Implement transceiver interface

This change introduces a new block 'TRX Interface', which is aimed
to provide an interface for external applications, such as Osmocom
MS side stack implementation - OsmocomBB. Currently one allows to
exchange raw GSM bursts between GR-GSM and other applications.

Moreover, there is a new 'trx.py' application, which implements a
simple follow graph, where all demodulated bursts are being sent
to external application via UDP link provided by 'TRX Interface'.
OsmoTRX (Osmocom's fork of OpenBTS transceiver) like control
interface is used to initialize, configure, start and stop the
application. Messages on this interface are human readable ASCII
strings, which contain a command and some related parameters.

15 files changed, 1062 insertions, 0 deletions

diff --git a/apps/trx/ctrl_if.py b/apps/trx/ctrl_if.py
new file mode 100644
index 0000000..43fc185
--- /dev/null
+++ b/apps/trx/ctrl_if.py
@@ -0,0 +1,157 @@
+#!/usr/bin/env python2
+# -*- coding: utf-8 -*-
+
+# GR-GSM based transceiver
+# Transceiver UDP interface
+#
+# (C) 2016-2017 by Vadim Yanitskiy <axilirator@gmail.com>
+#
+# 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 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, write to the Free Software Foundation, Inc.,
+# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+import socket
+import select
+
+class UDPServer:
+	def __init__(self, remote_addr, remote_port, bind_port):
+		self.sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+		self.sock.bind(('0.0.0.0', bind_port))
+		self.sock.setblocking(0)
+
+		# Save remote info
+		self.remote_addr = remote_addr
+		self.remote_port = remote_port
+
+	def loop(self):
+		r_event, w_event, x_event = select.select([self.sock], [], [])
+
+		# Check for incoming data
+		if self.sock in r_event:
+			data, addr = self.sock.recvfrom(128)
+			self.handle_rx(data)
+
+	def shutdown(self):
+		self.sock.close();
+
+	def send(self, data):
+		self.sock.sendto(data, (self.remote_addr, self.remote_port))
+
+	def handle_rx(self, data):
+		raise NotImplementedError
+
+class CTRLInterface(UDPServer):
+	def __init__(self, remote_addr, remote_port, bind_port, radio_if):
+		print("[i] Init TRX CTRL interface")
+		UDPServer.__init__(self, remote_addr, remote_port, bind_port)
+		self.tb = radio_if
+
+	def shutdown(self):
+		print("[i] Shutdown TRX CTRL interface")
+		UDPServer.shutdown(self)
+
+	def handle_rx(self, data):
+		if self.verify_req(data):
+			request = self.prepare_req(data)
+			self.parse_cmd(request)
+		else:
+			print("[!] Wrong data on CTRL interface")
+
+	def verify_req(self, data):
+		# Verify command signature
+		return data.startswith("CMD")
+
+	def prepare_req(self, data):
+		# Strip signature, paddings and \0
+		request = data[4:].strip().strip("\0")
+		# Split into a command and arguments
+		request = request.split(" ")
+		# Now we have something like ["TXTUNE", "941600"]
+		return request
+
+	def verify_cmd(self, request, cmd, argc):
+		# Check if requested command matches
+		if request[0] != cmd:
+			return False
+
+		# And has enough arguments
+		if len(request) - 1 != argc:
+			return False
+
+		# Check if all arguments are numeric
+		for v in request[1:]:
+			if not v.isdigit():
+				return False
+
+		return True
+
+	def parse_cmd(self, request):
+		response_code = "0"
+
+		# Power control
+		if self.verify_cmd(request, "POWERON", 0):
+			print("[i] Recv POWERON cmd")
+			if not self.tb.trx_started:
+				if self.tb.check_available():
+					print("[i] Starting transceiver...")
+					self.tb.trx_started = True
+					self.tb.start()
+				else:
+					print("[!] Transceiver isn't ready to start")
+					response_code = "-1"
+			else:
+				print("[!] Transceiver already started!")
+				response_code = "-1"
+		elif self.verify_cmd(request, "POWEROFF", 0):
+			print("[i] Recv POWEROFF cmd")
+			print("[i] Stopping transceiver...")
