diff --git a/src/Makefile b/src/Makefile
new file mode 100644
index 0000000..441696f
--- /dev/null
+++ b/src/Makefile
@@ -0,0 +1,13 @@
+LDFLAGS+=-losmocore -losmogsm
+CFLAGS+=-Wall
+
+all: e1_test
+
+%.o: %.c
+	$(CC) $(CFLAGS) -o $@ -c $^
+
+e1_test: osmo_e1.o crc4itu.o e1_test.o
+	$(CC) $(LDFLAGS) -o $@ $^
+
+clean:
+	@rm *.o e1_test
diff --git a/src/crc4itu.c b/src/crc4itu.c
new file mode 100644
index 0000000..fbefac8
--- /dev/null
+++ b/src/crc4itu.c
@@ -0,0 +1,28 @@
+#include <stdint.h>
+
+static const uint8_t crc4_table_byte[256] = {
+	0x0, 0x7, 0xe, 0x9, 0x5, 0x2, 0xb, 0xc, 0xa, 0xd, 0x4, 0x3, 0xf, 0x8, 0x1, 0x6,
+	0xd, 0xa, 0x3, 0x4, 0x8, 0xf, 0x6, 0x1, 0x7, 0x0, 0x9, 0xe, 0x2, 0x5, 0xc, 0xb,
+	0x3, 0x4, 0xd, 0xa, 0x6, 0x1, 0x8, 0xf, 0x9, 0xe, 0x7, 0x0, 0xc, 0xb, 0x2, 0x5,
+	0xe, 0x9, 0x0, 0x7, 0xb, 0xc, 0x5, 0x2, 0x4, 0x3, 0xa, 0xd, 0x1, 0x6, 0xf, 0x8,
+	0x6, 0x1, 0x8, 0xf, 0x3, 0x4, 0xd, 0xa, 0xc, 0xb, 0x2, 0x5, 0x9, 0xe, 0x7, 0x0,
+	0xb, 0xc, 0x5, 0x2, 0xe, 0x9, 0x0, 0x7, 0x1, 0x6, 0xf, 0x8, 0x4, 0x3, 0xa, 0xd,
+	0x5, 0x2, 0xb, 0xc, 0x0, 0x7, 0xe, 0x9, 0xf, 0x8, 0x1, 0x6, 0xa, 0xd, 0x4, 0x3,
+	0x8, 0xf, 0x6, 0x1, 0xd, 0xa, 0x3, 0x4, 0x2, 0x5, 0xc, 0xb, 0x7, 0x0, 0x9, 0xe,
+	0xc, 0xb, 0x2, 0x5, 0x9, 0xe, 0x7, 0x0, 0x6, 0x1, 0x8, 0xf, 0x3, 0x4, 0xd, 0xa,
+	0x1, 0x6, 0xf, 0x8, 0x4, 0x3, 0xa, 0xd, 0xb, 0xc, 0x5, 0x2, 0xe, 0x9, 0x0, 0x7,
+	0xf, 0x8, 0x1, 0x6, 0xa, 0xd, 0x4, 0x3, 0x5, 0x2, 0xb, 0xc, 0x0, 0x7, 0xe, 0x9,
+	0x2, 0x5, 0xc, 0xb, 0x7, 0x0, 0x9, 0xe, 0x8, 0xf, 0x6, 0x1, 0xd, 0xa, 0x3, 0x4,
+	0xa, 0xd, 0x4, 0x3, 0xf, 0x8, 0x1, 0x6, 0x0, 0x7, 0xe, 0x9, 0x5, 0x2, 0xb, 0xc,
+	0x7, 0x0, 0x9, 0xe, 0x2, 0x5, 0xc, 0xb, 0xd, 0xa, 0x3, 0x4, 0x8, 0xf, 0x6, 0x1,
+	0x9, 0xe, 0x7, 0x0, 0xc, 0xb, 0x2, 0x5, 0x3, 0x4, 0xd, 0xa, 0x6, 0x1, 0x8, 0xf,
+	0x4, 0x3, 0xa, 0xd, 0x1, 0x6, 0xf, 0x8, 0xe, 0x9, 0x0, 0x7, 0xb, 0xc, 0x5, 0x2,
+};
+
+uint8_t crc4itu(uint8_t crc, const uint8_t *data, unsigned int len)
+{
+	crc &= 0xf;
+	while (len--)
+		crc = crc4_table_byte[crc ^ *data++];
+	return crc;
+}
diff --git a/src/crc4itu.h b/src/crc4itu.h
new file mode 100644
index 0000000..ba8e79a
--- /dev/null
+++ b/src/crc4itu.h
@@ -0,0 +1,3 @@
+#pragma once
+
+uint8_t crc4itu(uint8_t crc, const uint8_t *data, unsigned int len);
diff --git a/src/e1_test.c b/src/e1_test.c
new file mode 100644
index 0000000..5fd416f
--- /dev/null
+++ b/src/e1_test.c
@@ -0,0 +1,141 @@
+#include <stdint.h>
+#include <unistd.h>
+#include <errno.h>
+#include <stdio.h>
+#include <string.h>
+
+#include <osmocom/core/utils.h>
+#include <osmocom/core/logging.h>
+#include <osmocom/core/application.h>
+#include <osmocom/gsm/gsm_utils.h>
+
+#include "osmo_e1.h"
+
+static struct osmo_e1_instance inst;
+static struct log_info log_info = {};
+
+/* pull data out of the transmitter and print hexdumps */
+static void pull_and_print(struct osmo_e1_instance *e1i)
+{
+	uint8_t buf[32];
+	osmo_e1_pull_tx_frame(e1i, buf);
+	printf("%s\n", osmo_hexdump(buf, sizeof(buf)));
+}
+
+static void data_cb(struct osmo_e1_instance_ts *e1t, struct msgb *msg)
+{
+	printf("Rx TS %u: %s\n", e1t->ts_nr, msgb_hexdump(msg));
+	msgb_free(msg);
+}
+
+static void notify_cb(struct osmo_e1_instance *e1i, enum osmo_e1_notify_event evt,
+			bool present, void *data)
+{
+	printf("NOTIFY: %s %s\n", osmo_e1_notify_event_name(evt), present ? "PRESENT" : "ABSENT");
+}
+
