,
+		},
+		.byte_mask = {
+			0xe0, 0x80, 0x80, 0x80,
+			0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00,
+		},
+		.byte_len = 20,
+	},
+};
+
+#if 0
+static struct sync_pattern rbs_ccu_sync_ind_16_pattern = {
+	.name ="RBS_CCU_SYNC_IND_16",
+	.byte_pattern = {
+		0x00, 0x00, 0x01, 0x00,
+		0x00, 0x00, 0x01, 0x00,
+		0x01, 0x00, 0x01, 0x00,
+	},
+	.byte_mask = {
+		0xff, 0xff, 0x01, 0x00,
+		0x01, 0x00, 0x01, 0x00,
+		0x01, 0x00, 0x01, 0x00,
+	},
+};
+
+static struct sync_pattern rbs_ccu_data_ind_16_pattern = {
+	.name ="RBS_CCU_DATA_IND_16",
+	.byte_pattern = {
+		0x00, 0x00, 0x01, 0x00,
+	},
+	.byte_mask = {
+		0xff, 0xff, 0x01, 0x00,
+	},
+};
+#endif
+
+
+static void expand_sync_pattern(struct sync_pattern *pat)
+{
+	osmo_pbit2ubit(pat->ubit_pattern, pat->byte_pattern, pat->byte_len*8);
+	osmo_pbit2ubit(pat->ubit_mask, pat->byte_mask, pat->byte_len*8);
+}
+
+static unsigned int count_one_bits(const ubit_t *in, unsigned int in_bits)
+{
+	unsigned int i, count = 0;
+
+	for (i = 0; i < in_bits; i++) {
+		if (in[i])
+			count++;
+	}
+	return count;
+}
+
+static void sync_pattern_register(struct sync_pattern *p)
+{
+	expand_sync_pattern(p);
+	p->bitcount = count_one_bits(p->ubit_mask, p->byte_len*8);
+}
+
+#if 0
+/*! correlate pattern with unpacked bits from buffer.
+ *  \param[in] pattern sync_pattern against which we shall compare
+ *  \param[in] bits unpacked bits to compare against pattern
+ *  \param[in] num_bits number of unpacked bits
+ *  \returns number of bits not matching pattern; -1 if insufficient bits available. */
+static int correlate_pattern_ubits(const struct sync_pattern *pattern,
+				    const ubit_t *bits, size_t num_bits)
+{
+	int i, num_wrong = 0;
+
+	if (num_bits < pattern->byte_len*8)
+		return -1; /* insufficient data */
+
+	for (i = 0; i < pattern->byte_len *8; i++) {
+		/* if mask doesn't contain '1', we can skip this octet */
+		if (!pattern->ubit_mask)
+			continue;
+		if (bits[i] != pattern->ubit_pattern[i])
+			num_wrong++;
+	}
+
+	return num_wrong;
+}
+#endif
+
+struct trau_rx_sync_state {
+	/*! call-back to be called for every TRAU frame (called with
+	 * bits=NULL in case of frame sync loss */
+	frame_out_cb_t out_cb;
+	/*! opaque user data; passed to out_cb */
+	void *user_data;
+
+	/*! history of received bits */
+	ubit_t history[MAX_TRAU_BYTES*8+1]; /* +1 not required, but helps to expose bugs */
+	/*! index of next-to-be-written ubit in history */
+	unsigned int history_idx;
+	/*! the pattern we are trying to sync to */
+	const struct sync_pattern *pattern;
+	/*! number of consecutive frames without sync */
+	unsigned int num_consecutive_errors;
+};
+
+/* correlate the history (up to the last received bit) against the pattern */
+static int correlate_history_against_pattern(struct trau_rx_sync_state *tss)
+{
+	const struct sync_pattern *pattern = tss->pattern;
+	int i, start, num_wrong = 0;
+
+	/* compute index of first bit in history array */
+	start = (ARRAY_SIZE(tss->history) + tss->history_idx - pattern->byte_len*8)
+		% ARRAY_SIZE(tss->history);
+
+	OSMO_ASSERT(ARRAY_SIZE(tss->history) >= pattern->byte_len*8);
+
+	for (i = 0; i < pattern->byte_len*8; i++) {
+		unsigned int pos = (start + i) % ARRAY_SIZE(tss->history);
+
+		/* if mask doesn't contain '1', we can skip this octet */
+		if (!pattern->ubit_mask[i])
+			continue;
+		if (tss->history[pos] != pattern->ubit_pattern[i])
+			num_wrong++;
+	}
+
+	return num_wrong;
+}
+
+/* add (append) one ubit to the history; wrap as needed */
+static void rx_history_add_bit(struct trau_rx_sync_state *tss, ubit_t bit)
+{
+	tss->history[tss->history_idx] = bit;
+	/* simply wrap around at the end */