+			self.tb.trx_started = False
+			# TODO: flush all buffers between blocks
+			self.tb.stop()
+		elif self.verify_cmd(request, "SETRXGAIN", 1):
+			print("[i] Recv SETRXGAIN cmd")
+			# TODO: check gain value
+			gain = int(request[1])
+			self.tb.set_gain(gain)
+
+		# Tuning Control
+		elif self.verify_cmd(request, "RXTUNE", 1):
+			print("[i] Recv RXTUNE cmd")
+			# TODO: check freq range
+			freq = int(request[1]) * 1000
+			self.tb.set_fc(freq)
+		elif self.verify_cmd(request, "TXTUNE", 1):
+			print("[i] Recv TXTUNE cmd")
+			# TODO: is not implemented yet
+
+		# Misc
+		elif self.verify_cmd(request, "ECHO", 0):
+			print("[i] Recv ECHO cmd")
+
+		# Wrong / unknown command
+		else:
+			print("[!] Wrong request on CTRL interface")
+			response_code = "-1"
+
+		# Anyway, we need to respond
+		self.send_response(request, response_code)
+
+	def send_response(self, request, response_code):
+		# Include status code, for example ["TXTUNE", "0", "941600"]
+		request.insert(1, response_code)
+		# Add the response signature, and join back to string
+		response = "RSP " + " ".join(request) + "\0"
+		# Now we have something like "RSP TXTUNE 0 941600"
+		self.send(response)
diff --git a/apps/trx/radio_if.py b/apps/trx/radio_if.py
new file mode 100644
index 0000000..41eb3e8
--- /dev/null
+++ b/apps/trx/radio_if.py
@@ -0,0 +1,184 @@
+#!/usr/bin/env python2
+# -*- coding: utf-8 -*-
+
+# GR-GSM based transceiver
+# Follow graph implementation
+#
+# (C) 2016-2017 by Vadim Yanitskiy <axilirator@gmail.com>
+#
+# 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 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, write to the Free Software Foundation, Inc.,
+# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+import pmt
+import time
+import grgsm
+import osmosdr
+
+from math import pi
+
+from gnuradio.eng_option import eng_option
+from gnuradio import eng_notation
+from gnuradio.filter import firdes
+from gnuradio import blocks
+from gnuradio import gr
+
+class RadioInterface(gr.top_block):
+	# PHY specific variables
+	samp_rate = 2000000
+	shiftoff = 400e3
+	subdev_spec = "" # TODO: use it
+	device_args = ""
+	fc = 941.6e6 # TODO: set ARFCN to 0?
+	gain = 30
+	ppm = 0
+
+	# Application state flags
+	trx_started = False
+	fc_set = False
+
+	def __init__(self, phy_args, phy_subdev_spec,
+				phy_sample_rate, phy_gain, phy_ppm,
+				trx_remote_addr, trx_base_port):
+		print("[i] Init Radio interface")
+
+		# TRX block specific variables
+		self.trx_remote_addr = trx_remote_addr
+		self.trx_base_port = trx_base_port
+
+		# PHY specific variables
+		self.subdev_spec = phy_subdev_spec
+		self.samp_rate = phy_sample_rate
+		self.device_args = phy_args
+		self.gain = phy_gain
+		self.ppm = phy_ppm
+
+		gr.top_block.__init__(self, "GR-GSM TRX")
+		shift_fc = self.fc - self.shiftoff
+
+		##################################################
+		# PHY Definition
+		##################################################
+		self.phy = osmosdr.source(
+			args = "numchan=%d %s" % (1, self.device_args))
+
+		self.phy.set_bandwidth(250e3 + abs(self.shiftoff), 0)
+		self.phy.set_center_freq(shift_fc, 0)
+		self.phy.set_sample_rate(self.samp_rate)
+		self.phy.set_freq_corr(self.ppm, 0)
+		self.phy.set_iq_balance_mode(2, 0)
+		self.phy.set_dc_offset_mode(2, 0)
+		self.phy.set_gain_mode(False, 0)
+		self.phy.set_gain(self.gain, 0)
+		self.phy.set_if_gain(20, 0)
+		self.phy.set_bb_gain(20, 0)
+		self.phy.set_antenna("", 0)
+
+		##################################################
+		# GR-GSM Magic
+		##################################################
+		self.blocks_rotator = blocks.rotator_cc(
+			-2 * pi * self.shiftoff / self.samp_rate)
+
+		self.gsm_input = grgsm.gsm_input(
+			ppm = self.ppm, osr = 4, fc = self.fc,
+			samp_rate_in = self.samp_rate)
+
+		self.gsm_receiver = grgsm.receiver(4, ([0]), ([]))
+
+		self.gsm_clck_ctrl = grgsm.clock_offset_control(
+			shift_fc, self.samp_rate, osr = 4)