+/* feed some random data into the E1 instance */
+static void tc_rx_random()
+{
+	uint8_t buf[32];
+	int i;
+
+	for (i = 0; i < 200; i++) {
+		osmo_get_rand_id(buf, sizeof(buf));
+		osmo_e1_rx_frame(&inst, buf);
+	}
+}
+
+static void tc_rx_align_basic()
+{
+	uint8_t buf[32];
+	int i;
+
+	for (i = 0; i < 80; i++) {
+		memset(buf, 0xff, sizeof(buf));
+		switch (i %2) {
+		case 0:
+			buf[0] = 0x9B;
+			break;
+		case 1:
+			buf[0] = 0x40;
+			break;
+		}
+		osmo_e1_rx_frame(&inst, buf);
+	}
+}
+
+static void tc_rx_align_mframe()
+{
+	uint8_t buf[32];
+	int i;
+
+	for (i = 0; i < 80; i++) {
+		memset(buf, 0xff, sizeof(buf));
+		switch (i % 16) {
+		case 0:
+		case 2:
+		case 4:
+		case 6:
+		case 8:
+		case 10:
+		case 12:
+		case 14:
+			buf[0] = 0x9B;
+			break;
+		case 1:
+		case 3:
+		case 7:
+		case 13:
+		case 15:
+			buf[0] = 0x40;
+			break;
+		case 5:
+		case 9:
+		case 11:
+			buf[0] = 0xc0;
+			break;
+		}
+		osmo_e1_rx_frame(&inst, buf);
+	}
+}
+
+
+static void tc_tx_idle()
+{
+	int i;
+	for (i = 0; i < 20; i++) {
+		pull_and_print(&inst);
+	}
+}
+
+int main(int argc, char **argv)
+{
+	int i;
+
+	osmo_init_logging2(NULL, &log_info);
+	osmo_e1_init();
+
+	osmo_e1_instance_init(&inst, "e1_test", &notify_cb, true, NULL);
+	for (i = 1; i < 32; i++) {
+		struct osmo_e1_instance_ts *e1t = osmo_e1_instance_ts(&inst, i);
+		osmo_e1_ts_config(e1t, &data_cb, 40, true);
+	}
+
+	printf("\nRx Random...\n");
+	osmo_e1_instance_reset(&inst);
+	tc_rx_random();
+
+	printf("\nAlign (Basic)...\n");
+	osmo_e1_instance_reset(&inst);
+	tc_rx_align_basic();
+
+	printf("\nAlign (Mframe)...\n");
+	osmo_e1_instance_reset(&inst);
+	tc_rx_align_mframe();
+
+	printf("\nTX Idle...\n");
+	osmo_e1_instance_reset(&inst);
+	tc_tx_idle();
+
+}
diff --git a/src/osmo_e1.c b/src/osmo_e1.c
new file mode 100644
index 0000000..2ad535e
--- /dev/null
+++ b/src/osmo_e1.c
@@ -0,0 +1,660 @@
+/* Osmocom Software Defined E1
+ *
+ * (C) 2018 by Harald Welte <laforge@gnumonks.org>
+ *
+ * Implements ITU-T Rec. G.704 Section 2.3
+ */
+
+#include <stdbool.h>
+#include <stdint.h>
+#include <unistd.h>
+#include <errno.h>
+#include <string.h>
+
+#include <osmocom/core/msgb.h>
+#include <osmocom/core/linuxlist.h>
+#include <osmocom/core/logging.h>
+#include <osmocom/core/fsm.h>
+
+#include "crc4itu.h"
+#include "osmo_e1.h"
+
+#define S(x)	(1 << (x))
+
+/* Frame Alignment Signal (BIT1 may be overwritten with CRC-4) */
+#define G704_E1_FAS	0x1B
+
+static inline bool is_correct_fas(uint8_t bt) {
+	if ((bt & 0x7F) == G704_E1_FAS)
+		return true;
+	else
+		return false;
+}
+
+/* are we in SMF II (true) or I (false) */
+static inline bool is_smf_II(const struct osmo_e1_tx_state *tx) {
+	if (tx->frame_nr >= 8)
+		return true;
+	return false;
+}
+
+static struct osmo_fsm e1_align_fsm;
+static void align_fsm_reset(struct osmo_e1_instance *e1i);
+
+static void notify_user(struct osmo_e1_instance *e1i, enum osmo_e1_notify_event evt,
+			bool present, void *priv)
+{
+	if (!e1i->notify_cb)
+		return;
+	e1i->notify_cb(e1i, evt, present, priv);
+}
+
+/*! Initialize a (caller-allocated) Osmocom E1 Instance
+ *  \param[inout] e1i E1 Instance to be initialized
+ *  \returns 0 on success, negative on error */
+int osmo_e1_instance_init(struct osmo_e1_instance *e1i, const char *name, e1_notify_cb cb,
+			  bool crc4_enabled, void *priv)
+{
+	int i;
+