+	tss->history_idx = (tss->history_idx + 1) % ARRAY_SIZE(tss->history);
+}
+
+/* append bits to history. We assume that this does NOT wrap */
+static void rx_history_add_bits(struct trau_rx_sync_state *tss, const ubit_t *bits, size_t n_bits)
+{
+	unsigned int frame_bits_remaining = tss->pattern->byte_len*8 - tss->history_idx;
+	OSMO_ASSERT(frame_bits_remaining >= n_bits);
+	memcpy(&tss->history[tss->history_idx], bits, n_bits);
+	tss->history_idx = tss->history_idx + n_bits;
+}
+
+/* align the history, i.e. next received bit is start of frame */
+static void rx_history_align(struct trau_rx_sync_state *tss)
+{
+	ubit_t tmp[sizeof(tss->history)];
+	size_t history_size = sizeof(tss->history);
+	size_t pattern_bits = tss->pattern->byte_len*8;
+	size_t first_bit = (history_size + tss->history_idx - pattern_bits) % history_size;
+	int i;
+
+	/* we need to shift the last received frame to the start of the history buffer;
+	 * do this in two steps: First copy to a local buffer on the stack, using modulo-arithmetic
+	 * as index into the history.  Second, copy it back to history */
+
+	for (i = 0; i < pattern_bits; i++)
+		tmp[i] = tss->history[(first_bit + i) % history_size];
+
+	memcpy(tss->history, tmp, history_size);
+	tss->history_idx = 0;
+}
+
+enum trau_sync_state {
+	WAIT_FRAME_ALIGN,
+	FRAME_ALIGNED,
+	/* if at least 3 consecutive frames with each at least one framing error have been received */
+	FRAME_ALIGNMENT_LOST,
+};
+
+enum trau_sync_event {
+	TRAUSYNC_E_RESET,
+	/*! a buffer of bits was received (msgb with ubits) */
+	TRAUSYNC_E_RX_BITS,
+};
+
+static const struct value_string trau_sync_event_names[] = {
+	{ TRAUSYNC_E_RESET, "RESET" },
+	{ TRAUSYNC_E_RX_BITS, "RX_BITS" },
+	{ 0, NULL }
+};
+
+
+static void trau_sync_wait_align(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	struct trau_rx_sync_state *tss = (struct trau_rx_sync_state *) fi->priv;
+	struct ubit_buf *ubb;
+
+	switch (event) {
+	case TRAUSYNC_E_RX_BITS:
+		ubb = data;
+		/* append every bit individually + check if we have sync */
+		while (ubb_length(ubb) > 0) {
+			ubit_t bit = ubb_pull_ubit(ubb);
+			int rc;
+
+			rx_history_add_bit(tss, bit);
+			rc = correlate_history_against_pattern(tss);
+			if (!rc) {
+				osmo_fsm_inst_state_chg(fi, FRAME_ALIGNED, 0, 0);
+				/* treat remainder of input bits in correct state */
+				osmo_fsm_inst_dispatch(fi, TRAUSYNC_E_RX_BITS, ubb);
+				return;
+			}
+		}
+		break;
+	default:
+		OSMO_ASSERT(0);
+	}
+}
+
+static void trau_sync_aligned_onenter(struct osmo_fsm_inst *fi, uint32_t prev_state)
+{
+	struct trau_rx_sync_state *tss = (struct trau_rx_sync_state *) fi->priv;
+	/* dispatch aligned frame to user */
+	rx_history_align(tss);
+	tss->out_cb(tss->user_data, tss->history, tss->pattern->byte_len*8);
+}
+
+static void trau_sync_aligned(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	struct trau_rx_sync_state *tss = (struct trau_rx_sync_state *) fi->priv;
+	struct ubit_buf *ubb;
+	int rc;
+
+	switch (event) {
+	case TRAUSYNC_E_RX_BITS:
+		ubb = data;
+		while (ubb_length(ubb)) {
+			unsigned int frame_bits_remaining = tss->pattern->byte_len*8 - tss->history_idx;
+			if (ubb_length(ubb) < frame_bits_remaining) {
+				/* frame not filled by this message; just add data */
+				rx_history_add_bits(tss, ubb_data(ubb), ubb_length(ubb));
+				ubb_pull(ubb, ubb_length(ubb));
+			} else {
+				/* append as many bits as are missing in the current frame */
+				rx_history_add_bits(tss, ubb_data(ubb), frame_bits_remaining);
+				ubb_pull(ubb, frame_bits_remaining);
+
+				/* check if we still have frame sync */
+				rc = correlate_history_against_pattern(tss);
+				if (rc > 0) {
+					tss->num_consecutive_errors++;