+
+		# TODO: implement configurable TS filter
+		self.gsm_ts_filter = grgsm.burst_timeslot_filter(0)
+
+		self.gsm_trx_if = grgsm.trx(self.trx_remote_addr,
+			str(self.trx_base_port))
+
+		##################################################
+		# Connections
+		##################################################
+		self.connect((self.phy, 0), (self.blocks_rotator, 0))
+		self.connect((self.blocks_rotator, 0), (self.gsm_input, 0))
+		self.connect((self.gsm_input, 0), (self.gsm_receiver, 0))
+
+		self.msg_connect((self.gsm_receiver, 'measurements'),
+			(self.gsm_clck_ctrl, 'measurements'))
+
+		self.msg_connect((self.gsm_clck_ctrl, 'ctrl'),
+			(self.gsm_input, 'ctrl_in'))
+
+		self.msg_connect((self.gsm_receiver, 'C0'),
+			(self.gsm_ts_filter, 'in'))
+
+		self.msg_connect((self.gsm_ts_filter, 'out'),
+			(self.gsm_trx_if, 'bursts'))
+
+	def check_available(self):
+		return self.fc_set
+
+	def shutdown(self):
+		print("[i] Shutdown Radio interface")
+		self.stop()
+		self.wait()
+
+	def get_args(self):
+		return self.args
+
+	def set_args(self, args):
+		self.args = args
+
+	def get_fc(self):
+		return self.fc
+
+	def set_fc(self, fc):
+		self.phy.set_center_freq(fc - self.shiftoff, 0)
+		self.gsm_input.set_fc(fc)
+		self.fc_set = True
+		self.fc = fc
+
+	def get_gain(self):
+		return self.gain
+
+	def set_gain(self, gain):
+		self.phy.set_gain(gain, 0)
+		self.gain = gain
+
+	def get_ppm(self):
+		return self.ppm
+
+	def set_ppm(self, ppm):
+		self.rtlsdr_source_0.set_freq_corr(ppm, 0)
+		self.ppm = ppm
+
+	def get_samp_rate(self):
+		return self.samp_rate
+
+	def set_samp_rate(self, samp_rate):
+		self.blocks_rotator.set_phase_inc(
+			-2 * pi * self.shiftoff / samp_rate)
+		self.gsm_input.set_samp_rate_in(samp_rate)
+		self.phy.set_sample_rate(samp_rate)
+		self.samp_rate = samp_rate
+
+	def get_shiftoff(self):
+		return self.shiftoff
+
+	def set_shiftoff(self, shiftoff):
+		self.blocks_rotator.set_phase_inc(
+			-2 * pi * shiftoff / self.samp_rate)
+		self.phy.set_bandwidth(250e3 + abs(shiftoff), 0)
+		self.phy.set_center_freq(self.fc - shiftoff, 0)
+		self.shiftoff = shiftoff
diff --git a/apps/trx/trx.py b/apps/trx/trx.py
new file mode 100755
index 0000000..25f76cf
--- /dev/null
+++ b/apps/trx/trx.py
@@ -0,0 +1,150 @@
+#!/usr/bin/env python2
+# -*- coding: utf-8 -*-
+
+# GR-GSM based transceiver
+#
+# (C) 2016-2017 by Vadim Yanitskiy <axilirator@gmail.com>
+#
+# 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 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, write to the Free Software Foundation, Inc.,
+# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+import signal
+import getopt
+import sys
+
+from ctrl_if import CTRLInterface
+from radio_if import RadioInterface
+
+COPYRIGHT = \
+	"Copyright (C) 2016-2017 by Vadim Yanitskiy <axilirator@gmail.com>\n" \
+	"License GPLv2+: GNU GPL version 2 or later " \
+	"<http://gnu.org/licenses/gpl.html>\n" \
+	"This is free software: you are free to change and redistribute it.\n" \
+	"There is NO WARRANTY, to the extent permitted by law.\n"
+
+class Application:
+	# Application variables
+	remote_addr = "127.0.0.1"
+	base_port = 5700
+
+	# PHY specific
+	phy_sample_rate = 2000000
+	phy_subdev_spec = False
+	phy_gain = 30
+	phy_args = ""
+	phy_ppm = 0
+
+	def __init__(self):
+		self.print_copyright()
+		self.parse_argv()
+
+		# Set up signal handlers
+		signal.signal(signal.SIGINT, self.sig_handler)
+
+	def run(self):
+		# Init Radio interface
+		self.radio = RadioInterface(self.phy_args, self.phy_subdev_spec,
+			self.phy_sample_rate, self.phy_gain, self.phy_ppm,
+			self.remote_addr, self.base_port)
+
+		# Init TRX CTRL interface
+		self.server = CTRLInterface(self.remote_addr,
+			self.base_port + 101, self.base_port + 1, self.radio)
+
+		print("[i] Init complete")
+
+		# Enter main loop
+		while True:
+			self.server.loop()
+
+	def shutdown(self):
+		print("[i] Shutting down...")