+	e1i->crc4_enabled = crc4_enabled;
+	e1i->notify_cb = cb;
+	e1i->tx.sa4_sa8 = 0x00;
+
+	e1i->priv = priv;
+
+	for (i = 1; i < ARRAY_SIZE(e1i->ts); i++) {
+		struct osmo_e1_instance_ts *e1t = &e1i->ts[i];
+		e1t->ts_nr = i;
+		e1t->inst = e1i;
+		INIT_LLIST_HEAD(&e1t->tx.queue);
+
+		e1t->rx.granularity = 256;
+	}
+
+	e1i->rx.fi = osmo_fsm_inst_alloc(&e1_align_fsm, NULL, e1i, LOGL_NOTICE, name);
+	if (!e1i->rx.fi)
+		return -1;
+
+	osmo_e1_instance_reset(e1i);
+
+	return 0;
+}
+
+/*! stop E1 timeslot; release any pending rx/tx buffers
+ *  \param[in] e1t Timeslot which we are to stop, disable and release buffers */
+void osmo_e1_ts_reset(struct osmo_e1_instance_ts *e1t)
+{
+	e1t->tx.underruns = 0;
+	msgb_queue_free(&e1t->tx.queue);
+
+	e1t->rx.enabled = false;
+	msgb_free(e1t->rx.msg);
+	e1t->rx.msg = NULL;
+}
+
+/*! stop E1 instance; stops all timeslots and releases any pending rx/tx buffers
+ *  \param[in] e1t E1 instance which we are to stop */
+void osmo_e1_instance_reset(struct osmo_e1_instance *e1i)
+{
+	int i;
+
+	align_fsm_reset(e1i);
+
+	e1i->tx.remote_alarm = false;
+	e1i->tx.crc4_error = false;
+	e1i->tx.frame_nr = 0;
+	e1i->tx.crc4_last_smf = 0;
+	e1i->tx.crc4 = 0;
+
+	e1i->rx.frame_nr = 0;
+	memset(&e1i->rx.ts0_history, 0, sizeof(e1i->rx.ts0_history));
+	e1i->rx.ts0_hist_len = 0;
+	e1i->rx.remote_alarm = false;
+	e1i->rx.remote_crc4_error = false;
+	e1i->rx.num_ts0_in_mframe_search = 0;
+
+	for (i = 1; i < ARRAY_SIZE(e1i->ts); i++) {
+		struct osmo_e1_instance_ts *e1t = &e1i->ts[i];
+		osmo_e1_ts_reset(e1t);
+	}
+}
+
+/*! obtain pointer to TS given by instance + timeslot number
+ *  \param[in] e1i E1 intance on which we work
+ *  \param[in] ts_nr E1 timeslot number (1..31)
+ *  \returns pointer to timeslot; NULL on error */
+struct osmo_e1_instance_ts *osmo_e1_instance_ts(struct osmo_e1_instance *e1i, uint8_t ts_nr)
+{
+	if (ts_nr == 0 || ts_nr >= ARRAY_SIZE(e1i->ts))
+		return NULL;
+
+	return &e1i->ts[ts_nr];
+}
+
+/*! configure an E1 timeslot
+ *  \param[in] e1t Timeslot which we are to configure
+ *  \param[in] granularity granularity (buffer size) to use on Rx
+ *  \param[in] enable enable (true) or disalble (false) receiving on this TS
+ *  \return 0 on success; negative on error */
+int osmo_e1_ts_config(struct osmo_e1_instance_ts *e1t, e1_data_cb cb, unsigned int granularity,
+		      bool enable)
+{
+	e1t->rx.data_cb = cb;
+	e1t->rx.enabled = enable;
+	e1t->rx.granularity = granularity;
+
+	return 0;
+}
+
+const struct value_string osmo_e1_notifv_evt_names[] = {
+	{ E1_NTFY_EVT_ALIGN_FRAME, "Aligned to Frame" },
+	{ E1_NTFY_EVT_ALIGN_CRC_MFRAME, "Aligned to CRC4-Multiframe" },
+	{ E1_NTFY_EVT_CRC_ERROR, "CRC Error detected (local)" },
+	{ E1_NTFY_EVT_REMOTE_CRC_ERROR, "CRC Error reported (remote)" },
+	{ E1_NTFY_EVT_REMOTE_ALARM, "Remote Alarm condition repoorted" },
+	{ 0, NULL }
+};
+
+/***********************************************************************
+ * Transmit Side
+ ***********************************************************************/
+
+/*! Enqueue a message buffer of to-be-transmitted data for a timeslot
+ *  \param[in] e1i E1 instance for which to enqueue
+ *  \param[in] ts_nr Timeslot number on which data is to be transmitted
+ *  \param[in] msg Message buffer storing the to-be-transmitted data
+ *  \returns 0 on success; negative in case of error.
