13 October 2016

CIS Navy 50Bd/200 FSK (T-600, BEE-36, CIS 36-50)

Sinchronous FSK 36-50Bd/200 system also known as CIS 36-50 and used by CIS Navy for their fleet broadcast. Transmissions start with an initial revs sequence transmitted at 36 Bd followed by traffic in 50 Bd mode. In case of more than one message, the initial 36 Bd reversals are not sent (Fig. 1).

Fig. 1
Frames (Fig. 3) are constructed from data blocks consisting of 7-bit elements: a packet of pay-load data of basically arbitrary length is surrounded by a start and an end sequence (EOM). Sometimes blocks of data already transmitted are observed to be repeated, verifying the contents by the recipient can be performed easily this way. Idle sequences of reversals, i.e. strictly alternating sequences of '0's and '1's, of 36 Bd and 50 Bd are used to introduce and to terminate a transmission or also (50 Bd only) to separate data blocks. A data block itself consists of the three sections start sequence, payload data and EOM sequence; all payload data consist of 5-bit characters coded into 7-bit sequence with a fixed ratio of '0's vs. '1's of 3 to 4 (or vice versa, depending on polarity of reception).  
The start sequence is transmitted after the last '0' bit of the idle sequence and consists of a 44 bit 42 bit sequence (usually  "100001010010111110000101001101011010101101") the ratio of three '0's to four '1's is not followed here to make the start sequence distinguishable from the actual data. The payload data is preceeded by a 70-bit Initialization Vector (ten 7-bit words) which is repeated twice. All subsequent data (arbitrary length) do not obey a special regularity anymore. The end sequence shows five equal 7-bit words "000100", again disregarding the bit ratio of the data section.

Fig. 3
Sometimes T600 is also used to forward flash messages in FSK/Morse mode as in Fig. 4 (transmitted by RDL HQ)

Fig. 4

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