PolAF, UUCP over RSX.25 to exchange HF email messages

I have already encountered the Rohde & Schwarz RSX.25 protocol in any transmissions of the German BPOL and Italian GdF, this time (just a fewer days ago) I spotted specified transmissions from the Polish AirForce (Siły Powietrzne - Ministerstwo Obrony Narodowej, MON) on 6884 KHz/USB where they usage R&S GM2100 proprietary waveforms as HF bearer and UUCP over RSX.25 to send PostMan II email messages (Figure 1). Transmission were recorded utilizing a Polish KiwiSDR [1].

Fig. 1

Particularly, 1 of the transmissions being analyzed refers to the nodes with ALE address WARSZAWA2 and BYDGOSZCZ:

TO WARSZAWA2 TIS BYDGOSZCZ
TO BYDGOSZCZ TIS WARSZAWA2
TO WARSZAWA2 TIS BYDGOSZCZ User Unique Function 00 07 (CMD USER UNIQUE WORD)

The utilized 2G-ALE protocol is the well-known standard 188-141A: the first thing that catches the eye is the usage of a User Unique Function (UUF) [2] with the value 00 07 (14-bit ASCII [nul][bel]) in the 3rd frame of the ALE handshake. User Unique Functions enable the transmission of a manufacturer-specific Unique Index which may be utilized for controlling the subsequent data transmission protocol; in this case, the value 0007 is most likely the peculiar "index" that R&S uses to signal UUCP/RSX.25 protocol to the receive node.

Data are sent utilizing the HF waveform "Signal Format", a alleged R&S proprietary advanced waveform provided by their GM2100/GM2200 HF modem. The utilized waveform is the rather common 2400Bd PSK8 occupying a 3 KHz bandwidth (Figure 2). With 8PSK the net data rate of the serial modem is 5400 bit/s, errors are at first corrected by FEC, which reduces net data rate to 2700 bit/s.

Fig. 2

The framing consists of a 192-symbol series preamble followed by 1 ore more data blocks each consisting of 64-symbols: 48 unknown symbols (coded data) + 16 known symbols ("test sequences"). The postamble terminates the data blocks and consists of a 64-symbol End Of Message sequence. but for the presence of an first TLC section(s), the full dimension is then a multiple of 64 symbols.

Fig. 3

Figure 4 shows the ACF/period of the GM2100 waveform: since the 2400 Baud, the ACF value of 133.33ms corresponds to a 320-symbol period, i.e. to 5 64-symbol data blocks.

Fig. 4

The dimension of 320 symbols is due to the fact that the 16-symbol test sequences are actually "segments" of a longer 80-symbol series and so they are 5 times repeated, as visible in Figure 4 (unless demodulation errors), hence the dimension of (48+16)×5=320 symbols, or 960 bit since each PSK8 symbol is mapped to a tri-bit series (000...111).

Fig. 5

After the removal of the HF waveform overhead, the well-known 8-bit patterns of RSX.25 appear (Figure 6). RSX.25 virtually stands for R&S adaptation of wired X.25 protocol to the HF radio channel,ie a modified AX.25 packet radio protocol.
Quoting R&S papers: "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 dimension of the frame data is variable and besides depends on radio-link quality: in channels of very good quality, a frame contains up to 250 data bytes, in powerfully disturbed channels 4 bytes. Errors escaping FEC are eliminated by the ARQ procedure of the RSX.25 protocol." [3]

Fig. 6