+ *
+ *  Ownership of \a msg is transferred from caller into this function, but only
+ *  in case of successful execution (return 0)!
+ */
+void osmo_e1_ts_enqueue(struct osmo_e1_instance_ts *e1t, struct msgb *msg)
+{
+	msgb_enqueue(&e1t->tx.queue, msg);
+}
+
+/* obtain a CRC4 bit for the current frame number */
+static uint8_t e1_pull_crc4_bit(struct osmo_e1_instance *e1i)
+{
+	/* If CRC-4 is disabled, all CRC bits shall be '1' */
+	if (e1i->crc4_enabled == 0) {
+		return 0x01;
+	} else {
+		/* CRC is transmitted MSB first */
+		switch (e1i->tx.frame_nr % 8) {
+		case 0:
+			return (e1i->tx.crc4_last_smf >> 3) & 1;
+		case 2:
+			return (e1i->tx.crc4_last_smf >> 2) & 1;
+		case 4:
+			return (e1i->tx.crc4_last_smf >> 1) & 1;
+		case 6:
+			return (e1i->tx.crc4_last_smf >> 0) & 1;
+		default:
+			OSMO_ASSERT(0);
+		}
+	}
+}
+
+/* pull a single to-be-transmitted byte for TS0 */
+static uint8_t e1_pull_ts0(struct osmo_e1_instance *e1i)
+{
+	uint8_t ret;
+
+	/* according to Table 5B/G.704 - CRC-4 multiframe structure */
+	if ((e1i->tx.frame_nr % 2) == 0) {
+		/* FAS */
+		ret = G704_E1_FAS | (e1_pull_crc4_bit(e1i) << 7);
+	} else {
+		switch (e1i->tx.frame_nr) {
+		case 1:
+		case 3:
+		case 7:
+			ret = 0x40;
+			break;
+		case 5:
+		case 9:
+		case 11:
+			ret = 0xC0;
+			break;
+		case 13:
+		case 15:
+			ret = 0x40;
+			if (e1i->tx.crc4_error)
+				ret |= 0x80;
+			break;
+		}
+		ret |= e1i->tx.sa4_sa8;
+		if (e1i->tx.remote_alarm)
+			ret |= 0x20;
+	}
+
+	/* re-set CRC4 at start of sub-multiframe */
+	if (e1i->tx.frame_nr == 0 || e1i->tx.frame_nr == 8) {
+		e1i->tx.crc4_last_smf = e1i->tx.crc4;
+		e1i->tx.crc4 = 0;
+	}
+
+	/* increment frame number modulo 16 */
+	e1i->tx.frame_nr = (e1i->tx.frame_nr + 1) % 16;
+
+	return ret;
+}
+
+/* pull a single to-be-transmitted byte for TS1..31 */
+static uint8_t e1_pull_tsN(struct osmo_e1_instance_ts *e1t)
+{
+	struct msgb *msg = llist_first_entry_or_null(&e1t->tx.queue, struct msgb, list);
+	uint8_t *cur;
+
+retry:
+	/* if there's no message to transmit */
+	if (!msg) {
+		e1t->tx.underruns++;
+		return 0xFF;
+	}
+	if (msgb_length(msg) <= 0) {
+		llist_del(&msg->list);
+		msgb_free(msg);
+		msg = llist_first_entry_or_null(&e1t->tx.queue, struct msgb, list);
+		goto retry;
+	}
+	cur = msgb_pull(msg, 1);
+	return *cur;
+}
+
+/* update the current in-progress CRC4 value with data from \a out_frame */
+static void e1_tx_update_crc4(struct osmo_e1_instance *e1i, const uint8_t *out_frame)
+{
+	uint8_t ts0;
+
+	ts0 = out_frame[0];
+	/* mask off the C bits */
+	if (is_correct_fas(ts0))
+		ts0 &= 0x7F;
+	e1i->tx.crc4 = crc4itu(e1i->tx.crc4, &ts0, 1);
+	/* add the remaining bytes/bits */
+	e1i->tx.crc4 = crc4itu(e1i->tx.crc4, out_frame+1, ARRAY_SIZE(e1i->ts)-1);
+}
+
+/*! Pull one to-be-transmitted E1 frame (256bits) from the E1 instance
+ *  \param e1i E1 instance for which the frame shall be generated
+ *  \param[out] out_frame callee-allocated buffer to which function stores 32 bytes
+ *  \returns 0 on success, negative on error */
+int osmo_e1_pull_tx_frame(struct osmo_e1_instance *e1i, uint8_t *out_frame)
+{
+	int i;
+
+	/* generate TS0 */
+	out_frame[0] = e1_pull_ts0(e1i);
+
+	/* generate TS1..31 */
+	for (i = 1; i < ARRAY_SIZE(e1i->ts); i++) {
+		struct osmo_e1_instance_ts *e1t = &e1i->ts[i];
+		/* get next to-be-transmitted byte from the TS */
