Fix EGSM900 downlink frequency: 935 MHz not 925 MHz

The EGSM900 band table had start=925e6 but ARFCNs 0-124 use downlink
frequencies starting at 935 MHz (DL = 935 + 0.2*ARFCN). The 925 MHz
value is the E-GSM extension band (ARFCNs 975-1023).

This caused grgsm_livemon to tune 10 MHz too low — ARFCN 22 tuned to
929.4 MHz instead of 939.4 MHz, receiving no GSM frames and producing
zero GSMTAP packets for tshark to capture.

Also adds EGSM900_EXT band (ARFCNs 975-1023, DL 925.2-934.8 MHz)
and diagnostic logging in the monitor thread to track raw tshark
line counts vs parsed packets.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

routes/gsm_spy.py

@@ -40,11 +40,13 @@ REGIONAL_BANDS = {
     'Europe': {
         'GSM800': {'start': 832e6, 'end': 862e6, 'arfcn_start': 438, 'arfcn_end': 511},  # E-GSM800 downlink
         'GSM850': {'start': 869e6, 'end': 894e6, 'arfcn_start': 128, 'arfcn_end': 251},  # Also used in some EU countries
-        'EGSM900': {'start': 925e6, 'end': 960e6, 'arfcn_start': 0, 'arfcn_end': 124},
+        'EGSM900': {'start': 935e6, 'end': 960e6, 'arfcn_start': 0, 'arfcn_end': 124},  # DL = 935 + 0.2*ARFCN
+        'EGSM900_EXT': {'start': 925.2e6, 'end': 935e6, 'arfcn_start': 975, 'arfcn_end': 1023},  # E-GSM extension
         'DCS1800': {'start': 1805e6, 'end': 1880e6, 'arfcn_start': 512, 'arfcn_end': 885}
     },
     'Asia': {
-        'EGSM900': {'start': 925e6, 'end': 960e6, 'arfcn_start': 0, 'arfcn_end': 124},
+        'EGSM900': {'start': 935e6, 'end': 960e6, 'arfcn_start': 0, 'arfcn_end': 124},  # DL = 935 + 0.2*ARFCN
+        'EGSM900_EXT': {'start': 925.2e6, 'end': 935e6, 'arfcn_start': 975, 'arfcn_end': 1023},  # E-GSM extension
         'DCS1800': {'start': 1805e6, 'end': 1880e6, 'arfcn_start': 512, 'arfcn_end': 885}
     }
 }
@@ -1795,6 +1797,7 @@ def monitor_thread(process, field_order=None):
 
     monitor_start_time = time.time()
     packets_captured = 0
+    lines_received = 0
     last_heartbeat = time.time()
 
     try:
@@ -1818,6 +1821,12 @@ def monitor_thread(process, field_order=None):
                     })
                 except queue.Full:
                     pass
+                # Periodic diagnostic: how many raw lines vs parsed
+                if lines_received > 0 or elapsed % 30 == 0:
+                    logger.info(
+                        f"Monitor stats: {lines_received} tshark lines received, "
+                        f"{packets_captured} parsed, fields={field_order}"
+                    )
 
             # Get output from queue with timeout
             try:
@@ -1828,6 +1837,11 @@ def monitor_thread(process, field_order=None):
             if msg_type == 'eof':
                 break  # EOF
 
+            lines_received += 1
+            # Log first 5 raw lines and then every 100th for diagnostics
+            if lines_received <= 5 or lines_received % 100 == 0:
+                logger.debug(f"tshark raw line #{lines_received}: {line.rstrip()!r}")
+
             parsed = parse_tshark_output(line, field_order)
             if parsed:
                 packets_captured += 1

tests/test_gsm_spy.py

@@ -187,14 +187,29 @@ class TestArfcnToFrequency:
 
     def test_egsm900_arfcn(self):
         """Test ARFCN in EGSM900 band."""
-        # EGSM900: ARFCN 0-124, 925-960 MHz
+        # EGSM900: ARFCN 0-124, DL = 935 + 0.2*ARFCN MHz
         arfcn = 0
         freq = arfcn_to_frequency(arfcn)
-        assert freq == 925000000  # 925 MHz
+        assert freq == 935000000  # 935.0 MHz
+
+        arfcn = 22
+        freq = arfcn_to_frequency(arfcn)
+        assert freq == 939400000  # 939.4 MHz
 
         arfcn = 124
         freq = arfcn_to_frequency(arfcn)
-        assert freq == 949800000  # 949.8 MHz
+        assert freq == 959800000  # 959.8 MHz
+
+    def test_egsm900_ext_arfcn(self):
+        """Test ARFCN in EGSM900 extension band."""
+        # EGSM900_EXT: ARFCN 975-1023, DL = 925.2 + 0.2*(ARFCN-975) MHz
+        arfcn = 975
+        freq = arfcn_to_frequency(arfcn)
+        assert freq == 925200000  # 925.2 MHz
+
+        arfcn = 1023
+        freq = arfcn_to_frequency(arfcn)
+        assert freq == 934800000  # 934.8 MHz
 
     def test_dcs1800_arfcn(self):
         """Test ARFCN in DCS1800 band."""