TODO: mention some of the known issues in the TODO list

[SaveMySugar/python3-savemysugar.git] / src / savemysugar / CallDistanceTransceiver.py

diff --git a/src/savemysugar/CallDistanceTransceiver.py b/src/savemysugar/CallDistanceTransceiver.py
index c251673..f04c533 100755 (executable)
--- a/src/savemysugar/CallDistanceTransceiver.py
+++ b/src/savemysugar/CallDistanceTransceiver.py
@@ -29,6 +29,143 @@ import logging
 import time
 
 
 import time
 
 

+class SymbolTime(object):
+    """
+    In theory the symbol distance (the distance to discriminate symbols) can
+    be arbitrary, and it's only bounded below by the stability of the period
+    time, but in practice it can be necessary to wait a predefined minimum
+    amount of time between periods because of technological limits of the
+    transmitting devices, we call this time the "minimum inter-symbol
+    distance".
+    """
+
+    # pylint: disable=too-few-public-methods
+    def __init__(self, period_min, period_max, multiplier,
+                 min_inter_symbol_distance=0.0):
+        assert multiplier >= 0
+        if (period_min == period_max) and (min_inter_symbol_distance == 0):
+            raise ValueError("If (period_min == period_max) a non-zero",
+                             "inter-symbol distance MUST be specified")
+
+        symbol_distance = 2 * (period_max - period_min)
+        if symbol_distance == 0:
+            symbol_distance = min_inter_symbol_distance
+
+        # The time the transmitter has to wait to disambiguate between this
+        # symbol and a different one.
+        self.dist = min_inter_symbol_distance + symbol_distance * multiplier
+
+        # The minimum time which represents the symbol at the receiving end.
+        self.min = min_inter_symbol_distance + period_min + \
+            symbol_distance * multiplier
+
+        # The maximum time which represents the symbol at the receiving end
+        self.max = min_inter_symbol_distance + period_min + \
+            symbol_distance * (multiplier + 1)
+
+
+class MorseDistanceModulator(object):
+    def __init__(self, period_min, period_max, pulse_min, pulse_max,
+                 inter_symbol_distance):
+        self.set_parameters(period_min, period_max,
+                            pulse_min, pulse_max,
+                            inter_symbol_distance)
+
+    def set_parameters(self, period_min, period_max, pulse_min, pulse_max,
+                       inter_symbol_distance):
+        self.period_min = period_min
+        self.period_max = period_max
+        self.pulse_min = pulse_min
+        self.pulse_max = pulse_max
+        self.inter_symbol_distance = inter_symbol_distance
+
+        self.dot_time = SymbolTime(period_min,
+                                   period_max, 0,
+                                   inter_symbol_distance)
+        self.dash_time = SymbolTime(period_min,
+                                    period_max, 1,
+                                    inter_symbol_distance)
+        self.signalspace_time = SymbolTime(period_min,
+                                           period_max, 2,
+                                           inter_symbol_distance)
+        self.wordspace_time = SymbolTime(period_min,
+                                         period_max, 3,
+                                         inter_symbol_distance)
+        self.eom_time = SymbolTime(period_min,
+                                   period_max, 4,
+                                   inter_symbol_distance)
+
+    def symbol_to_distance(self, symbol):
+        if symbol == ".":
+            return self.dot_time.dist
+        elif symbol == "-":
+            return self.dash_time.dist
+        elif symbol == " ":
+            return self.signalspace_time.dist
+        elif symbol == "/" or symbol == " / ":
+            return self.wordspace_time.dist
+        elif symbol == "EOM":
+            return self.eom_time.dist
+
+        raise ValueError("Unexpected symbol %s" % symbol)
+
+    def is_same_period(self, distance):
+        return distance > self.pulse_min and distance <= self.pulse_max
+
+    def distance_to_symbol(self, distance):
+        if distance > self.dot_time.min and \
+           distance <= self.dot_time.max:
+            return "."
+
+        if distance > self.dash_time.min and \
+           distance <= self.dash_time.max:
+            return "-"
+
+        if distance > self.signalspace_time.min and \
+           distance <= self.signalspace_time.max:
+            return " "
+
+        if distance > self.wordspace_time.min and \
+           distance <= self.wordspace_time.max:
+            return "/"
+
+        if distance > self.eom_time.min:
+            return "EOM"
+
+        raise ValueError("Unexpected distance %.2f" % distance)
+
+    def modulate(self, morse):
+        signals = morse.split(' ')
+        distances = []
+        for i, signal in enumerate(signals):
+            for symbol in signal:
+                distances.append(self.symbol_to_distance(symbol))
+
+            # Transmit a signal separator only when strictly necessary.
+            #
+            # Avoid it in these cases:
+            #  - after the last symbol, because EOM is going to ne transmitted
+            #    anyway and that will mark the end of the last symbol.
+            #  - between words, because the word separator act as a symbol
+            #    separator too.
+            if i != len(signals) - 1 and signals[i + 1] != "/" and signal != "/":
+                distances.append(self.symbol_to_distance(" "))
+
+        distances.append(self.symbol_to_distance("EOM"))
+
+        # Since the Morse signals are encoded in the distance between calls, an
+        # extra call is needed in order for receiver actually get the EOM and
+        # see that the transmission has terminated.
+        distances.append(0)
+
+        return distances
+
+
+def log_symbol(distance, symbol, extra_info=""):
+    logging.info("distance: %.2f Received \"%s\"%s", distance, symbol,
+                 extra_info)
+
+