+		out_frame[i] = e1_pull_tsN(e1t);
+	}
+	/* update our CRC4 computation */
+	e1_tx_update_crc4(e1i, out_frame);
+
+	return 0;
+}
+
+/***********************************************************************
+ * Receiver Side
+ ***********************************************************************/
+
+/* According to Figure 2 / ITU-T G.706 */
+enum e1_align_state {
+	/* Frame Alignment Search */
+	E1_AS_SEARCH_FRAME,
+	/* CRC multiframe alignment search */
+	E1_AS_SEARCH_CRC_MFRAME,
+	/* monitoring for incorrect frame alignment and error performance using CRC */
+	E1_AS_ALIGNED_CRC_MFRAME,
+	/* no CRC: just frame alignment loss check */
+	E1_AS_ALIGNED_BASIC,
+};
+
+enum e1_align_event {
+	/* received a TS0 octet */
+	E1_AE_RX_TS0,
+	E1_AE_RESET
+};
+
+static const struct value_string e1_align_evt_names[] = {
+	{ E1_AE_RX_TS0, "E1_AE_RX_TS0" },
+	{ E1_AE_RESET, "E1_AE_RESET" },
+	{ 0, NULL }
+};
+
+/* get a TS0 byte from the history. delta 0 == current, delte 1 == previous, ... */
+static uint8_t get_ts0_hist(struct osmo_e1_instance *e1i, uint8_t delta)
+{
+	return e1i->rx.ts0_history[((e1i->rx.frame_nr + 16)-delta) % 16];
+}
+
+/* ITU-T G.706 Section 4.1.1 */
+static bool frame_alignment_lost(struct osmo_e1_instance *e1i)
+{
+	if (e1i->rx.frame_nr % 2)
+		return false;
+
+	/* Frame alignment will be assumed to have been lost when three consecutive incorrect
+	 * frame alignment signals have been received. */
+	if (!is_correct_fas(get_ts0_hist(e1i, 0)) &&
+	    !is_correct_fas(get_ts0_hist(e1i, 2)) &&
+	    !is_correct_fas(get_ts0_hist(e1i, 4)))
+		return true;
+	else
+		return false;
+}
+
+/* ITU-T G.706 Section 4.1.2 */
+static bool frame_alignment_recovered(struct osmo_e1_instance *e1i)
+{
+	/* two consecutive FAS with one non-FAS interspersed */
+	if (is_correct_fas(get_ts0_hist(e1i, 0)) &&
+	    !is_correct_fas(get_ts0_hist(e1i, 1)) &&
+	    is_correct_fas(get_ts0_hist(e1i, 2)))
+		return true;
+	else
+		return false;
+}
+
+/* ITU-T G.706 Section 4.2 */
+static bool crc_mframe_alignment_achieved(struct osmo_e1_instance *e1i)
+{
+	/* if current TS0 byte is FAS, we cannot detect alignment */
+	if (is_correct_fas(get_ts0_hist(e1i, 0)))
+		return false;
+	if ((get_ts0_hist(e1i, 0) >> 7) == 1 &&
+	    (get_ts0_hist(e1i, 2) >> 7) == 1 &&
+	    (get_ts0_hist(e1i, 4) >> 7) == 0 &&
+	    (get_ts0_hist(e1i, 6) >> 7) == 1 &&
+	    (get_ts0_hist(e1i, 8) >> 7) == 0 &&
+	    (get_ts0_hist(e1i, 10) >> 7) == 0)
+		return true;
+	else
+		return false;
+}
+
+/* Get the CRC4 that was received from our Rx TS0 history */
+static uint8_t crc4_from_ts0_hist(struct osmo_e1_instance *e1i, bool smf2)
+{
+	uint8_t crc = 0;
+	uint8_t offset = 0;
+
+	if (smf2)
+		offset = 8;
+
+	crc |= (e1i->rx.ts0_history[0+offset] >> 7) << 3;
+	crc |= (e1i->rx.ts0_history[2+offset] >> 7) << 2;
+	crc |= (e1i->rx.ts0_history[4+offset] >> 7) << 1;
+	crc |= (e1i->rx.ts0_history[6+offset] >> 7) << 0;
+
+	return crc;
+}
+
+/* update the current in-progress CRC4 value with data from \a rx_frame */
+static void e1_rx_update_crc4(struct osmo_e1_instance *e1i, const uint8_t *rx_frame)
+{
+	uint8_t ts0;
+
+	ts0 = rx_frame[0];
+	/* mask off the C bits */
+	if (is_correct_fas(ts0))
+		ts0 &= 0x7F;
+	e1i->rx.crc4 = crc4itu(e1i->rx.crc4, &ts0, 1);
+	/* add the remaining bytes/bits */
+	e1i->rx.crc4 = crc4itu(e1i->rx.crc4, rx_frame+1, ARRAY_SIZE(e1i->ts)-1);
+}
