fake_trx/burst_fwd.py: use DATAMSG transformation API

As the DATAMSG classes were introduced, let's use them.
This approach abstracts one from dealing with raw bytes.

Also, now BurstForwarder randomizes both RSSI and ToA values,
as this feature is supported from-the-box by the DATAMSG_TRX2L1.

Change-Id: Ib15018eab749150e244914dab4b6e433ce0c9209

src/target/fake_trx/burst_fwd.py

@@ -22,6 +22,10 @@
 # with this program; if not, write to the Free Software Foundation, Inc.,
 # 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 
+import random
+
+from data_msg import *
+
 class BurstForwarder:
 	# Timeslot filter (drop everything by default)
 	ts_pass = None
@@ -30,40 +34,60 @@ class BurstForwarder:
 	bts_freq = None
 	bb_freq = None
 
+	# Constants
+	# TODO: add options to change this
+	RSSI_RAND_TRESHOLD = 10
+	RSSI_RAND_MIN = -90
+	RSSI_RAND_MAX = -60
+
+	# TODO: add options to change this
+	TOA_RAND_TRESHOLD = 0.3
+	TOA_RAND_BASE = 0.00
+
 	def __init__(self, bts_link, bb_link):
 		self.bts_link = bts_link
 		self.bb_link = bb_link
 
+		# Generate a random RSSI range
+		rssi = random.randint(self.RSSI_RAND_MIN, self.RSSI_RAND_MAX)
+		self.rssi_min = rssi - self.RSSI_RAND_TRESHOLD
+		self.rssi_max = rssi + self.RSSI_RAND_TRESHOLD
+
+		# Generate a random ToA range
+		self.toa_min = self.TOA_RAND_BASE - self.TOA_RAND_TRESHOLD
+		self.toa_max = self.TOA_RAND_BASE + self.TOA_RAND_TRESHOLD
+
 	def set_slot(self, ts):
 		if ts > 0 and ts < 8:
 			self.ts_pass = ts
 		else:
 			raise ValueError("Incorrect index for timeslot filter")
 
-	def process_payload(self, data):
-		payload = bytearray(data)
-		length = len(payload)
+	# Converts a L12TRX message to TRX2L1 message
+	def transform_msg(self, msg_raw):
+		# Attempt to parse a message
+		try:
+			msg_l12trx = DATAMSG_L12TRX()
+			msg_l12trx.parse_msg(bytearray(msg_raw))
+		except:
+			print("[!] Dropping unhandled DL message...")
+			return None
 
-		# HACK: set fake RSSI value (-53)
-		payload[5] = 0x35
+		# Compose a new message for L1
+		msg_trx2l1 = msg_l12trx.gen_trx2l1()
 
-		# HACK: add fake TOA value (6th and 7th bytes)
-		payload[6:2] = [0x00, 0x00]
+		# Randomize both RSSI and ToA values
+		msg_trx2l1.rssi = msg_trx2l1.rand_rssi(
+			min = self.rssi_min, max = self.rssi_max)
+		msg_trx2l1.toa = msg_trx2l1.rand_toa(
+			min = self.toa_min, max = self.toa_max)
 
-		# Convert ubits to {255..0}
-		for i in range(8, length):
-			payload[i] = 255 if payload[i] else 0
-
-		# WTF: append two unused bytes at the end
-		payload[length:2] = [0x00, 0x00]
-
-		return payload
+		return msg_trx2l1
 
 	# Downlink handler: BTS -> BB
 	def bts2bb(self):
 		# Read data from socket
 		data, addr = self.bts_link.sock.recvfrom(512)
-		payload = self.process_payload(data)
 
 		# BB is not connected / tuned
 		if self.bb_freq is None:
@@ -73,10 +97,22 @@ class BurstForwarder:
 		if self.bb_freq != self.bts_freq:
 			return None
 
-		# Timeslot filter
-		if payload[0] != self.ts_pass:
+		# Process a message
+		msg = self.transform_msg(data)
+		if msg is None:
 			return None
 
+		# Timeslot filter
+		if msg.tn != self.ts_pass:
+			return None
+
+		# Validate and generate the payload
+		payload = msg.gen_msg()
+
+		# Append two unused bytes at the end
+		# in order to keep the compatibility
+		payload += bytearray(2)
+
 		# Send burst to BB
 		self.bb_link.send(payload)
 
@@ -84,7 +120,6 @@ class BurstForwarder:
 	def bb2bts(self):
 		# Read data from socket
 		data, addr = self.bb_link.sock.recvfrom(512)
-		payload = self.process_payload(data)
 
 		# BTS is not connected / tuned
 		if self.bts_freq is None:
@@ -94,5 +129,17 @@ class BurstForwarder:
 		if self.bb_freq != self.bts_freq:
 			return None
 
+		# Process a message
+		msg = self.transform_msg(data)
+		if msg is None:
+			return None
+
+		# Validate and generate the payload
+		payload = msg.gen_msg()
+
+		# Append two unused bytes at the end
+		# in order to keep the compatibility
+		payload += bytearray(2)
+
 		# Send burst to BTS
 		self.bts_link.send(payload)