 class CallDistanceTransceiver(object):
     """Transmit Morse messages using the distance between calls.
 
 class CallDistanceTransceiver(object):
     """Transmit Morse messages using the distance between calls.
 


@@ -50,78 +187,100 @@ class CallDistanceTransceiver(object):
     fine.
 
     Plus, supporting communications between analog modems is cool :)
     fine.
 
     Plus, supporting communications between analog modems is cool :)

-

     """
 
     def __init__(self, modem,
     """
 
     def __init__(self, modem,

-                 call_setup_average=8.5, call_setup_uncertainty=1.5,
-                 ring_distance_average=5, ring_uncertainty=0.2):
+                 call_setup_time_min=7, call_setup_time_max=15,
+                 ring_time_min=4.8, ring_time_max=5.2,
+                 add_inter_call_distance=True):

         """Encode the Morse symbols using the distance between calls.
 
         Args:
         """Encode the Morse symbols using the distance between calls.
 
         Args:

-            call_setup_average: the time between when the transmitter dials
-                the number and the receiver reports RING notifications. This is
-                needed in order to be sure than the receiver has received
-                _at_least_ one ring. The default chosen here should work fine
-                even with old analog modems, provided that there is some more
-                distance between two calls.
-            call_setup_uncertainty: uncertainty of call_setup_average,
-                a tolerance value
-            ring_distance_average: the time between two consecutive RINGs
+
+            call_setup_time_min: the minimum time between  when the transmitter
+                dials the number and the receiver reports RING notifications.
+            call_setup_time_max: the maximum time between when the transmitter
+                dials the number and the receiver reports RING notifications.
+                Waiting this time after dialing ensures that the receiver has
+                received _at_least_ one ring. The default chosen here have been
+                tested to work fine with old analog modems, provided that there
+                is some more distance between two calls, and with Android
+                receivers.
+            ring_time_min: the minimum time between two consecutive RINGs
+                in the same call.
+            ring_time_max: the maximum time between two consecutive RINGs

                 in the same call, the standard interval is about 5 seconds, but
                 in the same call, the standard interval is about 5 seconds, but

-                some tolerance is needed to account for line delays.
-                This is needed in order to ignore rings in the same call, and
-                differentiate two different calls.
-            ring_uncertainty: uncertainty of ring_distance_average, a tolerance
-                value
-        """
+                line and/or software delays can make it vary.  This is needed
+                in order to ignore multiple ring in the same call when multiple
+                RING in the same call are notified, and then be able to
+                discriminate between two different calls.
+            add_inter_call_distance: specify if it is needed to wait an extra
+                fixed time between calls.
+            """

 
         self.modem = modem
         self.translator = MorseTranslator()
 
 
         self.modem = modem
         self.translator = MorseTranslator()
 