+
+/* FSM State handler */
+static void e1_align_search_frame(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	struct osmo_e1_instance *e1i = (struct osmo_e1_instance *) fi->priv;
+
+	if (frame_alignment_recovered(e1i)) {
+		/* if we detected the 2nd FAS, we must be in FN 2 (or at least FN%2=0 */
+		e1i->rx.frame_nr = 2;
+		notify_user(e1i, E1_NTFY_EVT_ALIGN_FRAME, true, NULL);
+		osmo_fsm_inst_state_chg(fi, E1_AS_SEARCH_CRC_MFRAME, 0, 0);
+	}
+}
+
+/* FSM State handler */
+static void e1_align_search_crc_mframe(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	struct osmo_e1_instance *e1i = (struct osmo_e1_instance *) fi->priv;
+
+	if (crc_mframe_alignment_achieved(e1i)) {
+		/* if we detected the 6-bit CRC multiframe signal, we must be in FN 11 */
+		e1i->rx.frame_nr = 11;
+		/* FIXME: "at least two valid CRC multiframe alignment signals can be located within
+		 * 8 ms, the time separating two CRC multiframe alignment signals being 2 ms or a
+		 * multiple of 2 ms" */
+		notify_user(e1i, E1_NTFY_EVT_ALIGN_CRC_MFRAME, true, NULL);
+		osmo_fsm_inst_state_chg(fi, E1_AS_ALIGNED_CRC_MFRAME, 0, 0);
+	} else {
+		/* if no mframe alignment is established within 8ms (64 frames), fall back */
+		if (e1i->rx.num_ts0_in_mframe_search >= 64) {
+			e1i->rx.num_ts0_in_mframe_search = 0;
+			osmo_fsm_inst_state_chg(fi, E1_AS_SEARCH_FRAME, 0, 0);
+		}
+		e1i->rx.num_ts0_in_mframe_search++;
+	}
+}
+
+static void e1_aligned_common(struct osmo_e1_instance *e1i)
+{
+	uint8_t inb = get_ts0_hist(e1i, 0);
+
+	/* All non-FAS frames contain "A" bit in TS0 */
+	if (!is_correct_fas(inb & 0x7F)) {
+		bool old_alarm = e1i->rx.remote_alarm;
+		/* frame not containing the frame alignment signal */
+		if (inb & 0x20)
+			e1i->rx.remote_alarm = true;
+		else
+			e1i->rx.remote_alarm = false;
+		if (old_alarm != e1i->rx.remote_alarm)
+			notify_user(e1i, E1_NTFY_EVT_REMOTE_ALARM, e1i->rx.remote_alarm, NULL);
+	}
+}
+
+/* FSM State handler */
+static void e1_aligned_crc_mframe(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	struct osmo_e1_instance *e1i = (struct osmo_e1_instance *) fi->priv;
+
+	if (frame_alignment_lost(e1i)) {
+		osmo_fsm_inst_state_chg(fi, E1_AS_SEARCH_FRAME, 0, 0);
+		return;
+	}
+
+	if (e1i->crc4_enabled) {
+		uint8_t crc_rx;
+		bool crc4_error;
+
+		/* check if we just received a complete CRC4 */
+		switch (e1i->rx.frame_nr) {
+		case 7:
+		case 15:
+			crc_rx = crc4_from_ts0_hist(e1i, e1i->rx.frame_nr == 15 ? true : false);
+			if (crc_rx != e1i->rx.crc4_last_smf)
+				crc4_error = true;
+			else
+				crc4_error = false;
+			if (crc4_error != e1i->tx.crc4_error) {
+				notify_user(e1i, E1_NTFY_EVT_CRC_ERROR, crc4_error, NULL);
+				e1i->tx.crc4_error = crc4_error;
+			}
+			/* rotate computed CRC4 one further */
+			e1i->rx.crc4_last_smf = e1i->rx.crc4;
+			e1i->rx.crc4 = 0;
+			break;
+		default:
+			break;
+		}
+
+		/* check if the remote side reports any CRC errors */
+		switch (e1i->rx.frame_nr) {
+		case 13:
+		case 15:
+			crc4_error = false;
+			if ((get_ts0_hist(e1i, 0) >> 7) == 0)
+				crc4_error = true;
+			if (crc4_error != e1i->rx.remote_crc4_error) {
+				notify_user(e1i, E1_NTFY_EVT_REMOTE_CRC_ERROR, crc4_error, NULL);
+				e1i->rx.remote_crc4_error = crc4_error;
+			}
+			break;
+		}
+	}
+
+	e1_aligned_common(e1i);
+}
+
+/* FSM State handler */
