diff --git a/.gitignore b/.gitignore
index cbe0733..10c6143 100644
--- a/.gitignore
+++ b/.gitignore
@@ -33,6 +33,7 @@ src/libhagelbarger/libhagelbarger.a
 src/libdtmf/libdtmf.a
 src/libtimer/libtimer.a
 src/libselect/libselect.a
+src/libph_socket/libph_socket.a
 src/libsamplerate/libsamplerate.a
 src/libscrambler/libscrambler.a
 src/libemphasis/libemphasis.a
diff --git a/configure.ac b/configure.ac
index 0cab408..90cabee 100644
--- a/configure.ac
+++ b/configure.ac
@@ -75,6 +75,7 @@ AC_OUTPUT(
     src/libdtmf/Makefile
     src/libtimer/Makefile
     src/libselect/Makefile
+    src/libph_socket/Makefile
     src/libsamplerate/Makefile
     src/libscrambler/Makefile
     src/libemphasis/Makefile
diff --git a/src/Makefile.am b/src/Makefile.am
index 3cf15bd..a3fbe66 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -16,6 +16,7 @@ SUBDIRS = \
 	libdtmf \
 	libtimer \
 	libselect \
+	libph_socket \
 	libsamplerate \
 	libscrambler \
 	libemphasis \
diff --git a/src/libph_socket/Makefile.am b/src/libph_socket/Makefile.am
new file mode 100644
index 0000000..60fa592
--- /dev/null
+++ b/src/libph_socket/Makefile.am
@@ -0,0 +1,7 @@
+AM_CPPFLAGS = -Wall -Wextra -g $(all_includes)
+
+noinst_LIBRARIES = libph_socket.a
+
+libph_socket_a_SOURCES = \
+	ph_socket.c
+
diff --git a/src/libph_socket/ph_socket.c b/src/libph_socket/ph_socket.c
new file mode 100644
index 0000000..28ec30f
--- /dev/null
+++ b/src/libph_socket/ph_socket.c
@@ -0,0 +1,339 @@
+/* PH-socket, a lightweight ISDN physical layer interface
+ *
+ * (C) 2022 by Andreas Eversberg <jolly@eversberg.eu>
+ * All Rights Reserved
+ *
+ * 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 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 General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <errno.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <sys/socket.h>
+#include "../libtimer/timer.h"
+#include "../libselect/select.h"
+#include "../libdebug/debug.h"
+#include "ph_socket.h"
+
+static int ph_socket_listen_cb(struct osmo_fd *ofd, unsigned int __attribute__((unused)) what);
+static int ph_socket_connect_cb(struct osmo_fd *ofd, unsigned int __attribute__((unused)) what);
+static void ph_socket_timeout_cb(void *data);
+
+static void open_connection(ph_socket_t *s)
+{
+	uint8_t enable = PH_CTRL_UNBLOCK;
+	int rc, flags;
+
+	if (s->connect_ofd.fd > 0)
+		return;
+
+	LOGP(DPH, LOGL_DEBUG, "Trying to connect to PH-socket server.\n");
+	rc = socket(PF_UNIX, SOCK_STREAM, 0);
+	if (rc < 0) {
+		LOGP(DPH, LOGL_ERROR, "Failed to create UNIX socket.\n");
+		osmo_timer_schedule(&s->retry_timer, SOCKET_RETRY_TIMER, 0);
+		return;
+	}
+	s->connect_ofd.fd = rc;
+	s->connect_ofd.data = s;
+	s->connect_ofd.when = BSC_FD_READ;
+	s->connect_ofd.cb = ph_socket_connect_cb;
+	osmo_fd_register(&s->connect_ofd);
+	/* set nonblocking io, because we do multiple reads when handling read event */
+	flags = fcntl(s->connect_ofd.fd, F_GETFL);
+	flags |= O_NONBLOCK;
+	fcntl(s->connect_ofd.fd, F_SETFL, flags);
+	/* connect */
+	rc = connect(s->connect_ofd.fd, (struct sockaddr *)&s->sock_address, sizeof(s->sock_address));
+	if (rc < 0 && errno != EAGAIN) {