+		self.server.shutdown()
+		self.radio.shutdown()
+
+	def print_copyright(self):
+		print(COPYRIGHT)
+
+	def print_help(self):
+		s  = " Usage: " + sys.argv[0] + " [options]\n\n" \
+			 " Some help...\n" \
+			 "  -h --help         this text\n\n"
+
+		# TRX specific
+		s += " TRX interface specific\n" \
+			 "  -s --remote-addr  Set remote address (default 127.0.0.1)\n" \
+			 "  -p --base-port    Set base port number (default 5700)\n\n"
+
+		# PHY specific
+		s += " Radio interface specific\n" \
+			 "  -a --device-args  Set device arguments\n" \
+			 "  -s --sample-rate  Set PHY sample rate (default 2000000)\n" \
+			 "  -S --subdev-spec  Set PHY sub-device specification\n" \
+			 "  -g --gain         Set PHY gain (default 30)\n" \
+			 "     --ppm          Set PHY frequency correction (default 0)\n"
+
+		print(s)
+
+	def parse_argv(self):
+		try:
+			opts, args = getopt.getopt(sys.argv[1:],
+				"a:p:i:s:S:g:h",
+				["help", "remote-addr=", "base-port=", "device-args=",
+				"gain=", "subdev-spec=", "sample-rate=", "ppm="])
+		except getopt.GetoptError as err:
+			# Print(help and exit)
+			self.print_help()
+			print("[!] " + str(err))
+			sys.exit(2)
+
+		for o, v in opts:
+			if o in ("-h", "--help"):
+				self.print_help()
+				sys.exit(2)
+
+			# TRX specific
+			elif o in ("-i", "--remote-addr"):
+				self.remote_addr = v
+			elif o in ("-p", "--base-port"):
+				if int(v) >= 0 and int(v) <= 65535:
+					self.base_port = int(v)
+				else:
+					print("[!] The port number should be in range [0-65536]")
+					sys.exit(2)
+
+			# PHY specific
+			elif o in ("-a", "--device-args"):
+				self.phy_args = v
+			elif o in ("-g", "--gain"):
+				self.phy_gain = int(v)
+			elif o in ("-S", "--subdev-spec"):
+				self.phy_subdev_spec = v
+			elif o in ("-s", "--sample-rate"):
+				self.phy_sample_rate = int(v)
+			elif o in ("--ppm"):
+				self.phy_ppm = int(v)
+
+	def sig_handler(self, signum, frame):
+		print("Signal %d received" % signum)
+		if signum is signal.SIGINT:
+			self.shutdown()
+			sys.exit(0)
+
+def main():
+	Application().run()
+
+if __name__ == '__main__':
+	main()
diff --git a/grc/CMakeLists.txt b/grc/CMakeLists.txt
index 2b94539..045d6a5 100644
--- a/grc/CMakeLists.txt
+++ b/grc/CMakeLists.txt
@@ -23,6 +23,7 @@ add_subdirectory(demapping)
 add_subdirectory(receiver)
 add_subdirectory(flow_control)
 add_subdirectory(misc_utils)
+add_subdirectory(trx_interface)
 install(FILES
     gsm_block_tree.xml DESTINATION share/gnuradio/grc/blocks
 )
diff --git a/grc/trx_interface/CMakeLists.txt b/grc/trx_interface/CMakeLists.txt
new file mode 100644
index 0000000..752b7a7
--- /dev/null
+++ b/grc/trx_interface/CMakeLists.txt
@@ -0,0 +1,22 @@
+# Copyright 2011,2012 Free Software Foundation, Inc.
+#
+# This file is part of GNU Radio
+#
+# GNU Radio 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, or (at your option)
+# any later version.
+#
+# GNU Radio 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 GNU Radio; see the file COPYING.  If not, write to
+# the Free Software Foundation, Inc., 51 Franklin Street,
+# Boston, MA 02110-1301, USA.
+
+install(FILES
+    gsm_trx.xml DESTINATION share/gnuradio/grc/blocks
+)
diff --git a/grc/trx_interface/gsm_trx.xml b/grc/trx_interface/gsm_trx.xml
new file mode 100644
index 0000000..cf882ef
--- /dev/null
+++ b/grc/trx_interface/gsm_trx.xml
@@ -0,0 +1,68 @@
+<?xml version="1.0"?>
+<block>
+  <name>TRX interface</name>
+  <key>grgsm_trx_interface</key>
+  <import>import grgsm</import>
+  <make>grgsm.trx($remote_addr, $base_port)</make>
+
+  <param>
+    <name>base_port</name>
+    <key>base_port</key>
+    <value>5700</value>
+    <type>string</type>
+  </param>
+
+  <param>
+    <name>remote_addr</name>
+    <key>remote_addr</key>
+    <value>127.0.0.1</value>
+    <type>string</type>
+  </param>
+
+  <sink>
+    <name>bursts</name>
+    <type>message</type>
+  </sink>
+
+  <doc>
+      OsmoTRX like UDP interface for external applications.