+					if (tss->num_consecutive_errors >= 3) {
+						tss->history_idx = 0;
+						/* send NULL frame to user */
+						tss->out_cb(tss->user_data, NULL, 0);
+						osmo_fsm_inst_state_chg(fi, FRAME_ALIGNMENT_LOST, 1, T_SYNC);
+						osmo_fsm_inst_dispatch(fi, TRAUSYNC_E_RX_BITS, ubb);
+						return;
+					}
+				} else
+					tss->num_consecutive_errors = 0;
+
+				/* dispatch aligned frame to user */
+				tss->out_cb(tss->user_data, tss->history, tss->history_idx);
+				tss->history_idx = 0;
+			}
+		}
+		break;
+	default:
+		OSMO_ASSERT(0);
+	}
+}
+
+static void trau_sync_alignment_lost(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	/* we try to restore sync for some amount of time before generating an error */
+
+	switch (event) {
+	case TRAUSYNC_E_RX_BITS:
+		trau_sync_wait_align(fi, event, data);
+		break;
+	default:
+		OSMO_ASSERT(0);
+	}
+}
+
+static void trau_sync_allstate(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	switch (event) {
+	case TRAUSYNC_E_RESET:
+		osmo_fsm_inst_state_chg(fi, WAIT_FRAME_ALIGN, 0, 0);
+		break;
+	default:
+		OSMO_ASSERT(0);
+	}
+}
+
+static int trau_sync_timeout(struct osmo_fsm_inst *fi)
+{
+	switch (fi->T) {
+	case T_SYNC:
+		/* if Tsync expires before frame synchronization is
+		 * again obtained the TRAU initiates sending of the
+		 * urgent alarm pattern described in clause 4.10.2. */
+		osmo_fsm_inst_state_chg(fi, WAIT_FRAME_ALIGN, 0, 0);
+		break;
+	default:
+		OSMO_ASSERT(0);
+	}
+	return 0;
+}
+
+static const struct osmo_fsm_state trau_sync_states[] = {
+	[WAIT_FRAME_ALIGN] = {
+		.name = "WAIT_FRAME_ALIGN",
+		.in_event_mask = S(TRAUSYNC_E_RX_BITS),
+		.out_state_mask = S(FRAME_ALIGNED),
+		.action = trau_sync_wait_align,
+	},
+	[FRAME_ALIGNED] = {
+		.name = "FRAME_ALIGNED",
+		.in_event_mask = S(TRAUSYNC_E_RX_BITS),
+		.out_state_mask = S(FRAME_ALIGNMENT_LOST) | S(WAIT_FRAME_ALIGN),
+		.action = trau_sync_aligned,
+		.onenter = trau_sync_aligned_onenter,
+	},
+	[FRAME_ALIGNMENT_LOST] = {
+		.name = "FRAME_ALIGNMENT_LOST",
+		.in_event_mask = S(TRAUSYNC_E_RX_BITS),
+		.out_state_mask = S(WAIT_FRAME_ALIGN) | S(FRAME_ALIGNED),
+		.action = trau_sync_alignment_lost,
+	},
+};
+
+static struct osmo_fsm trau_sync_fsm = {
+	.name = "trau_sync",
+	.states = trau_sync_states,
+	.num_states = ARRAY_SIZE(trau_sync_states),
+	.allstate_event_mask = S(TRAUSYNC_E_RESET),
+	.allstate_action = trau_sync_allstate,
+	.timer_cb = trau_sync_timeout,
+	.log_subsys = DLGLOBAL,
+	.event_names = trau_sync_event_names,
+};
+
+
+struct osmo_fsm_inst *
+osmo_trau_sync_alloc(void *ctx, const char *name, frame_out_cb_t frame_out_cb,
+		     enum osmo_tray_sync_pat_id pat_id, void *user_data)
+{
+	struct trau_rx_sync_state *tss;
+	struct osmo_fsm_inst *fi;
+
+	if (pat_id >= ARRAY_SIZE(sync_patterns))
+		return NULL;
+
+	fi = osmo_fsm_inst_alloc(&trau_sync_fsm, ctx, NULL, LOGL_NOTICE, name);
+	if (!fi)
+		return NULL;
+	tss = talloc_zero(fi, struct trau_rx_sync_state);
+	if (!tss) {
+		osmo_fsm_inst_term(fi, OSMO_FSM_TERM_ERROR, NULL);
+		return NULL;
+	}
+	fi->priv = tss;
+
+	tss->out_cb = frame_out_cb;
+	tss->user_data = user_data;
+	/* FIXME: this must be configurable */
+	tss->pattern = &sync_patterns[pat_id];
+
+	return fi;
+}
+
+void osmo_trau_sync_rx_ubits(struct osmo_fsm_inst *fi, const ubit_t *bits, size_t n_bits)
+{
+	struct ubit_buf ubb;
+	ubb_init(&ubb, bits, n_bits);
+	osmo_fsm_inst_dispatch(fi, TRAUSYNC_E_RX_BITS, &ubb);
+}
+
+static void __attribute__((constructor)) on_dso_load_sync(void)
+{
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(sync_patterns); i++)
+		sync_pattern_register(&sync_patterns[i]);
+	osmo_fsm_register(&trau_sync_fsm);
+}