+		if (!s->connect_failed)
+			LOGP(DPH, LOGL_NOTICE, "Failed to connect UNIX socket, retrying...\n");
+		close(s->connect_ofd.fd);
+		s->connect_failed = 1;
+		osmo_fd_unregister(&s->connect_ofd);
+		s->connect_ofd.fd = 0;
+		osmo_timer_schedule(&s->retry_timer, SOCKET_RETRY_TIMER, 0);
+		return;
+	}
+	s->connect_failed = 0;
+	LOGP(DPH, LOGL_INFO, "Connection to PH-socket server.\n");
+	/* reset rx buffer */
+	s->rx_header_index = 0;
+	s->rx_data_index = 0;
+	/* indicate established socket connection */
+	s->ph_socket_rx_msg(s, 0, PH_PRIM_CTRL_IND, &enable, 1);
+}
+
+static void close_connection(ph_socket_t *s)
+{
+	struct socket_msg_list *ml;
+	uint8_t disable = PH_CTRL_BLOCK;
+
+	if (s->connect_ofd.fd <= 0)
+		return;
+
+	LOGP(DPH, LOGL_INFO, "Connection from PH-socket closed.\n");
+
+	/* indicate loss of socket connection */
+	s->ph_socket_rx_msg(s, 0, (s->listen_ofd.fd > 0) ? PH_PRIM_CTRL_REQ : PH_PRIM_CTRL_IND, &disable, 1);
+
+	osmo_fd_unregister(&s->connect_ofd);
+	close(s->connect_ofd.fd);
+	s->connect_ofd.fd = 0;
+
+	while ((ml = s->tx_list)) {
+		s->tx_list = ml->next;
+		free(ml);
+	}
+	s->tx_list_tail = &s->tx_list;
+	if (s->rx_msg) {
+		free(s->rx_msg);
+		s->rx_msg = NULL;
+	}
+
+	if (s->listen_ofd.fd <= 0) {
+		/* set timer, so that retry is delayed */
+		osmo_timer_schedule(&s->retry_timer, SOCKET_RETRY_TIMER, 0);
+	}
+}
+
+int ph_socket_init(ph_socket_t *s, void (*ph_socket_rx_msg)(ph_socket_t *s, int channel, uint8_t prim, uint8_t *data,
+		int length), void *priv, const char *socket_name, int server)
+{
+	int rc;
+
+	memset(s, 0, sizeof(*s));
+	s->name = socket_name;
+	s->ph_socket_rx_msg = ph_socket_rx_msg;
+	s->priv = priv;
+	s->tx_list_tail = &s->tx_list;
+
+	memset(&s->sock_address, 0, sizeof(s->sock_address));
+	s->sock_address.sun_family = AF_UNIX;
+	strcpy(s->sock_address.sun_path+1, socket_name);
+
+	s->retry_timer.data = s;
+	s->retry_timer.cb = ph_socket_timeout_cb;
+
+	if (server) {
+		rc = socket(PF_UNIX, SOCK_STREAM, 0);
+		if (rc < 0) {
+			LOGP(DPH, LOGL_ERROR, "Failed to create UNIX socket.\n");
+			return rc;
+		}
+		s->listen_ofd.fd = rc;
+		s->listen_ofd.data = s;
+		s->listen_ofd.when = BSC_FD_READ;
+		s->listen_ofd.cb = ph_socket_listen_cb;
+		osmo_fd_register(&s->listen_ofd);
+
+		rc = bind(s->listen_ofd.fd, (struct sockaddr *)(&s->sock_address), sizeof(s->sock_address));
+		if (rc < 0) {
+			LOGP(DPH, LOGL_ERROR, "Failed to bind UNIX socket with path '%s' (errno = %d (%s)).\n",
+					s->name, errno, strerror(errno));
+			return rc;
+		}
+
+		rc = listen(s->listen_ofd.fd, 1);
+		if (rc < 0) {
+			LOGP(DPH, LOGL_ERROR, "Failed to listen to UNIX socket with path '%s' (errno = %d (%s)).\n",
+					s->name, errno, strerror(errno));
+			return rc;
+		}
+	} else
+		open_connection(s);
+
+	LOGP(DPH, LOGL_INFO, "Created PH-socket at '%s'.\n", s->name);
+
+	return 0;
+}
+
+void ph_socket_exit(ph_socket_t *s)
+{
+	LOGP(DPH, LOGL_INFO, "Destroyed PH-socket.\n");
+
+	close_connection(s);
+	if (s->listen_ofd.fd > 0) {
+		osmo_fd_unregister(&s->listen_ofd);
+		close(s->listen_ofd.fd);
+		s->listen_ofd.fd = 0;
+	}
+
+	if (osmo_timer_pending(&s->retry_timer))