-        self.destination_number = ""
-
-        self.call_setup_time = call_setup_average + call_setup_uncertainty
-        self.rings_distance = ring_distance_average + ring_uncertainty
-
-        # In theory the symbol distance, the distance between calls which
-        # represent symbols, can be arbitrary, but in practice it's better to
-        # wait at least the duration of a ring between terminating one call and
-        # initiating the next call, as pick-up and hang-up can take some time
-        # with old analog modems.
-        symbol_distance = self.rings_distance
-
-        def symbol_time(multiplier):
-            return self.call_setup_time + symbol_distance * multiplier
-
-        self.dot_time = symbol_time(1)
-        self.dash_time = symbol_time(2)
-        self.signalspace_time = symbol_time(3)
-        self.wordspace_time = symbol_time(4)
-        self.eom_time = symbol_time(5)
-
-        self.ring_uncertainty = ring_uncertainty
-        self.symbol_uncertainty = symbol_distance / 2.
-
-        logging.debug("dot time:         transmit: %.2f receive: (%.2f, %.2f)",
-                      self.dot_time,
-                      (self.dot_time - self.symbol_uncertainty),
-                      (self.dot_time + self.symbol_uncertainty))
-        logging.debug("dash time:        transmit: %.2f receive: (%.2f, %.2f)",
-                      self.dash_time,
-                      (self.dash_time - self.symbol_uncertainty),
-                      (self.dash_time + self.symbol_uncertainty))
-        logging.debug("signalspace time: transmit: %.2f receive: (%.2f, %.2f)",
-                      self.signalspace_time,
-                      (self.signalspace_time - self.symbol_uncertainty),
-                      (self.signalspace_time + self.symbol_uncertainty))
-        logging.debug("wordspace time:   transmit: %.2f receive: (%.2f, %.2f)",
-                      self.wordspace_time,
-                      (self.wordspace_time - self.symbol_uncertainty),
-                      (self.wordspace_time + self.symbol_uncertainty))
-        logging.debug("EOM time:         transmit: %.2f receive: (%.2f, +inf)",
-                      self.eom_time,
-                      (self.eom_time - self.symbol_uncertainty))
+        self.call_setup_time_max = call_setup_time_max
+
+        if add_inter_call_distance:
+            # Analog modems don't like to dial a new call immediately after
+            # they hung up the previous one; an extra delay of about one
+            # ring time makes them happy.
+            inter_symbol_distance = ring_time_max
+        else:
+            inter_symbol_distance = 0
+
+        self.modulator = MorseDistanceModulator(call_setup_time_min,
+                                                call_setup_time_max,
+                                                ring_time_min,
+                                                ring_time_max,
+                                                inter_symbol_distance)
+
+        logging.debug("call setup time between %.2f and %.2f "
+                      "--------- dot transmit time: %.2f + %.2f "
+                      "receive time: between %.2f and %.2f",
+                      self.modulator.period_min,
+                      self.modulator.period_max,
+                      self.modulator.period_max,
+                      self.modulator.dot_time.dist,
+                      self.modulator.dot_time.min,
+                      self.modulator.dot_time.max)
+        logging.debug("call setup time between %.2f and %.2f "
+                      "-------- dash transmit time: %.2f + %.2f "
+                      "receive time: between %.2f and %.2f",
+                      self.modulator.period_min,
+                      self.modulator.period_max,
+                      self.modulator.period_max,
+                      self.modulator.dash_time.dist,
+                      self.modulator.dash_time.min,
+                      self.modulator.dash_time.max)
+        logging.debug("call setup time between %.2f and %.2f "
+                      "- signalspace transmit time: %.2f + %.2f "
+                      "receive time: between %.2f and %.2f",
+                      self.modulator.period_min,
+                      self.modulator.period_max,
+                      self.modulator.period_max,
+                      self.modulator.signalspace_time.dist,
+                      self.modulator.signalspace_time.min,
+                      self.modulator.signalspace_time.max)
+        logging.debug("call setup time between %.2f and %.2f "
+                      "--- wordspace transmit time: %.2f + %.2f "
+                      "receive time: between %.2f and %.2f",
+                      self.modulator.period_min,
+                      self.modulator.period_max,
+                      self.modulator.period_max,
+                      self.modulator.wordspace_time.dist,
+                      self.modulator.wordspace_time.min,
+                      self.modulator.wordspace_time.max)
+        logging.debug("call setup time between %.2f and %.2f "
+                      "--------- EOM transmit time: %.2f + %.2f "
+                      "receive time: between %.2f and inf",
+                      self.modulator.period_min,
+                      self.modulator.period_max,
+                      self.modulator.period_max,
+                      self.modulator.eom_time.dist,
+                      self.modulator.eom_time.min)