+
+      There are three UDP sockets: CLCK, CTRL and DATA.
+      Give a base port B (5700 by default), the SCH clock
+      interface is at port P=B. The TRX-side control interface
+      is on port P=B+100+1 and the data interface is on an odd
+      numbered port P=B+100+2.
+
+      Indications on the SCH Clock Interface (CLCK)
+      The SCH clock interface is output only (from the radio).
+      CLOCK gives the current value of the BTS clock to
+      be used by external applications. The indications are
+      sent whenever a transmission packet arrives that is too
+      late or too early.
+
+      Commands on Control Interface (CTRL)
+      The control interface uses a command-response protocol.
+      Commands are NULL-terminated ASCII strings. Each command
+      has a corresponding response. This interface isn't handled
+      by this particular block, and should be implemented outside.
+
+      Messages on the Data Interface
+      Messages on the data interface carry one radio burst per
+      UDP message.
+
+      Received Data Burst:
+      1 byte timeslot index
+      4 bytes GSM frame number, big endian
+      1 byte RSSI in -dBm
+      2 bytes correlator timing offset in 1/256 symbol steps,
+          2's-comp, big endian
+      148 bytes soft symbol estimates, 0 -&gt; definite "0",
+          255 -&gt; definite "1"
+
+      Transmit Data Burst:
+      1 byte timeslot index
+      4 bytes GSM frame number, big endian
+      1 byte transmit level wrt ARFCN max, -dB (attenuation)
+      148 bytes output symbol values, 0 &amp; 1
+  </doc>
+</block>
diff --git a/include/grgsm/CMakeLists.txt b/include/grgsm/CMakeLists.txt
index dc9b191..beca954 100644
--- a/include/grgsm/CMakeLists.txt
+++ b/include/grgsm/CMakeLists.txt
@@ -33,3 +33,4 @@ add_subdirectory(receiver)
 add_subdirectory(misc_utils)
 add_subdirectory(qa_utils)
 add_subdirectory(flow_control)
+add_subdirectory(trx_interface)
diff --git a/include/grgsm/trx_interface/CMakeLists.txt b/include/grgsm/trx_interface/CMakeLists.txt
new file mode 100644
index 0000000..8d35ef7
--- /dev/null
+++ b/include/grgsm/trx_interface/CMakeLists.txt
@@ -0,0 +1,25 @@
+# Copyright 2011,2012 Free Software Foundation, Inc.
+#
+# This file is part of GNU Radio
+#
+# GNU Radio 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, or (at your option)
+# any later version.
+#
+# GNU Radio 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 GNU Radio; see the file COPYING.  If not, write to
+# the Free Software Foundation, Inc., 51 Franklin Street,
+# Boston, MA 02110-1301, USA.
+
+########################################################################
+# Install public header files
+########################################################################
+install(FILES
+    trx.h DESTINATION include/grgsm/trx_interface
+)
diff --git a/include/grgsm/trx_interface/trx.h b/include/grgsm/trx_interface/trx.h
new file mode 100644
index 0000000..7821389
--- /dev/null
+++ b/include/grgsm/trx_interface/trx.h
@@ -0,0 +1,59 @@
+/* -*- c++ -*- */
+/* @file
+ * @author Vadim Yanitskiy <axilirator@gmail.com>
+ * @section LICENSE
+ * 
+ * Gr-gsm 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, or (at your option)
+ * any later version.
+ * 
+ * Gr-gsm 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 gr-gsm; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ * 
+ */
+
+#ifndef INCLUDED_GRGSM_TRX_H
+#define INCLUDED_GRGSM_TRX_H
+
+#include <grgsm/api.h>
+#include <gnuradio/sync_block.h>
+
+namespace gr {
+  namespace grgsm {
+
+    /*!
+     * \brief <+description of block+>
+     * \ingroup grgsm
+     *
+     */
+    class GRGSM_API trx : virtual public gr::block
+    {
+     public:
+      typedef boost::shared_ptr<trx> sptr;
+
+      /*!
+       * \brief Return a shared_ptr to a new instance of grgsm::trx.