+static void e1_aligned_basic(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	struct osmo_e1_instance *e1i = (struct osmo_e1_instance *) fi->priv;
+
+	if (frame_alignment_lost(e1i)) {
+		osmo_fsm_inst_state_chg(fi, E1_AS_SEARCH_FRAME, 0, 0);
+		return;
+	}
+
+	e1_aligned_common(e1i);
+}
+
+static const struct osmo_fsm_state e1_align_states[] = {
+	[E1_AS_SEARCH_FRAME] = {
+		.name = "SEARCH_FRAME",
+		.in_event_mask = S(E1_AE_RX_TS0),
+		.out_state_mask = S(E1_AS_SEARCH_FRAME) |
+				  S(E1_AS_SEARCH_CRC_MFRAME) |
+				  S(E1_AS_ALIGNED_BASIC),
+		.action = e1_align_search_frame,
+	},
+	[E1_AS_SEARCH_CRC_MFRAME] = {
+		.name = "SEARCH_CRC_MFRAME",
+		.in_event_mask = S(E1_AE_RX_TS0),
+		.out_state_mask = S(E1_AS_SEARCH_FRAME) |
+				  S(E1_AS_SEARCH_CRC_MFRAME) |
+				  S(E1_AS_ALIGNED_CRC_MFRAME),
+		.action = e1_align_search_crc_mframe,
+	},
+	[E1_AS_ALIGNED_CRC_MFRAME] = {
+		.name = "ALIGNED_CRC_MFRAME",
+		.in_event_mask = S(E1_AE_RX_TS0),
+		.out_state_mask = S(E1_AS_SEARCH_FRAME) |
+				  S(E1_AS_SEARCH_CRC_MFRAME) |
+				  S(E1_AS_ALIGNED_CRC_MFRAME),
+		.action = e1_aligned_crc_mframe,
+	},
+	[E1_AS_ALIGNED_BASIC] = {
+		.name = "ALIGNED_BASIC",
+		.in_event_mask = S(E1_AE_RX_TS0),
+		.out_state_mask = S(E1_AS_SEARCH_FRAME),
+		.action = e1_aligned_basic,
+	},
+};
+
+static void e1_allstate(struct osmo_fsm_inst *fi, uint32_t event, void *data)
+{
+	struct osmo_e1_instance *e1i = (struct osmo_e1_instance *) fi->priv;
+
+	switch (event) {
+	case E1_AE_RESET:
+		e1i->rx.num_ts0_in_mframe_search = 0;
+		osmo_fsm_inst_state_chg(fi, E1_AS_SEARCH_FRAME, 0, 0);
+		break;
+	}
+}
+
+static struct osmo_fsm e1_align_fsm = {
+	.name = "e1-align",
+	.states = e1_align_states,
+	.num_states = ARRAY_SIZE(e1_align_states),
+	.allstate_event_mask = S(E1_AE_RESET),
+	.allstate_action = e1_allstate,
+	.log_subsys = DLGLOBAL,
+	.event_names = e1_align_evt_names,
+};
+
+static void align_fsm_reset(struct osmo_e1_instance *e1i)
+{
+	osmo_fsm_inst_dispatch(e1i->rx.fi, E1_AE_RESET, NULL);
+}
+
+static void e1_rx_hist_add(struct osmo_e1_instance *e1i, uint8_t inb)
+{
+	e1i->rx.ts0_history[e1i->rx.frame_nr] = inb;
+	if (e1i->rx.ts0_hist_len < 16)
+		e1i->rx.ts0_hist_len++;
+}
+
+static void e1_rx_ts0(struct osmo_e1_instance *e1i, uint8_t inb)
+{
+	/* append just-received byte to the TS0 receive history buffer */
+	e1_rx_hist_add(e1i, inb);
+
+	/* notify the FSM that a new TS0 byte was received */
+	osmo_fsm_inst_dispatch(e1i->rx.fi, E1_AE_RX_TS0, NULL);
+
+	e1i->rx.frame_nr = (e1i->rx.frame_nr + 1) % 16;
+}
+
+static void e1_rx_tsN(struct osmo_e1_instance_ts *e1t, uint8_t inb)
+{
+	struct msgb *msg;
+
+	if (!e1t->rx.enabled)
+		return;
+
+	if (!e1t->rx.msg)
+		e1t->rx.msg = msgb_alloc(e1t->rx.granularity, "E1 Rx");
+	msg = e1t->rx.msg;
+	OSMO_ASSERT(msg);
+
+	msgb_put_u8(msg, inb);
+	if (msgb_tailroom(msg) <= 0) {
+		if (!e1t->rx.data_cb)
+			msgb_free(msg);
+		else
+			e1t->rx.data_cb(e1t, msg);
+		e1t->rx.msg = NULL;
+	}
+}
+
+/*! Receive a single E1 frame of 32x8 (=256) bits
+ *  \param e1i E1 instance for which the frame was received
+ *  \param[in] in_frame caller-provided buffer of 32 octets
+ *
+ *  The idea is that whoever calls us will already have done the bit-alignment,
+ *  i.e. the first bit of TS0 of the frame will be octet-aligned and hence the
+ *  entire 256bit buffer is provided as octet-aligned 32bytes in \a in_frame.
