diff --git a/src/global.h b/src/global.h
index ea3fe43f..4f445a3c 100755
--- a/src/global.h
+++ b/src/global.h
@@ -68,8 +68,8 @@
    much higher DSL network latencies. A length of 6 ms seems to be optimal */
 #define BLOCK_DURATION_MS				6 /* ms */
 
-#define BLOCK_SIZE_SAMPLES				(BLOCK_DURATION_MS * SAMPLE_RATE / 1000)
-#define SND_CRD_BLOCK_SIZE_SAMPLES		(BLOCK_DURATION_MS * SND_CRD_SAMPLE_RATE / 1000)
+#define BLOCK_SIZE_SAMPLES				( BLOCK_DURATION_MS * SAMPLE_RATE / 1000 )
+#define SND_CRD_BLOCK_SIZE_SAMPLES		( BLOCK_DURATION_MS * SND_CRD_SAMPLE_RATE / 1000 )
 
 /* maximum network buffer size (which can be chosen by slider) */
 #define MAX_NET_BUF_SIZE_NUM_BL			12 /* number of blocks */
@@ -102,6 +102,20 @@
 #define _MAXSHORT						32767
 #define _MAXBYTE						255 /* binary: 11111111 */
 #define _MINSHORT						(-32768)
+
+#if HAVE_STDINT_H
+# include <stdint.h>
+#elif HAVE_INTTYPES_H
+# include <inttypes.h>
+#elif defined ( _WIN32 )
+typedef unsigned __int32 uint32_t;
+typedef unsigned __int16 uint16_t;
+typedef unsigned __int8  uint8_t;
+#else
+typedef unsigned int   uint32_t;
+typedef unsigned short uint16_t;
+typedef unsigned char  uint8_t;
+#endif
 
 
 /* Definitions for window message system ------------------------------------ */
diff --git a/src/protocol.cpp b/src/protocol.cpp
index 4753d020..df82d071 100755
--- a/src/protocol.cpp
+++ b/src/protocol.cpp
@@ -4,6 +4,20 @@
  * Author(s):
  *	Volker Fischer
  *
+
+Protocol message definition
+
+   +-----------+------------+-----------------+--------------+-------------+
+   | 2 byte ID | 1 byte cnt | 2 byte length n | n bytes data | 2 bytes CRC |
+   +-----------+------------+-----------------+--------------+-------------+
+
+- message ID defined by the defines PROTMESSID_x
+- cnt: counter which is incremet for each message and wraps around at 255
+- length n in bytes of the data
+- actual data, dependent on message type
+- 16 bits CRC, calculating over the entire message, is transmitted inverted
+  Generator polynom: G_16(x) = x^16 + x^12 + x^5 + 1, initial state: all ones
+
  *
  ******************************************************************************
  *
diff --git a/src/protocol.h b/src/protocol.h
index dbcdfbd3..c734855a 100755
--- a/src/protocol.h
+++ b/src/protocol.h
@@ -28,6 +28,13 @@
 #include "global.h"
 
 
+/* Definitions ****************************************************************/
+// protocol message IDs
+#define PROTMESSID_ACKN				 0 // acknowledge
+#define PROTMESSID_JITT_BUF_SIZE	10 // jitter buffer size
+#define PROTMESSID_PING				11 // for measuring ping time
+
+
 /* Classes ********************************************************************/
 class CProtocol
 {
@@ -58,6 +65,16 @@ public:
 protected:
 
 };
+
+class CProtMessage
+{
+public:
+	CProtMessage () {}
+	virtual ~CProtMessage () {}
+
+protected:
+
+};
 