+		osmo_timer_del(&s->retry_timer);
+}
+
+static int ph_socket_listen_cb(struct osmo_fd *ofd, unsigned int __attribute__((unused)) what)
+{
+	ph_socket_t *s = ofd->data;
+	struct sockaddr_un sock_address;
+	uint8_t enable = PH_CTRL_UNBLOCK;
+	int rc, flags;
+
+	socklen_t sock_len = sizeof(sock_address);
+	/* see if there is an incoming connection */
+	rc = accept(s->listen_ofd.fd, (struct sockaddr *)&sock_address, &sock_len);
+	if (rc > 0) {
+		if (s->connect_ofd.fd > 0) {
+			LOGP(DPH, LOGL_ERROR, "Rejecting incoming connection, because we already have a client "
+					"connected!\n");
+			close(rc);
+		} else {
+			LOGP(DPH, LOGL_INFO, "Connection from PH-socket client.\n");
+			s->connect_ofd.fd = rc;
+			s->connect_ofd.data = s;
+			s->connect_ofd.when = BSC_FD_READ;
+			s->connect_ofd.cb = ph_socket_connect_cb;
+			osmo_fd_register(&s->connect_ofd);
+			/* set nonblocking io, because we do multiple reads when handling read event */
+			flags = fcntl(s->connect_ofd.fd, F_GETFL);
+			flags |= O_NONBLOCK;
+			fcntl(s->connect_ofd.fd, F_SETFL, flags);
+			/* reset rx buffer */
+			s->rx_header_index = 0;
+			s->rx_data_index = 0;
+			/* indicate established socket connection */
+			s->ph_socket_rx_msg(s, 0, PH_PRIM_CTRL_REQ, &enable, 1);
+		}
+	}
+
+	return 0;
+}
+
+static int ph_socket_connect_cb(struct osmo_fd *ofd, unsigned __attribute__((unused)) int what)
+{
+	ph_socket_t *s = ofd->data;
+	int rc;
+
+	if (what & BSC_FD_READ) {
+rx_again:
+		if (!s->rx_msg)
+			s->rx_msg = calloc(1, sizeof(*s->rx_msg));
+		if (s->rx_header_index < (int)sizeof(s->rx_msg->msg.header)) {
+			/* read header until complete */
+			rc = recv(s->connect_ofd.fd, ((uint8_t *)&s->rx_msg->msg.header) + s->rx_header_index,
+					sizeof(s->rx_msg->msg.header) - s->rx_header_index, 0);
+			if (rc > 0) {
+				s->rx_header_index += rc;
+				goto rx_again;
+			} else if (rc == 0 || errno != EAGAIN) {
+				close_connection(s);
+				return 0;
+			}
+		} else if (s->rx_data_index < s->rx_msg->msg.header.length) {
+			/* read data until complete */
+			rc = recv(s->connect_ofd.fd, s->rx_msg->msg.data + s->rx_data_index,
+					s->rx_msg->msg.header.length - s->rx_data_index, 0);
+			if (rc > 0) {
+				s->rx_data_index += rc;
+				goto rx_again;
+			} else if (rc == 0 || errno != EAGAIN) {
+				close_connection(s);
+				return 0;
+			}
+		} else {
+			/* process and free message */
+			if (s->rx_msg->msg.header.prim != PH_PRIM_DATA_REQ
+			 && s->rx_msg->msg.header.prim != PH_PRIM_DATA_IND
+			 && s->rx_msg->msg.header.prim != PH_PRIM_DATA_CNF) {
+				LOGP(DPH, LOGL_DEBUG, "message 0x%02x channel %d from socket\n",
+						s->rx_msg->msg.header.prim, s->rx_msg->msg.header.channel);
+				if (s->rx_msg->msg.header.length)
+					LOGP(DPH, LOGL_DEBUG, " -> data:%s\n", osmo_hexdump(s->rx_msg->msg.data,
+								s->rx_msg->msg.header.length));
+			}
+			s->ph_socket_rx_msg(s, s->rx_msg->msg.header.channel, s->rx_msg->msg.header.prim,
+					s->rx_msg->msg.data, s->rx_msg->msg.header.length);
+			free(s->rx_msg);
+			s->rx_msg = NULL;
+			/* reset rx buffer */
+			s->rx_header_index = 0;
+			s->rx_data_index = 0;
+		}
+	}
+
+	if (what & BSC_FD_WRITE) {
