15 October 2018

email over HF using RSX.25 and GM2X00 waveforms (R&S PostMan II)

I had already met the "combined" Rohde & Schwarz RSX.25 ARQ protocol + GM2X00 waveforms some time ago, in that case UUCP sessions were exchanged between German BPOL patrol boats and the ashore station server in order to transmit and store the positions of the vessels: the analysis is posted here
This time I was lucky to spot transmissions where RSX.25 ARQ protocol and GM2X00 waveforms are used by the R&S message handling system "PostMan II" to send emails between Italian GdF patrol boats and between patrol boats and ashore commands. Such transmissions, along with 188-141 2G-ALE messages exchanges, can be heard by monitoring some known frequencies such as 6450 (noised by close S4285 transmissions), 8190 (sometimes with co-channel interference from Saudi-AF and Israeli-Ny), and 12431 all usb. Unfortunately it could be a long and fruitless monitoring since emails transmissions are not frequent.

R&S PostMan II is a combined hardware and software product, the hardware platform is a communication server running on the Linux operating system and also controlling the connected radios. Notice that PostManII uses the advanced waveforms provided by GM2X00 HF modems since STANAG/MIL-STD waveforms cannot be used together with the RSX.25 protocol; if interoperability is needed the transmission method defined in STANAG-5066 protocol and 110A/4539 HF waveforms can be applied.

Fig. 1 - R&S PostMan II email gateway

Commands and associated files coming from PostMan, who sits at application layer, are segmented and encapsulated into RSX.25 frames which in turn are transported on air by GM2X000 HF waveforms; therefore in order to get the PostMan bitstream you need to demodulate the received signal, extract RSX.25 frames, remove their encapsulation and finally reassembly the segments into a single file (here termed "infoData.tmp"): what I got is shown in Figure 2.

Fig. 2 - infodata.tmp file, PostMan commands togheter with the (Bzip2 compressed) email file
Now let's have a close look to the first bytes of the Hex/ASCII coded infoData.tmp file, i.e. to the PostMan commands (Fig. 3).


OC0008 pm2mrs -CR D.000t 8 0666 simo@oltramonti.gdf.it 0x3da rs*ail -v2 -f simo@oltramonti.gdf.it simo@cagliari.gdf.it # Ú#º¦BZh94AY&SYèså  ...

R&S PostMan II commands
pm2mrs = PostMan II messenger(?) R&S
-CR  = (?)
rs*ail -v2 -f = rsmail -v2 (version2) -f (?)

email addresses
simo@oltramonti.gdf.it = sender, patrol vessel "Oltramonti"
simo@cagliari.gdf.it = recipient, ashore station Cagliari

Bzip2 4 bytes header
BZ = Signature (0x425A magic number)
h = Bzip2 (h is for Huffman coding)
9 =  increments of 100 kB block-size uncompressed

... compressed email file bytes follow (I did not unpack it!)

Some comments:
1) Since PostMan offers  e-mail, fax and file transfer, I think that the additional command rsmail (most likely R&S mail) that follows the pm2mrs invocation just specifies the email service.
2) In order to reduce the transmission time, both the emails text body and attached files always undergo data compression.
3) For what concerns the email addresses simo@<ALE-address>.gdf.it, it seems that each radio (with its own name, ALE address and PostMan address) belongs to one station so that each station has one unique e-mail domain name where the ALE-address is also the hostname. All the email addresses have the same local-part "simo": I do not know if it's the PostMan default username (just like "user" for Harris RF-6750 mail gateway) or if it's the special GdF entity/role/staff with delegation to the message handling system.
4) The uncoloured strings are not constant in the PostMan sessions I heard, they could be related to the underlying Linux OS layer or to some other parameters (eg, 0x3da could be a file length). Below the strings of another captured (and a bit distorted) PostMan session:
C004 pm2mrs   -CR Dû;004 06D5   simo@cappelletti.gdf.it 0x565 rsmail -v2 -f simo@cappelle tti.gdf.it simo@roma.gdf.it

As said above, PostMan command and files are segmented and encapsulated into RSX.25 frames which in turn are transported on air by GM2X000 HF waveforms. 
The RSX.25 protocol is the R&S adaptation of wired X.25 protocol to the HF radio channel. 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 depending on radio-link quality and being adapted at regular intervals. RSX.25 has a typical 8-bit period (Fig. 4) with recognizable patterns and is visible once removed the overhead due the GM2X00 "advanced" HF serial waveform.

Fig. 4 - typical RSX.25 bitstream
GM2X00 HF waveforms are based on a PSK-8 constellation modulated at a symbol-rate of 2400Bd. The frame structure consists of an initial 192 symbol sequence followed by a data block consisting 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 the one of the data frames but it contains a stop-code sequence instead of information data.

By the way, these are the R&S HF equipments used in the two stations (*):
P.V. 5 "Oltramonti" patrol boat [1]
- XK2500 500W, antenna dipole Whip 8 mt mod. STA80 + tuner FK855C3
- XK2100L 150W, antenna dipole + tuner HX002M1
"Cagliari" Aeronaval Group, Operational Control Room
- XK2900 1 KW, antenna HX002
- XK859C1 1 KW, antenna HX002

(*) these informations are publicly available from the GdF website:
[1] http://www.naviecapitani.it/.../GDF/G%205%20Oltramonti.htm

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