T-600 is a proprietary modem used by the Russian Navy.
T-600 uses synchronous BFSK modulation without a fixed baud/shift combination, although symbol rates of 50 & 100 Bd and shifts of 200 & 250 Hz are most common. Assuming USB demodulation, the lower frequency is space (0), and the higher frequency is mark (1).
The broadcasts are one-way and follow a simple structure. They usually begin with bit reversals (101010...) followed by a 44-bit start of message sequence, which depends on the variant:
- Naval variant: 11100001010010111110000101001101011010101101
- RDL variant: 11101011111010110100000101000001100110110011
- Also used: 11101011111010110100000101000001011010101101
T-600 uses an unidentified 7-bit alphabet, where each character always has 4 marks and 3 spaces. Only 32 of 35 possible combinations are used, likely implying a 5-bit alphabet underneath (the unused combinations are 0110110, 0111010, and 1010110). After the above start of message sequence, a 10-character (70-bit) session key is repeated twice, followed by a variable-length message. All messages are presumed to be online encrypted. End of message is marked by 00010000001000000100000010000001000.
Multiple messages can be sent consecutively; in that case, brief bit reversals are sent, then the entire above procedure is repeated. After delivering all messages, the modem may either power off, send bit reversals, or standby on a carrier.
50Bd/200Hz T-600 (Naval variant) sending reversals, delivering two messages, then sending reversals
50Bd/200Hz T-600 (RDL variant) powering on, delivering three messages, then powering off
FSK Morse followed by 50Bd/200Hz T-600 (RDL variant) ("XXX XXX" indicates a "Monolith" type message)