 
 #endif /* !defined(PROTOCOL_H__3B123453_4344_BB2392354455IUHF1912__INCLUDED_) */
diff --git a/src/util.cpp b/src/util.cpp
index 5ed27c04..33582b42 100755
--- a/src/util.cpp
+++ b/src/util.cpp
@@ -70,23 +70,53 @@ double CSignalLevelMeter::MicLevel ()
 		return -100000.0; /* large negative value */
 	}
 }
-
-
-/* Global functions implementation ********************************************/
-void DebugError(const char* pchErDescr, const char* pchPar1Descr, 
-				const double dPar1, const char* pchPar2Descr,
-				const double dPar2)
-{
-	FILE* pFile = fopen("DebugError.dat", "a");
-	fprintf(pFile, pchErDescr); fprintf(pFile, " ### ");
-	fprintf(pFile, pchPar1Descr); fprintf(pFile, ": ");
-	fprintf(pFile, "%e ### ", dPar1);
-	fprintf(pFile, pchPar2Descr); fprintf(pFile, ": ");
-	fprintf(pFile, "%e\n", dPar2);
-	fclose(pFile);
-	printf("\nDebug error! For more information see test/DebugError.dat\n");
-	exit(1);
-}
+
+
+/* CRC ---------------------------------------------------------------------- */
+void CCRC::Reset ()
+{
+	/* Init state shift-register with ones. Set all registers to "1" with
+	   bit-wise not operation */
+	iStateShiftReg = ~uint32_t ( 0 );
+}
+
+void CCRC::AddByte ( const uint8_t byNewInput )
+{
+	for ( int i = 0; i < 8; i++ )
+	{
+		/* Shift bits in shift-register for transistion */
+		iStateShiftReg <<= 1;
+
+		/* Take bit, which was shifted out of the register-size and place it
+		   at the beginning (LSB)
+		   (If condition is not satisfied, implicitely a "0" is added) */
+		if ( ( iStateShiftReg & iBitOutMask) > 0 )
+		{
+			iStateShiftReg |= 1;
+		}
+
+		/* Add new data bit to the LSB */
+		if ( ( byNewInput & ( 1 << ( 8 - i - 1 ) ) ) > 0 )
+		{
+			iStateShiftReg ^= 1;
+		}
+
+		/* Add mask to shift-register if first bit is true */
+		if ( iStateShiftReg & 1 )
+		{
+			iStateShiftReg ^= iPoly;
+		}
+	}
+}
+
+uint32_t CCRC::GetCRC()
+{
+	/* Return inverted shift-register (1's complement) */
+	iStateShiftReg = ~iStateShiftReg;
+
+	/* Remove bit which where shifted out of the shift-register frame */
+	return iStateShiftReg & ( iBitOutMask - 1 );
+}
 
 
 /******************************************************************************\
@@ -316,3 +346,22 @@ CLlconHelpMenu::CLlconHelpMenu ( QWidget* parent ) : QPopupMenu ( parent )
 	insertSeparator();
 	insertItem ( tr ( "&About..." ), this, SLOT ( OnHelpAbout () ) );
 }
+
+
+/******************************************************************************\
+* Global functions implementation                                              *
+\******************************************************************************/
+void DebugError(const char* pchErDescr, const char* pchPar1Descr, 
+				const double dPar1, const char* pchPar2Descr,
+				const double dPar2)
+{
+	FILE* pFile = fopen("DebugError.dat", "a");
+	fprintf(pFile, pchErDescr); fprintf(pFile, " ### ");
+	fprintf(pFile, pchPar1Descr); fprintf(pFile, ": ");
+	fprintf(pFile, "%e ### ", dPar1);
+	fprintf(pFile, pchPar2Descr); fprintf(pFile, ": ");
+	fprintf(pFile, "%e\n", dPar2);
+	fclose(pFile);
+	printf("\nDebug error! For more information see test/DebugError.dat\n");
+	exit(1);
+}
diff --git a/src/util.h b/src/util.h
index 4a97268c..01f04b13 100755
--- a/src/util.h
+++ b/src/util.h
@@ -381,6 +381,26 @@ protected:
 };
 
 
+/* CRC ---------------------------------------------------------------------- */
+class CCRC
+{
+public:
+	CCRC () : iPoly ( ( 1 << 5 ) | ( 1 << 12 ) ), iBitOutMask ( 1 << 16 )
+		{ Reset (); }
+	virtual ~CCRC () {}
+
+	void Reset ();
+	void AddByte ( const uint8_t byNewInput );
+	bool CheckCRC ( const uint32_t iCRC ) { return iCRC == GetCRC(); }
+	uint32_t GetCRC ();
+
+protected:
+	uint32_t iBitOutMask;
+	uint32_t iPoly;
+	uint32_t iStateShiftReg;
+};
+
+
 /* Time conversion ---------------------------------------------------------- */
 // needed for ping measurement
 class CTimeConv