27 July 2020

CIS-81-81 (81-81, Moroz) T-206

A friend of mine sent me an interesting comment about the 6 and 12 bit formats used in CIS-81-81 mode, likely they refer to a former version of 81-81 and clear-text transmissions:

6-bit format (40.5 Bd): idle and traffic

12-bit format (81 Bd), two cahannels "a" and "b", bit-interleaved:

a) Channel a and b idle
b) Channel a traffic
c) Channel b traffic
d) Channel a and b traffic

(bits 2 and 5 are sent in inverted polarity).

July, 21 
Since in the last few weeks I have encountered mostly FSK2 signals (mostly from the East), I decided to take a slightly deeper look at "old" FSK2 modes such as CIS-81-81 and CIS-11 (the latter in a next post).
CIS-81-81, also designated as 81-81 (1) , T206-M 3M1 device, is the dual channel version of CIS-40.5 mode, hence the designation 81-81; "Moroz" ("FROST" in English) is just an old nickname for the mode [1]. Most likely, when operating in two channels the data are transmitted using time division multiplexing. Both 40.5 and 81 are quite "old" systems, most likely also in use by some ex USSR republic. Speculation is that this older 81-81 system, as defined above, is no longer in use and that the newer 81-81 system is most likely to be derived from CIS-14 or CIS-27 system (81-29 designation?) [2]. Some old reports from WUN logs indicate different users of these systems [2].

Shift           Common User
125             Navy   
500             Railway Authority
200,250,500,    Military

So far, I have at my disposal CIS-81 samples in two different shift patterns:

81Bd 250
Fig. 1
It's the 81-81 most interesting waveform since its demodulated stream exhibits a 24-bit pattern due to the  length of the idle signal which consists of 12 mark and 12 space bits (notice that CIS-40.5 250 uses 12-bit idle signal consisting of 6 mark and 6 space bits). The sequence generated by x^11+x^9+1 (PRBS11)  is used to scramble to the following data block or symply as "test" message; then the idle signal is repeated in opposite polarity (Fig. 3). Traffic in the channels is encrypted. It's worth noting that the polynomial and the idle signal "format" are the same in the 250Hz shift patterns of CIS 40.5 and 81 Baud.

Fig. 2 - CIS-81-81 250 


81Bd 500
Although the lack of the idle signals (at least in this recording), the demodulated stream is very interesting since it too can be descrambled using  the polynomial x^11+x^9+1 (Fig. 5).

T-206 MT "WESNA" (T-206 MT, T-206 2MT, T-206 3M und T-206 3M1)
T-206 is a Soviet teletype and data encryption family devices (not a modem!) used in stationary and mobile operation [3].

Fig. 5 - T206 MT [3]

The system has it roots in the synchronous, manual Baudot printing telegraph system whcih was used extensively already in Imperial Russia. The telegraph network of Imperial Russia was of course inherited by the Soviet Union, where the political leadership very soon realized that good telecommunications were at the roots of its modernization project. During and after WWII teleprinting was extended to radio links using either A1 or F1 modulation. It was therefore natural to adapt the old Baudot system to radio transmission and reception. As the system was synchronous its frame format needed a means to tell whether the link was active or idle, consequently a status bit was added in front of the five data bits yielding a 6-bit frame. For 2-channel systems this was naturally expanded to 2 x (1+5) = 12 bits. The system would then be transmitted as frame or bit interleaved. These systems used the first version of the International Telegraph Alphabet (ITA 1).

https://yadi.sk/d/WQikSYo77Z6oBA (250Hz shift, #1)
https://yadi.sk/d/bd81CpHyBFDuCQ (250Hz shift, #2) 
https://yadi.sk/d/-Q-4JagM1eeBHw (500Hz shift)                     

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