+		if (s->tx_list) {
+			/* some frame in tx list, so try sending it */
+			rc = send(s->connect_ofd.fd, ((uint8_t *)&s->tx_list->msg.header),
+					sizeof(s->tx_list->msg.header) + s->tx_list->msg.header.length, 0);
+			if (rc > 0) {
+				struct socket_msg_list *ml;
+				if (rc != (int)sizeof(s->tx_list->msg.header) + s->tx_list->msg.header.length) {
+					LOGP(DPH, LOGL_ERROR, "Short write, please fix handling!\n");
+				}
+				/* remove list entry */
+				ml = s->tx_list;
+				s->tx_list = ml->next;
+				if (s->tx_list == NULL)
+					s->tx_list_tail = &s->tx_list;
+				free(ml);
+			} else if (rc == 0 || errno != EAGAIN) {
+				close_connection(s);
+				return 0;
+			}
+		} else
+			s->connect_ofd.when &= ~BSC_FD_WRITE;
+	}
+
+	return 0;
+}
+
+static void ph_socket_timeout_cb(void *data)
+{
+	ph_socket_t *s = data;
+
+	open_connection(s);
+}
+
+void ph_socket_tx_msg(ph_socket_t *s, int channel, uint8_t prim, uint8_t *data, int length)
+{
+	struct socket_msg_list *tx_msg;
+
+	if (prim != PH_PRIM_DATA_REQ
+	 && prim != PH_PRIM_DATA_IND
+	 && prim != PH_PRIM_DATA_CNF) {
+		LOGP(DPH, LOGL_DEBUG, "message 0x%02x channel %d to socket\n", prim, channel);
+		if (length)
+			LOGP(DPH, LOGL_DEBUG, " -> data:%s\n", osmo_hexdump(data, length));
+	}
+
+	if (length > (int)sizeof(tx_msg->msg.data)) {
+		LOGP(DPH, LOGL_NOTICE, "Frame from HDLC process too large for socket, dropping!\n");
+		return;
+	}
+
+	if (s->connect_ofd.fd <= 0) {
+		LOGP(DPH, LOGL_NOTICE, "Dropping message for socket, socket is closed!\n");
+		return;
+	}
+
+	tx_msg = calloc(1, sizeof(*tx_msg));
+	tx_msg->msg.header.channel = channel;
+	tx_msg->msg.header.prim = prim;
+	if (length) {
+		tx_msg->msg.header.length = length;
+		memcpy(tx_msg->msg.data, data, length);
+	}
+	/* move message to list */
+	*s->tx_list_tail = tx_msg;
+	s->tx_list_tail = &tx_msg->next;
+	s->connect_ofd.when |= BSC_FD_WRITE;
+}
+
diff --git a/src/libph_socket/ph_socket.h b/src/libph_socket/ph_socket.h
new file mode 100644
index 0000000..97293c3
--- /dev/null
+++ b/src/libph_socket/ph_socket.h
@@ -0,0 +1,107 @@
+
+#include <sys/un.h>
+
+/*
+ * Procedure:
+ *
+ * If socket connection is establised, a PH_PRIM_CTRL_REQ message with
+ * PH_CTRL_ENABLE information is received by the socket server user.
+ * If the socket connection is lost, a PH_PRIM_CTRL_REQ message with
+ * PH_CTRL_DISABLE information is received by the user.
+ * 
+ * If socket connection is establised, a PH_PRIM_CTRL_IND message with
+ * PH_CTRL_ENABLE information is received by the socket client user.
+ * If the socket connection is lost, a PH_PRIM_CTRL_IND message with
+ * PH_CTRL_DISABLE information is received by the user.
+ *
+ * The socket server should enable or disable interface depending on
+ * the PH_CTRL_ENABLE / PH_CTRL_DISABLE information.
+ *
+ * The socket client user shall keep track of last PH_PRIM_ACT_IND /
+ * PH_PRIM_DEACT_IND message and treat a PH_PRIM_CTRL_IND message with
+ * PH_CTRL_DISABLE information as a deactivation of all channels that
+ * were activated. Also it shall reject every PH_RIM_ACT_REQ with a
+ * PH_PRIM_DACT_IND, if the socket is currently unavailable.