+       *
+       * To avoid accidental use of raw pointers, grgsm::trx's
+       * constructor is in a private implementation
+       * class. grgsm::trx::make is the public interface for
+       * creating new instances.
+       */
+      static sptr make(
+        const std::string &remote_addr,
+        const std::string &base_port);
+    };
+
+  } // namespace grgsm
+} // namespace gr
+
+#endif /* INCLUDED_GRGSM_TRX_H */
+
diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index 680c41f..5d3f461 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -69,6 +69,8 @@ list(APPEND grgsm_sources
     qa_utils/message_source_impl.cc
     qa_utils/message_sink_impl.cc
     decryption/decryption_impl.cc
+    trx_interface/udp_socket.cc
+    trx_interface/trx_impl.cc
 )
 
 
diff --git a/lib/trx_interface/trx_impl.cc b/lib/trx_interface/trx_impl.cc
new file mode 100644
index 0000000..9b9c484
--- /dev/null
+++ b/lib/trx_interface/trx_impl.cc
@@ -0,0 +1,154 @@
+/* -*- c++ -*- */
+/* @file
+ * @author Vadim Yanitskiy <axilirator@gmail.com>
+ * @section LICENSE
+ * 
+ * Gr-gsm 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, or (at your option)
+ * any later version.
+ * 
+ * Gr-gsm 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 gr-gsm; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ * 
+ */
+
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#include <gnuradio/io_signature.h>
+#include <boost/lexical_cast.hpp>
+
+#include "udp_socket.h"
+#include "trx_impl.h"
+
+namespace gr {
+  namespace grgsm {
+
+    trx::sptr
+    trx::make(
+      const std::string &remote_addr,
+      const std::string &base_port)
+    {
+      int base_port_int = boost::lexical_cast<int> (base_port);
+
+      return gnuradio::get_initial_sptr
+        (new trx_impl(remote_addr, base_port_int));
+    }
+
+    /*
+     * The private constructor
+     */
+    trx_impl::trx_impl(const std::string &remote_addr, int base_port)
+    : gr::block("trx",
+        gr::io_signature::make(0, 0, 0),
+        gr::io_signature::make(0, 0, 0))
+    {
+        message_port_register_in(pmt::mp("bursts"));
+        set_msg_handler(pmt::mp("bursts"),
+          boost::bind(&trx_impl::handle_dl_burst, this, _1));
+
+        // Prepare port numbers
+        std::string clck_src_port = boost::lexical_cast<std::string> (base_port + 0);
+        std::string clck_dst_port = boost::lexical_cast<std::string> (base_port + 100);
+        std::string data_src_port = boost::lexical_cast<std::string> (base_port + 2);
+        std::string data_dst_port = boost::lexical_cast<std::string> (base_port + 102);
+
+        // Init DATA interface
+        d_data_sock = new udp_socket(remote_addr, data_src_port, data_dst_port);
+        d_clck_sock = new udp_socket(remote_addr, clck_src_port, clck_dst_port);
+    }
+
+    /*
+     * Our virtual destructor.
+     */
+    trx_impl::~trx_impl()
+    {
+        // Release all UDP sockets and free memory
+        delete d_data_sock;
+        delete d_clck_sock;
+    }
+
+    /*
+     * Create an UDP payload with clock indication
+     */
+    void
+    trx_impl::clck_ind_send(uint32_t frame_nr)
+    {
+      char buf[20];
+      size_t n;
+
+      n = snprintf(buf, 20, "IND CLOCK %u", frame_nr);
+      d_clck_sock->udp_send((uint8_t *) buf, n + 1);
+    }
+
+    /*
+     * Create an UDP payload with burst bits
+     * and some channel data.
+     */
+    void
+    trx_impl::burst_pack(pmt::pmt_t msg, uint8_t *buf)
+    {
+      pmt::pmt_t header_plus_burst = pmt::cdr(msg);
+
+      // Extract GSMTAP header from message
+      gsmtap_hdr *header = (gsmtap_hdr *)
+        pmt::blob_data(header_plus_burst);
+
+      // Pack timeslot index
+      buf[0] = header->timeslot;
+
+      // Extract frame number
+      uint32_t frame_nr = be32toh(header->frame_number);
+
+      // HACK: send clock indications every 51-th frame
+      if (frame_nr % 51 == 0)
+        clck_ind_send(frame_nr);
+
+      // Pack frame number
+      buf[1] = (frame_nr >> 24) & 0xff;
+      buf[2] = (frame_nr >> 16) & 0xff;
+      buf[3] = (frame_nr >>  8) & 0xff;
+      buf[4] = (frame_nr >>  0) & 0xff;
+
+      // Pack RSSI (-dBm)
+      buf[5] = -(uint8_t) header->signal_dbm;
+
+      // Pack correlator timing offset (TOA)
+      // FIXME: where to find this value?
