6 November 2021

a note about CIS Navy FSK (T-600)

My friend Nicola, whom I thank for the collaboration, reported to me an inaccuracy in the post of April 16, 2021 "CIS Navy FSK 50Bd/250 (T-600)", more precisely regarding the 44-bit sequence which is sent after the reversals:


The 44-bit sync sequence is in fact a 42-bit sequence (six 7-bit characters). The reason is that the transition from idle to traffic condition is signalled by a violation of the bit reversal structure so that a '1' is inserted instead of a '0' when the systems transits to traffic condition, ie the end is '...0101011' and not '01010' as given in my post (figure 1). That initial sync sequence of six 7-bit characters is also a violation of the 4:3 ratio. This ensures that sync is reliable. To use violation as signalling is quite common in many protocols, e.g. Ethernet LAN protocols.

Fig. 1

Generally speaking, one should notice that 'primitive' block protocols as the ones used by the Russian Navy will have this general structure:
- Call and acknowledgement provided by morse coded session
- Bit sync provided by bit reversals with or without a final violation
- Character sync provided by an initial Unique Word (or sync sequence, the designation is a matter of semantics)
- Possibly, but not necessarily a header (address, length, type of message etc.)
- Data, including possible initialization vectors or session keys
- End-of-Message
- End-of-Transmission, which could be provided by yet another morse session


  1. There are two distinct sync sequences used. The first one (before 4M/3S) is the
    The first is used by "RDL" and the second seems to be used by the non-"RDL" stations.

  2. thanks for your comment about the sync sequences