diff --git a/M17Control.cpp b/M17Control.cpp
index 0b5f0fd..4b2cfdc 100644
--- a/M17Control.cpp
+++ b/M17Control.cpp
@@ -78,7 +78,6 @@ m_rfErrs(0U),
 m_rfBits(1U),
 m_rfLSF(),
 m_rfLSFn(0U),
-m_rfFN(0U),
 m_netLSF(),
 m_netLSFn(0U),
 m_rssiMapper(rssiMapper),
@@ -189,7 +188,6 @@ bool CM17Control::writeModem(unsigned char* data, unsigned int len)
 			m_aveRSSI   = m_rssi;
 			m_rssiCount = 1U;
 			m_rfLSFn    = 0U;
-			m_rfFN      = 0U;
 
 #if defined(DUMP_M17)
 			openFile();
@@ -238,7 +236,6 @@ bool CM17Control::writeModem(unsigned char* data, unsigned int len)
 			m_maxRSSI   = m_rssi;
 			m_aveRSSI   = m_rssi;
 			m_rssiCount = 1U;
-			m_rfFN      = 0U;
 
 #if defined(DUMP_M17)
 			openFile();
@@ -257,11 +254,9 @@ bool CM17Control::writeModem(unsigned char* data, unsigned int len)
 		unsigned char frame[M17_FN_LENGTH_BYTES + M17_PAYLOAD_LENGTH_BYTES];
 		unsigned int errors = conv.decodeData(data + 2U + M17_SYNC_LENGTH_BYTES + M17_LICH_FRAGMENT_FEC_LENGTH_BYTES, frame);
 
-		m_rfFN = (frame[0U] << 8) + (frame[1U] << 0);
+		uint16_t fn = (frame[0U] << 8) + (frame[1U] << 0);
 
-		LogDebug("M17, audio: FN: %u, errs: %u/272 (%.1f%%)", m_rfFN & 0x7FFFU, errors, float(errors) / 2.72F);
-
-		m_rfFN++;
+		LogDebug("M17, audio: FN: %u, errs: %u/272 (%.1f%%)", fn & 0x7FFFU, errors, float(errors) / 2.72F);
 
 		m_rfBits += 272U;
 		m_rfErrs += errors;
@@ -324,8 +319,10 @@ bool CM17Control::writeModem(unsigned char* data, unsigned int len)
 		if (m_rfLSFn >= 6U)
 			m_rfLSFn = 0U;
 
-		// Only check for the EOT marker if the frame has a reasonable BER
-		if ((m_rfFN & 0x8000U) == 0x8000U) {
+		bool bValid = (fn & 0x7FFFU) < (210U * 25U);		// 210 seconds maximum
+		bool bEnd   = (fn & 0x8000U) == 0x8000U;
+
+		if (bValid && bEnd) {
 			std::string source = m_rfLSF.getSource();
 			std::string dest   = m_rfLSF.getDest();
 
diff --git a/M17Control.h b/M17Control.h
index e8eab8e..5339d33 100644
--- a/M17Control.h
+++ b/M17Control.h
@@ -68,7 +68,6 @@ private:
 	unsigned int               m_rfBits;
 	CM17LSF                    m_rfLSF;
 	unsigned int               m_rfLSFn;
-	unsigned int               m_rfFN;
 	CM17LSF                    m_netLSF;
 	unsigned int               m_netLSFn;
 	CRSSIInterpolator*         m_rssiMapper;
diff --git a/M17Defines.h b/M17Defines.h
index 85c07ff..dd2fe37 100644
--- a/M17Defines.h
+++ b/M17Defines.h
@@ -26,7 +26,6 @@ const unsigned int M17_FRAME_LENGTH_BYTES   = M17_FRAME_LENGTH_BITS / 8U;
 
 const unsigned char M17_LINK_SETUP_SYNC_BYTES[] = {0x55U, 0xF7U};
 const unsigned char M17_STREAM_SYNC_BYTES[]     = {0xFFU, 0x5DU};
-const unsigned char M17_PACKET_SYNC_BYTES[]     = {0x75U, 0xFFU};
 
 const unsigned int  M17_SYNC_LENGTH_BITS  = 16U;
 const unsigned int  M17_SYNC_LENGTH_BYTES = M17_SYNC_LENGTH_BITS / 8U;
diff --git a/Sync.cpp b/Sync.cpp
index 5ec1f88..6d38bbd 100644
--- a/Sync.cpp
+++ b/Sync.cpp
@@ -1,5 +1,5 @@
 /*
- *   Copyright (C) 2015,2016,2018,2020 by Jonathan Naylor G4KLX
+ *   Copyright (C) 2015,2016,2018,2020,2021 by Jonathan Naylor G4KLX
  *
  *   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
@@ -99,9 +99,3 @@ void CSync::addM17StreamSync(unsigned char* data)
 	::memcpy(data, M17_STREAM_SYNC_BYTES, M17_SYNC_LENGTH_BYTES);
 }
 
-void CSync::addM17PacketSync(unsigned char* data)
-{
-	assert(data != NULL);
-
-	::memcpy(data, M17_PACKET_SYNC_BYTES, M17_SYNC_LENGTH_BYTES);
-}
diff --git a/Sync.h b/Sync.h
index 2ca7818..6190ae1 100644
--- a/Sync.h
+++ b/Sync.h
@@ -1,5 +1,5 @@
 /*
- *   Copyright (C) 2015,2016,2018,2020 by Jonathan Naylor G4KLX
+ *   Copyright (C) 2015,2016,2018,2020,2021 by Jonathan Naylor G4KLX
  *
  *   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
@@ -35,7 +35,6 @@ public:
 
 	static void addM17LinkSetupSync(unsigned char* data);
 	static void addM17StreamSync(unsigned char* data);
-	static void addM17PacketSync(unsigned char* data);
 
 private:
 };
diff --git a/Version.h b/Version.h
index 25555d8..c4fd038 100644
--- a/Version.h
+++ b/Version.h
@@ -19,6 +19,6 @@
 #if !defined(VERSION_H)
 #define	VERSION_H
 
-const char* VERSION = "20210817";
+const char* VERSION = "20210818";
 
 #endif