T-600 is one of several code names for a proprietary FSK broadcast system used by the Russian Navy.
This system doesn't have a fixed symbol rate and frequency shift. On HF, the typical configurations are 50 Bd & 200 Hz, 50 Bd & 250 Hz, and 100 Bd & 500 Hz. On VLF, 50 Bd & 75 Hz is the sole configuration used.
50 Bd & 200 Hz ship-to-shore variant sending reversals, delivering two messages, then sending reversals
50 Bd & 200 Hz RDL variant powering on, delivering three messages, then powering off
FSK Morse followed by the 50 Bd & 200 Hz RDL variant ("XXX XXX" indicates a "Monolith" type message)
Broadcasts made using this system follow a rigid structure. It begins with brief reversals (010101...) followed by a 44-bit start of message sequence, which may be one of the following:
The system casts 5-bit characters into 7-bit sequences, with each sequence always containing 4 marks and 3 spaces. The start of message sequence is followed by a 10-character (70-bit) session key repeated twice, and then by the message proper. End of message is marked using "0001000 0001000 0001000 0001000 0001000."