 
         self.previous_ring_time = -1
         self.previous_call_time = -1
 
         self.previous_ring_time = -1
         self.previous_call_time = -1


@@ -131,17 +290,13 @@ class CallDistanceTransceiver(object):
 
         self.end_of_message = False
 
 
         self.end_of_message = False
 

-    def log_symbol(self, distance, symbol, extra_info=""):
-        logging.info("distance: %.2f Received \"%s\" %s", distance, symbol,
-                     extra_info)
-
-    def receive_character(self):
+    def receive_symbol(self):

         current_ring_time = time.time()
 
         if self.previous_ring_time == -1:
             self.previous_ring_time = current_ring_time
             self.previous_call_time = current_ring_time
         current_ring_time = time.time()
 
         if self.previous_ring_time == -1:
             self.previous_ring_time = current_ring_time
             self.previous_call_time = current_ring_time

-            self.log_symbol(0, "", "(The very first ring)")
+            log_symbol(0, "", "(The very first ring)")

             return
 
         ring_distance = current_ring_time - self.previous_ring_time
             return
 
         ring_distance = current_ring_time - self.previous_ring_time


@@ -149,7 +304,7 @@ class CallDistanceTransceiver(object):
         self.previous_ring_time = current_ring_time
 
         # Ignore multiple rings in the same call
         self.previous_ring_time = current_ring_time
 
         # Ignore multiple rings in the same call

-        if abs(ring_distance - self.rings_distance) < self.ring_uncertainty:
+        if self.modulator.is_same_period(ring_distance):

             logging.debug("multiple rings in the same call, distance: %.2f",
                           ring_distance)
             return
             logging.debug("multiple rings in the same call, distance: %.2f",
                           ring_distance)
             return


@@ -157,47 +312,36 @@ class CallDistanceTransceiver(object):
         call_distance = current_ring_time - self.previous_call_time
         self.previous_call_time = current_ring_time
 
         call_distance = current_ring_time - self.previous_call_time
         self.previous_call_time = current_ring_time
 

-        if abs(call_distance - self.dot_time) < self.symbol_uncertainty:
-            self.log_symbol(call_distance, '.')
-            self.morse_message += "."
+        try:
+            symbol = self.modulator.distance_to_symbol(call_distance)
+        except ValueError as err:
+            logging.error("%s", err)
+            logging.error("Check the transmitter and receiver parameters")

             return
 
             return
 

-        if abs(call_distance - self.dash_time) < self.symbol_uncertainty:
-            self.log_symbol(call_distance, '-')
-            self.morse_message += "-"
-            return
-
-        if abs(call_distance - self.signalspace_time) < self.symbol_uncertainty:
+        extra_info = ""
+        if symbol in [" ", "/", "EOM"]:

             signal = self.morse_message.strip().split(' ')[-1]
             character = self.translator.signal_to_character(signal)
             signal = self.morse_message.strip().split(' ')[-1]
             character = self.translator.signal_to_character(signal)

-            self.log_symbol(call_distance, ' ', "got \"%s\"" % character)
-            self.morse_message += " "
-            return
+            extra_info = " got \"%s\"" % character

 
 

-        if abs(call_distance - self.wordspace_time) < self.symbol_uncertainty:
-            signal = self.morse_message.strip().split(' ')[-1]
-            character = self.translator.signal_to_character(signal)
-            self.log_symbol(call_distance, '/', "got \"%s\"" % character)
-            self.morse_message += " / "
-            return
+        log_symbol(call_distance, symbol, extra_info)

 
 

-        if call_distance >= self.eom_time - self.symbol_uncertainty:
-            signal = self.morse_message.strip().split(' ')[-1]
-            character = self.translator.signal_to_character(signal)
-            self.log_symbol(call_distance, 'EOM', "got \"%s\"" % character)
+        if symbol != "EOM":
+            # Add spaces around the wordspace symbol to make it easier to split
+            # the Morse message in symbols later on
+            if symbol == "/":
+                symbol = " / "
+            self.morse_message += symbol
+        else:

             self.end_of_message = True
             self.previous_ring_time = -1
             self.previous_call_time = -1
             self.end_of_message = True
             self.previous_ring_time = -1
             self.previous_call_time = -1