+      buf[6] = 0;
+      buf[7] = 0;
+
+      // Extract bits {0..1} from message
+      // Despite GR-GSM uses int8_t, they are not real sbits {-127..127}
+      uint8_t *burst = (uint8_t *)
+        (pmt::blob_data(header_plus_burst)) + sizeof(gsmtap_hdr);
+
+      // Convert to transceiver interface specific bits {255..0}
+      for (int i = 0; i < 148; i++)
+        buf[8 + i] = burst[i] ? 255 : 0;
+    }
+
+    void
+    trx_impl::handle_dl_burst(pmt::pmt_t msg)
+    {
+      // 8 bytes of header + 148 bytes of burst
+      uint8_t buf[156];
+
+      // Compose a new UDP payload with burst
+      burst_pack(msg, buf);
+
+      // Send a burst
+      d_data_sock->udp_send(buf, 156);
+    }
+
+  } /* namespace grgsm */
+} /* namespace gr */
diff --git a/lib/trx_interface/trx_impl.h b/lib/trx_interface/trx_impl.h
new file mode 100644
index 0000000..41a4788
--- /dev/null
+++ b/lib/trx_interface/trx_impl.h
@@ -0,0 +1,54 @@
+/* -*- c++ -*- */
+/* @file
+ * @author Vadim Yanitskiy <axilirator@gmail.com>
+ * @section LICENSE
+ * 
+ * Gr-gsm 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, or (at your option)
+ * any later version.
+ * 
+ * Gr-gsm 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 gr-gsm; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ * 
+ */
+
+#ifndef INCLUDED_GRGSM_TRX_IMPL_H
+#define INCLUDED_GRGSM_TRX_IMPL_H
+
+#include <stddef.h>
+
+#include <grgsm/gsmtap.h>
+#include <grgsm/trx_interface/trx.h>
+
+namespace gr {
+  namespace grgsm {
+
+    class trx_impl : public trx
+    {
+     private:
+      udp_socket *d_data_sock;
+      udp_socket *d_clck_sock;
+
+      void clck_ind_send(uint32_t frame_nr);
+      void burst_pack(pmt::pmt_t msg, uint8_t *buf);
+
+     public:
+      trx_impl(const std::string &remote_addr, int base_port);
+      ~trx_impl();
+
+      void handle_dl_burst(pmt::pmt_t msg);
+    };
+
+  } // namespace grgsm
+} // namespace gr
+
+#endif /* INCLUDED_GRGSM_TRX_IMPL_H */
+
diff --git a/lib/trx_interface/udp_socket.cc b/lib/trx_interface/udp_socket.cc
new file mode 100644
index 0000000..4bbf206
--- /dev/null
+++ b/lib/trx_interface/udp_socket.cc
@@ -0,0 +1,117 @@
+/* -*- c++ -*- */
+/*
+ * Copyright 2013 Free Software Foundation, Inc.
+ *
+ * This file is part of GNU Radio
+ *
+ * GNU Radio 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, or (at your option)
+ * any later version.
+ *
+ * GNU Radio 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 GNU Radio; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#include <gnuradio/thread/thread.h>
+#include <gnuradio/io_signature.h>
+#include <gnuradio/blocks/pdu.h>
+#include <pmt/pmt.h>
+
+#include <boost/lexical_cast.hpp>
+#include "udp_socket.h"
+
+using boost::asio::ip::udp;
+
+namespace gr {
+  namespace grgsm {
+
+    udp_socket::udp_socket(
+      const std::string &remote_addr,
+      const std::string &src_port,
+      const std::string &dst_port)
+    {
+      // Resolve remote host address
+      udp::resolver resolver(d_io_service);
+
+      udp::resolver::query rx_query(
+        udp::v4(), remote_addr, src_port,
+        boost::asio::ip::resolver_query_base::passive);
+      udp::resolver::query tx_query(
+        udp::v4(), remote_addr, dst_port,
+        boost::asio::ip::resolver_query_base::passive);
+
+      d_udp_endpoint_rx = *resolver.resolve(rx_query);
+      d_udp_endpoint_tx = *resolver.resolve(tx_query);
+
+      // Create a socket
+      d_udp_socket.reset(new udp::socket(d_io_service, d_udp_endpoint_rx));
+
+      // Setup read handler
+      d_udp_socket->async_receive_from(
+        boost::asio::buffer(d_rxbuf), d_udp_endpoint_rx,
+        boost::bind(&udp_socket::handle_udp_read, this,
