20 February 2018

unid 128-bit secondary protocol

Interesting (proprietary?) adaptive HF modem spotted on 6670.0 and 7998.5 KHz/USB. The modem uses a waveform set from 188-110A and STANAG-4539 and switches from 300bps up to 4800bps modes with constant modulation rate of 2400 symbols/sec. The data transfer phase follows the 188-141A handshake between the ALE calls:  
- CAMP,OUTPOST (in the sample recorded on 6670.0 Khz)
- HORBEN, CAMP (in the sample recorded on 7798.5 KHz)
(these callsigns are unknown to me, some DXers attribute these callsigns to the Swiss Emergency Network)
Although it is the same network, as can be supposed from the callsigns, it's interesting to note that in the sample recorded on 6670 KHz the 110A waveform exhibits four initial unmodulated tones  at 500, 1200, 1700 and 2600 KHz which are non provided in the standard.

Me and J-4538 investigated the bitstreams after the removal of the overheads due to the bearer HF waveforms and we found a 128-bit period secondary protocol (Figure 1)  no matter 188-110A or 4539!

Fig.1 - the 128-bit period of the secondary protocol
a possible transmission frame could be (Fig. 2):
a. frame sync 
b. Message Indicator (MI), 128-bit encryption specific block, repeated five times
c. phasing/idling
d. encrypted data

Fig. 2

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