MESSAGE FORMAT
Preamble:Identifies the message, synchronization of message
Prekey: 16 char for MU to establish bit and character sync
Bit Ambiguity: two char (+,*) to establish bit recognition correct
Char Synchronization: Two consecutive char are sent to establish char synchronization
Start of Heading: Char sent to indicate start of message heading also block check sequence
Mode: To be used in future (RTCA)
Address: Identifies destination. MU matches the address. Down link identifies aircraft originating
Acknowledgement: MU clears from memory last down link message only after receiving ACK message. Upon receiving NAK ground will retransmit
Label: Identifies message type like ‘QT’ for departure report, ‘5D’ ATIS request etc.
Up link Block Identifier: Changes char every time a new message or general message is sent
End of Preamble: Identified by characters ‘ETX’ for message containing no TEXT and ‘STX’ for message containing TEXT.
Text: Free talk included + others
Suffix: End of Text(ETX)/End of Block(ETB)
Block Check Seq: Gen. ACK or NAK