+ *
+ * PH_PRIM_CTRL_REQ and PH_PRIM_CTRL_IND messages with PH_CTRL_ENABLE
+ * and PH_CTRL_DISABLE informations are not assoicated with a channel
+ * number. The socket sender shall set it to 0, the receiver shall
+ * ignore it.
+ *
+ * A missing MODE in PH_PRIM_ACT_REQ is interepreted as default:
+ * HDLC on D-channel, TRANS on B-channel.
+ *
+ * Each packet on the socket shall have the follwoing header:
+ *   uint8_t channel;
+ *   uint8_t prim;
+ *   uint16_t length;
+ *
+ * The length shall be in host's endian on UN sockets and in network
+ * endian on TCP sockets and not being transmitted on UDP sockets.
+ *
+ * 0 to 65535 bytes shall follow the header, depending on the length
+ * information field.
+ */
+
+/* all primitives */
+#define PH_PRIM_DATA_REQ	0x00	/* any data sent to channel from upper layer */
+#define PH_PRIM_DATA_IND	0x01	/* any data received from channel to upper layer */
+#define PH_PRIM_DATA_CNF	0x02	/* confirm data sent to channel */
+
+#define PH_PRIM_CTRL_REQ	0x04	/* implementation specific requests towards interface */
+#define PH_PRIM_CTRL_IND	0x05	/* implementation specific indications from interface */
+
+#define PH_PRIM_ACT_REQ		0x08	/* activation request of channel, mode is given as payload */
+#define PH_PRIM_ACT_IND		0x09	/* activation indication that the channel is now active */
+
+#define PH_PRIM_DACT_REQ	0x0c	/* deactivation request of channel */
+#define PH_PRIM_DACT_IND	0x0d	/* deactivation indication that the channel is now inactive */
+
+/* one byte sent activation request */
+#define PH_MODE_TRANS		0x00	/* raw data is sent via B-channel */
+#define PH_MODE_HDLC		0x01	/* HDLC transcoding is performed via B-channel */
+
+/* one byte sent with control messages */
+#define PH_CTRL_BLOCK		0x00	/* disable (block) interface, when socket is disconnected */
+#define PH_CTRL_UNBLOCK		0x01	/* enable (unblock) interface, when socket is connected */
+#define PH_CTRL_LOOP_DISABLE	0x04	/* disable loopback */
+#define PH_CTRL_LOOP1_ENABLE	0x05	/* enable LT transceier loopback */
+#define PH_CTRL_LOOP2_ENABLE	0x06	/* enable NT transceier loopback */
+#define PH_CTRL_LOOP_ERROR	0x10	/* frame error report (loopback test) */
+#define PH_CTRL_VIOLATION_LT	0x11	/* code violation received by LT */
+#define PH_CTRL_VIOLATION_NT	0x12	/* code violation received by NT */
+
+struct socket_msg {
+	struct {
+		uint8_t channel;
+		uint8_t prim;
+		uint16_t length;
+	} header;
+	uint8_t data[65536];
+} __attribute__((packed));
+
+struct socket_msg_list {
+	struct socket_msg_list *next;
+	struct socket_msg msg;
+};
+
+#define SOCKET_RETRY_TIMER	2
+
+typedef struct ph_socket {
+	const char *name;
+	void (*ph_socket_rx_msg)(struct ph_socket *s, int channel, uint8_t prim, uint8_t *data, int length);
+	void *priv;
+	struct sockaddr_un sock_address;
+	struct osmo_fd listen_ofd;		/* socket to listen to incoming connections */
+	struct osmo_fd connect_ofd;		/* socket of incoming connection */
+	int connect_failed;			/* used to print a failure only once */
+	struct osmo_timer_list retry_timer;	/* timer to connect again */
+	struct socket_msg_list *tx_list, **tx_list_tail;
+	struct socket_msg_list *rx_msg;
+	int rx_header_index;
+	int rx_data_index;
+} ph_socket_t;
+
+int ph_socket_init(ph_socket_t *s, void (*ph_socket_rx_msg)(ph_socket_t *s, int channel, uint8_t prim, uint8_t *data, int length), void *priv, const char *socket_name, int server);
+void ph_socket_exit(ph_socket_t *s);
+void ph_socket_tx_msg(ph_socket_t *s, int channel, uint8_t prim, uint8_t *data, int length);