-            return
-
-        # if the code made it up to here, something fishy is going on
-        logging.error("Unexpected distance: %.2f", ring_distance)
-        logging.error("Check the transmitter and receiver parameters")

 
     def receive_loop(self):
         while not self.end_of_message:
             self.modem.get_response("RING")
 
     def receive_loop(self):
         while not self.end_of_message:
             self.modem.get_response("RING")

-            self.receive_character()
+            self.receive_symbol()

             logging.debug("Current message: %s", self.morse_message)
 
         self.end_of_message = False
             logging.debug("Current message: %s", self.morse_message)
 
         self.end_of_message = False


@@ -207,123 +351,118 @@ class CallDistanceTransceiver(object):
     def get_text(self):
         return self.text_message
 
     def get_text(self):
         return self.text_message
 

-    def transmit_symbol(self, symbol):
-        if symbol == ".":
-            sleep_time = self.dot_time
-        elif symbol == "-":
-            sleep_time = self.dash_time
-        elif symbol == " ":
-            sleep_time = self.signalspace_time
-        elif symbol == "/":
-            sleep_time = self.wordspace_time
-        elif symbol == "EOM":
-            sleep_time = self.eom_time
-        elif symbol is None:
-            # To terminate the transmission just call and hangup, with no extra
-            # distance
-            sleep_time = self.call_setup_time
-
-        logging.info("Dial and wait %.2f seconds (transmitting '%s')",
-                     sleep_time, symbol)
-
-        # Dial, then wait self.call_setup_time to make sure the receiver gets
-        # at least one RING, and then hangup and sleep the remaining time
-        self.modem.send_command("ATDT" + self.destination_number)
-        time.sleep(self.call_setup_time)
+    def transmit_symbol(self, destination_number, symbol, sleep_time):
+        logging.info("Dial and wait %.2f = %.2f + %.2f seconds "
+                     "(transmitting '%s')",
+                     self.call_setup_time_max + sleep_time,
+                     self.call_setup_time_max,
+                     sleep_time,
+                     symbol)
+
+        # Dial, then wait self.call_setup_time_max to make sure the receiver
+        # gets at least one RING, and then hangup and sleep the time needed to
+        # transmit a symbol.
+        self.modem.send_command("ATDT" + destination_number + ";")
+        time.sleep(self.call_setup_time_max)

         self.modem.send_command("ATH")
         self.modem.get_response()
         self.modem.send_command("ATH")
         self.modem.get_response()

-        time.sleep(sleep_time - self.call_setup_time)
-
-    def transmit_signal(self, signal):
-        logging.debug("Transmitting signal: %s", signal)
-        for symbol in signal:
-            self.transmit_symbol(symbol)
+        time.sleep(sleep_time)

 
     def transmit(self, message, destination_number):
 
     def transmit(self, message, destination_number):

-        self.destination_number = destination_number
-

         morse_message = self.translator.text_to_morse(message)
         morse_message = self.translator.text_to_morse(message)

-        signals = morse_message.split()
-        print(signals)
+        distances = self.modulator.modulate(morse_message)

 
         logging.debug("Starting the transmission")
 
         logging.debug("Starting the transmission")

-        for i, signal in enumerate(signals):
-            logging.debug("Transmitting '%s' as '%s'", message[i], signal)
-            self.transmit_signal(signal)
-
-            # Transmit a signal separator only when strictly necessary:
-            #  - after the last symbol, we are going to transmit an EOM
-            #    anyway, and that will mark the end of the last symbol.
-            #  - between words the word separator act as a symbol separator
-            #    too.
-            if i != len(signals) - 1 and signals[i + 1] != "/" and signal != "/":
-                self.transmit_symbol(" ")
+        for i, distance in enumerate(distances):
+            # Use 'None' for the last call
+            if i == len(distances) - 1:
+                symbol = None
+            else:
+                total_sleep_time = self.call_setup_time_max + distance
+                symbol = self.modulator.distance_to_symbol(total_sleep_time)

 
 

-        self.transmit_symbol("EOM")
-
-        # Since the Morse signals are encoded in the distance between calls, an
-        # extra call is needed in order for receiver actually get the EOM and
-        # see that the transmission has terminated.
-        self.transmit_symbol(None)
+            self.transmit_symbol(destination_number, symbol, distance)

 
     def estimate_transmit_duration(self, message):
         morsemessage = self.translator.text_to_morse(message)
 
     def estimate_transmit_duration(self, message):
         morsemessage = self.translator.text_to_morse(message)