+ */
+int osmo_e1_rx_frame(struct osmo_e1_instance *e1i, const uint8_t *in_frame)
+{
+	int i;
+
+	e1_rx_update_crc4(e1i, in_frame);
+
+	e1_rx_ts0(e1i, in_frame[0]);
+
+	for (i = 1; i < ARRAY_SIZE(e1i->ts); i++) {
+		struct osmo_e1_instance_ts *e1t = &e1i->ts[i];
+		e1_rx_tsN(e1t, in_frame[1]);
+	}
+
+	return 0;
+}
+
+int osmo_e1_init(void)
+{
+	return osmo_fsm_register(&e1_align_fsm);
+}
diff --git a/src/osmo_e1.h b/src/osmo_e1.h
new file mode 100644
index 0000000..f39a308
--- /dev/null
+++ b/src/osmo_e1.h
@@ -0,0 +1,110 @@
+#pragma once
+
+#include <stdint.h>
+#include <stdbool.h>
+#include <osmocom/core/msgb.h>
+#include <osmocom/core/linuxlist.h>
+#include <osmocom/core/fsm.h>
+
+struct osmo_e1_tx_state {
+	bool remote_alarm;
+	bool crc4_error;
+	/* lower 5 bits: Sa4..Sa8 */
+	uint8_t sa4_sa8;
+	/* frame number 0..15 */
+	uint8_t frame_nr;
+	uint8_t crc4_last_smf;
+	uint8_t crc4;
+};
+
+struct osmo_e1_rx_state {
+	uint8_t frame_nr;
+	/* history of rceived TS0 octets */
+	uint8_t ts0_history[16];
+	uint8_t ts0_hist_len;
+	/* was a remote alarm received? */
+	bool remote_alarm;
+	bool remote_crc4_error;
+	/* number of TS0 bytes received since entering CRC mframe search */
+	uint8_t num_ts0_in_mframe_search;
+	struct osmo_fsm_inst *fi;
+	/* computed CRC4 */
+	uint8_t crc4_last_smf;
+	uint8_t crc4;
+};
+
+enum osmo_e1_notify_event {
+	E1_NTFY_EVT_ALIGN_FRAME,
+	E1_NTFY_EVT_ALIGN_CRC_MFRAME,
+	E1_NTFY_EVT_CRC_ERROR,
+	E1_NTFY_EVT_REMOTE_CRC_ERROR,
+	E1_NTFY_EVT_REMOTE_ALARM,
+};
+
+struct osmo_e1_instance_ts;
+struct osmo_e1_instance;
+typedef void (*e1_data_cb)(struct osmo_e1_instance_ts *ts, struct msgb *msg);
+typedef void (*e1_notify_cb)(struct osmo_e1_instance *e1i, enum osmo_e1_notify_event evt,
+			     bool present, void *data);
+
+struct osmo_e1_instance_ts {
+	/* timeslot number */
+	uint8_t ts_nr;
+	/* back-pointer to e1 instance */
+	struct osmo_e1_instance *inst;
+	struct {
+		/* queue of pending to-be-transmitted messages */
+		struct llist_head queue;
+		unsigned long underruns;
+	} tx;
+	struct {
+		bool enabled;
+		/* how many bytes to buffer before calling call-back */
+		unsigned int granularity;
+		/* current receive buffer */
+		struct msgb *msg;
+		e1_data_cb data_cb;
+		/* private data, relevant to user */
+		void *priv;
+	} rx;
+};
+
+struct osmo_e1_instance {
+	/* list; currently not used yet */
+	struct llist_head list;
+
+	/* is CRC4 generation + parsing enabled? */
+	bool crc4_enabled;
+	/* notification call-back function */
+	e1_notify_cb notify_cb;
+
+	/* Rx + Tx related state */
+	struct osmo_e1_tx_state tx;
+	struct osmo_e1_rx_state rx;
+
+	/* our 32 timeslots (only 1..32 are used) */
+	struct osmo_e1_instance_ts ts[32];
+
+	/* private data, relevant to user */
+	void *priv;
+};
+
+extern const struct value_string osmo_e1_notifv_evt_names[];
+
+static inline const char *osmo_e1_notify_event_name(enum osmo_e1_notify_event evt) {
+	return get_value_string(osmo_e1_notifv_evt_names, evt);
+}
+
+int osmo_e1_init(void);
+struct osmo_e1_instance_ts *osmo_e1_instance_ts(struct osmo_e1_instance *e1i, uint8_t ts_nr);
+int osmo_e1_instance_init(struct osmo_e1_instance *e1i, const char *name, e1_notify_cb cb,
+			  bool crc4_enabled, void *priv);
+void osmo_e1_instance_reset(struct osmo_e1_instance *e1i);
+int osmo_e1_ts_config(struct osmo_e1_instance_ts *e1t, e1_data_cb cb, unsigned int granularity,
+		      bool enable);
+void osmo_e1_ts_reset(struct osmo_e1_instance_ts *e1t);
+
+
+void osmo_e1_ts_enqueue(struct osmo_e1_instance_ts *e1t, struct msgb *msg);
+int osmo_e1_pull_tx_frame(struct osmo_e1_instance *e1i, uint8_t *out_frame);
+int osmo_e1_rx_frame(struct osmo_e1_instance *e1i, const uint8_t *in_frame);