+          boost::asio::placeholders::error,
+          boost::asio::placeholders::bytes_transferred));
+
+      // Start server
+      d_thread = gr::thread::thread(
+        boost::bind(&udp_socket::run_io_service, this));
+    }
+
+    udp_socket::~udp_socket()
+    {
+      // Stop server
+      d_io_service.stop();
+      d_thread.interrupt();
+      d_thread.join();
+    }
+
+    void
+    udp_socket::run_io_service(void)
+    {
+      d_io_service.run();
+    }
+
+    void
+    udp_socket::udp_send(uint8_t *data, size_t len)
+    {
+      d_udp_socket->send_to(
+        boost::asio::buffer(data, len),
+        d_udp_endpoint_tx);
+    }
+
+    void
+    udp_socket::handle_udp_read(
+      const boost::system::error_code& error,
+      size_t bytes_transferred)
+    {
+      if (error)
+        return;
+
+      pmt::pmt_t vector = pmt::init_u8vector(bytes_transferred,
+        (const uint8_t *) &d_rxbuf[0]);
+      pmt::pmt_t pdu = pmt::cons(pmt::PMT_NIL, vector);
+
+      // TODO: call handler here
+
+      d_udp_socket->async_receive_from(
+        boost::asio::buffer(d_rxbuf), d_udp_endpoint_rx,
+        boost::bind(&udp_socket::handle_udp_read, this,
+          boost::asio::placeholders::error,
+          boost::asio::placeholders::bytes_transferred));
+    }
+
+  } /* namespace blocks */
+}/* namespace gr */
diff --git a/lib/trx_interface/udp_socket.h b/lib/trx_interface/udp_socket.h
new file mode 100644
index 0000000..2f424c9
--- /dev/null
+++ b/lib/trx_interface/udp_socket.h
@@ -0,0 +1,65 @@
+/* -*- c++ -*- */
+/*
+ * Copyright 2013 Free Software Foundation, Inc.
+ *
+ * This file is part of GNU Radio
+ *
+ * GNU Radio 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, or (at your option)
+ * any later version.
+ *
+ * GNU Radio 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 GNU Radio; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#ifndef INCLUDED_GRGSM_TRX_UDP_SOCKET_H
+#define INCLUDED_GRGSM_TRX_UDP_SOCKET_H
+
+#include <gnuradio/thread/thread.h>
+
+#include <boost/array.hpp>
+#include <boost/asio.hpp>
+#include <pmt/pmt.h>
+
+namespace gr {
+  namespace grgsm {
+
+    class udp_socket
+    {
+    private:
+      boost::asio::io_service d_io_service;
+      std::vector<char> d_rxbuf;
+      gr::thread::thread d_thread;
+      bool d_started;
+      bool d_finished;
+
+      boost::asio::ip::udp::endpoint d_udp_endpoint_rx;
+      boost::asio::ip::udp::endpoint d_udp_endpoint_tx;
+      boost::shared_ptr<boost::asio::ip::udp::socket> d_udp_socket;
+
+      void handle_udp_read(const boost::system::error_code& error,
+        size_t bytes_transferred);
+      void run_io_service(void);
+
+    public:
+      udp_socket(
+        const std::string &remote_addr,
+        const std::string &src_port,
+        const std::string &dst_port);
+      ~udp_socket();
+
+      void udp_send(uint8_t *data, size_t len);
+    };
+
+  } /* namespace blocks */
+} /* namespace gr */
+
+#endif /* INCLUDED_GRGSM_TRX_UDP_SOCKET_H */
diff --git a/swig/grgsm_swig.i b/swig/grgsm_swig.i
index 68911ae..6013cb1 100644
--- a/swig/grgsm_swig.i
+++ b/swig/grgsm_swig.i
@@ -41,6 +41,7 @@
 #include "grgsm/misc_utils/message_file_source.h"
 #include "grgsm/misc_utils/msg_to_tag.h"
 #include "grgsm/misc_utils/controlled_fractional_resampler_cc.h"
+#include "grgsm/trx_interface/trx.h"
 %}
 
 %include "grgsm/receiver/receiver.h"
@@ -117,3 +118,5 @@ GR_SWIG_BLOCK_MAGIC2(gsm, burst_source);
 GR_SWIG_BLOCK_MAGIC2(gsm, message_source);
 %include "grgsm/qa_utils/message_sink.h"
 GR_SWIG_BLOCK_MAGIC2(gsm, message_sink);
+%include "grgsm/trx_interface/trx.h"
+GR_SWIG_BLOCK_MAGIC2(grgsm, trx);