T-600 is a proprietary modem used by the Russian Navy, and the Russian RDL strategic network.

T-600 uses synchronous BFSK modulation without a fixed baud/shift combination, although symbol rates of 50Bd & 100Bd, and shifts of 200Hz & 250Hz 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

T-600 uses an unidentified 7-bit alphabet, where each character always has 4 marks and 3 spaces. 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 0001000 0001000 0001000 0001000 0001000 0000000.

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)


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