22 June 2018

redefining T-207 checksums

T-207 is a multiplexed two channels "system" and can be connected to several modems therefore it can be found in several FSK waveforms. Since the lack of official documentation it's difficult to say much more about the (former Soviet) T-207: guys from radioscanner talk about "equipment" as a in-line ciphering device, ex DDR STASI archives refer to T-207 as an encryption algorithm; probably the name of the algorithm has been used to indicate the system that implements it, altough its characteristic "checksums" are also recognizable in clear text transmissions.

AFAIK, T-207 has two frame formats:  
14 bits: 2 x 6 bits + 2 parity bits
28 bits: 2 x (2 x 6 bits + 2 parity bits)

Using the 14-bit frame format, the two channels A and B may be splitted as:
1A 2A 3A 4A 5A 6A 1B 2B 3B 4B 5B 6B PP (47.5, 50, 84.21, 94 Baud waveforms) 
or interleaved
1A 1B 2A 2B 3A 3B 4A 4B 5A 5B 6A 6B PP (100 Bd waveforms and CIS-14)
 
The 28-bit frame format is used in 200 Baud waveforms and supports up to 4 channels, splitted as:
1A 1C 2A 2C 3A 3C 4A 4C 5A 5C 6A 6C PP 1B 1D 2B 2D 3B 3D 4B 4D 5B 5D 6B 6D PP

As said in the previous posts, T-207 detection had to be manually spotted by processing the demodulated bitstream and checking if it matches the criteria described in this post in radioscanner forum: you have to count the number of "1" bits in the first 12 columns then check if the 13-14 bits have a value among the expected ones.
The Octave script shown here has been improved and now it detects the presence of T-207 checksums in a given bit stream and for each permutation of the checksum bits. I run the script against several waveforms and the results are very interesting.
So far, I found two checksum modes termed "3" and "20":
  

and three waveforms (50Bd/1000, 100Bd/500, VFT 6x100Bd/120)  that can be coded with both the two checksums:


It's worth noting a CIS-14 96Bd/500 transmission which transposrts data only in channel B: it's probably a test since data are in clear-text mode



The Octave script T202_detect.m can be downloaded from:
https://github.com/hcab14/.../T207_detect.m