19 November 2016

R&S proprietary ARQ-protocol RSX.25 over GM-2100 HF modem

transmission received on 06450.0 KHz on USB at 0753 UTC. The ending part in MIL 188-141A, which terminates the link, reveals that the user is the Italian "Guardia di Finanza - GdF", ALE addresses are CAGLIARI (Coastal Naval Station in Cagliari, the sender) and the patrol boat OLTRAMONTI (the receive peer). 
Data are sent using the HF waveform "Signal Format", a so-called Rohde & Schwarz proprietary waveform originated by the HF modem GM2100, in combination with the proprietary ARQ-protocol RSX.25. LAN/WAN interconnections are most likely managed by the "Message Handling System PostMan", this also from R&S.
RSX.25 literally stands for R&S adaptation of wired X.25 protocol to the HF radio channel, it derives from the packet protocol X.25 and cannot be used togheter with STANAG and MIL-STD HF waveforms such as S-4039, S-4285 and 199-110A; for these waveforms, radio protocols such as STANAG 5066 are available (quoting from R&S data sheets [1]).

The HF waveform GM2100 "Signal Format" exhibits the usual 1800Hz carrier with PSK-8 modulation at 2400 symbols/sec (Fig. 1)

Figs. 2a, 2b shows the structure of a frame with R&S Signal Format and transmitted by the HF modem GM2100. The preamble consisting of a fixed 192 symbol sequence enables the receive station to synchronize with correct timing and phase. The following data block consists of 64-symbols frames each composed of 48 unknown (data) symbols + 16 known symbols (probe). The postamble, terminating the data block, has a structure which is basically the same as that of the data frames but it contains a stop-code sequence instead of information data.
Fig. 2b (from a different recording)

Most likely the length of the interleaver or the scrambler generates the 133.33msec ACF spikes, i.e. five data blocks which make 960 bit or 320 symbols period (Fig.3).
The RSX.25 protocol permits all types of digital data to be transmitted, eg for a printer, digital camera, camcorder or fax unit. RSX.25 organizes the data to be transmitted in packets, which are successively transferred to the data modem. The packets contain a variable number of frames, the number per packet depending on radio-link quality and being adapted at regular intervals.
The data transmitted in a packet are distributed among the frames. The length of the frame data is variable and also depends on radio-link quality. In channels of very good quality, a frame contains 250 data bytes, in strongly disturbed channels 4 bytes. The length of the transmitted data is continually adapted to link quality

RSX.25 has a typical period of 8-bit period with recognizable patterns and is visible once removed the overhead due the Signal Format waveform (Fig. 4).


RSX.25 use the BZIP arvhivier so the received data can be extracted to ASCII files. Although the "data protection" feature is used, some portions as the e-mail addresses or other headers are in clear-text and thus easily identified, as it was easy to predict, ALE and e-mail addresses match.

Below some examples from Italian GdF - Guardia di Finanza (Fig. 5) and Bundes Polizei (Fig. 6), it's worth noting the X.25 login procedure in the latter

Fig. 6

Some useful links:


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