-        signals = morsemessage.split()
+        logging.debug(morsemessage)

 
 

-        logging.debug(signals)
+        distances = self.modulator.modulate(morsemessage)

 
         transmitting_time = 0
 
         transmitting_time = 0

-        for i, signal in enumerate(signals):
-            logging.debug("signal: %s", signal)
-
-            for symbol in signal:
-                if symbol == ".":
-                    transmitting_time += self.dot_time
-                elif symbol == "-":
-                    transmitting_time += self.dash_time
-                elif symbol == "/":
-                    transmitting_time += self.wordspace_time
-
-                if i != len(signals) - 1 and signals[i + 1] != "/" and symbol != "/":
-                    transmitting_time += self.signalspace_time
+        for distance in distances:
+            transmitting_time += self.call_setup_time_max
+            transmitting_time += distance

 
 

-        transmitting_time += self.eom_time
-
-        logging.debug("Estimated transmitting time: %d seconds",
+        logging.debug("Estimated transmitting time: %.2f seconds",

                       transmitting_time)
 
 
                       transmitting_time)
 
 

-def test_send_receive():
+def test_transmit_receive():

     logging.basicConfig(level=logging.DEBUG)
     logging.basicConfig(level=logging.DEBUG)

-    call_setup_time = 2
-    call_setup_uncertainty = 0.4
-    ring_time = 1
-    ring_uncertainty = 0.3
+    call_setup_time_min = 0
+    call_setup_time_max = 0.01
+    ring_time_min = 0
+    ring_time_max = 0
+
+    import random

 
     class DummyModem(object):
 
     class DummyModem(object):

-        """Always receive a '.' and then a '/', which result in 'E '."""
+        """Always receive a '.', a '/' and then EOM, which results in 'E '."""

 
         def __init__(self):
             self.ring_count = 0
 
 
         def __init__(self):
             self.ring_count = 0
 

+            # Take trasmission times from a transceiver
+            self.transceiver = None
+
+            random.seed(None)
+

         def send_command(self, command):
             pass
 
         def send_command(self, command):
             pass
 

-        def get_response(self, response):
-            if self.ring_count % 2:
+        def get_response(self, response=None):
+            # pylint: disable=unused-argument
+
+            setup_time = random.uniform(self.transceiver.modulator.period_min,
+                                        self.transceiver.modulator.period_max)
+
+            if self.ring_count == 0:
+                # dummy ring
+                pass
+            elif self.ring_count == 1:

                 # received a '.'
                 # received a '.'

-                time.sleep(call_setup_time + (ring_time + ring_uncertainty))
-            else:
+                dot_time = self.transceiver.modulator.dot_time.dist
+                time.sleep(setup_time + dot_time)
+            elif self.ring_count == 2:

                 # received a '/'
                 # received a '/'

-                time.sleep(call_setup_time + (ring_time + ring_uncertainty) * 4)
+                wordspace_time = self.transceiver.modulator.wordspace_time.dist
+                time.sleep(setup_time + wordspace_time)
+            else:
+                # received an 'EOM'
+                eom_time = self.transceiver.modulator.eom_time.dist
+                time.sleep(setup_time + eom_time)

 
             self.ring_count += 1
 
             self.ring_count += 1

+            self.ring_count %= 4
+
+    modem = DummyModem()
+
+    xcv = CallDistanceTransceiver(modem,
+                                  call_setup_time_min, call_setup_time_max,
+                                  ring_time_min, ring_time_max, True)
+
+    modem.transceiver = xcv
+
+    xcv.transmit("CODEX PARIS", "0")

 
 

-    xcv = CallDistanceTransceiver(DummyModem(), call_setup_time,
-                                  call_setup_uncertainty, ring_time,
-                                  ring_uncertainty)
-    xcv.receive_loop()
+    while True:
+        modem.ring_count = 0
+        xcv.receive_loop()
+        print()
+        print("Message received!")
+        print("\"%s\"" % xcv.get_text(), flush=True)

 
 
 if __name__ == "__main__":
 
 
 if __name__ == "__main__":

-    test_send_receive()
+    test_transmit_receive()