CARB-data formats in CTA, IDR, NSS, PBB and TBB transmissions

As a final stage of my ITA2 tour I wanted to take a look at how CARB (Channel Availability and Receipt Broadcast) data are sent. By the way, CARB transmissions typically consist of clear-text informations on the frequencies available for ship-shore traffic and are used to perform a channel-link before a message could be sent. 

Most stations (CTA, IDR, NSS, TBB) use STANAG-4285 600bps/Long while PBB uses asynchronous FSK 75Bd/850 for its CARBs; all the messages are originated in ITA2 (Baudot) and sent using the 5N1 framing.

Given the use of the 5N1 framing, a sharp 7-bit period bitstream is expected but it happens only in transmissions from IDR and NSS in which the start and stop bits are well identified and with clear-cut outlines (Figure 1)

Fig. 1

whereas in the other cases (CTA,TBB and PBB) bits are mixed up and a strict 5N1 format is impossible to see (Figs 2,3)

Fig. 2

Fig. 3

In TBB transmissions the CARB data are sent within a 75-bit frame, with a variable spacing between the ITA2 characters (spaces consist of 1-value bits). It's just the variable spacing that makes clear 5N1 visualization impossible. To clean up the bitstream and remove the start and stop bits I used the "Del CT" tool provided by the BEE software, "Del CT" can be used to extract a character length of 5,6,7,8 bits (ITA2 and ITA5): in this case I used the "5 bit" setting. The tool searches and removes the framing, returning the initial ITA2 characters of the CARB strings (Fig. 4)

Fig. 4

The same procedure was used for CTA transmissions, in which the CARB frame length is 74 bits (Figure 5).

Fig. 5

Unlike the above, IDR and NSS use a fixed spacing between the characters and this allows the clear 5N1 visualization depicted in Figure 1 (a perfect 7-bit period frame).

Fig. 6

In cases like this it is not necessary to use the "Del CT" tool and the start and stop bits can be removed by hand (figure 7).

Fig. 7

The CARB transmissions from PBB, although they use asynchronous FSK and not the S-4285 waveform, adopt the same way of TBB and CTA: data are sent within a 64-bit frames and with variable spacing between characters. The five ITA2 bits are recovered from the demodulated bitstream by using "Del CT" tool (Figure 8).

Fig. 8

And... don't be fooled by your eyes: bits are sent serially from left to right and row by row!

Fig. 9

DDH7/ DDH9/DDK9 ITA2 FSK and Sorcerer

Following the previous post, I tried the asynchronous FSK decoder of Sorcerer on some 5N1.5 bitstreams transmitted by DWD: precisely DDH7, DDH9 and DDK9. All transmissions are FSK 50Bd/450Hz with 5N1.5 framing, as evidenced by the SA rasters in Figure 1.

 

Fig. 1

I added the generic decoder for synchronous FSK demodulator in order to see and process the bitstreams on which Sorcerer works. As already underlined, bit editors work with an integer number of bits since they can't represent an half bit, so a 5N1.5 bitstream is depicted as 15-bit frame obtained by aggregating two consecutive frames (ie, 7.5 x 2). The raster method of SA allows to detect  the length of 1.5 bit because it works in the time domain. Figure 2 indicates that in all three cases, Sorcerer correctly demodulates the signals.

Fig. 2

Unfortunately the asynchronous FSK decoder for ITA2 does not provide the 5N1.5 framing and therefore there are some uncertainties in the decoding; using 5N2 framing you get gibberish (Figure 3). It's worth noting that in the previous post a similar not-standard framing was correctly decoded by the S-4285 decoder using 5N1 either 5N2 settings.

Fig. 3

In these cases, asynchronous FSK with 1.5 framing, the best choice is Fldigi which provides this non-standard length of the stop bit (Figure 4). 

Fig. 4

French Navy FUE, STANAG-4285 asynchronous ITA2 framing

updated
Looking at milcomm logs in the web, I noted that the STANAG-4285 clear-text test transmissions of the French Navy call "FUE" from Brest are logged with different indications of the ITA2 data-format (here called as "framing"): sometimes 5N1 and sometimes 5N2, as also reported in some youtube video clips [1].
I tuned their 6348.0 KHz/USB frequency and recorded a sample of these transmissions, then I processed the wav file using two S-4285 well known decoders: sorcerer and multiPSK. It is interesting to note that the two programs correctly decode the test messages using both 5N1 and 5N2 framing settings (Figs. 1,2) and this is certainly curious because seven and eight bits are not the same thing.

Fig. 1

 

Fig. 2

Looking at the bit outputs produced by the two decoders, the stream is structured in five bits of data (ITA2 coding, commonly referred to as Baudot) sandwiched between one start bit and two stop bits, ie a 5N2 framing (Fig. 03).

Fig. 3 - sorcerer/multiPSK bitstream output

If the recorded transmission (S-4285 from FUE) were a "pure" 5N1 then things would be different since decoding with the 5N2 setting would produce errors, as in the case of a 5N1 CARB transmission from the Italian Navy IDR (Fig. 4).

 

Fig. 4 - decoding of a pure 5N1 transmission from Italian Navy "IDR"

Then, I tried to process the wav file from FUE using the HARRIS RF-5710A modem, configured for S-4285 600bps/L, connected to Realterm, a serial terminal program [2] (not a decoder!): the DTE port of the modem was in synchronous mode. These tests show that Realterm detects framing errors when the 5N2 setting is used, whilst the reception  is correct when the 5N1 setting is used (Fig. 5): indeed a different result if compared to that seen with sorcerer and multiPSK where the 5N2 setting produces right decodings (Fig. 6).

Fig. 5 - 5710A -> Realterm

Fig. 6 - decoding of the streams from Realterm

So it seems that not 5N1 neither 5N2 are used. By the way, Figure 7 shows some possible ITA2 framing derived from ASCII-bits output of sorcerer.

Fig. 7

That said, it's seems that we face a non-standard framing: 1 start bit, 5 ITA2 data bits, and <1-2> stop bits... or perhaps there is something odd in these decoders. I try a possible explanation.

Sorcerer and MultiPSK, two software modems/decoders, even if they do not provide the exact framing, they are able to recognize this data format by synchronizing their framing window to the incoming stream, say a "dynamic window", so that we get correct decodings using 5N1 or 5N2 settings. Realterm, just an RS-232 terminal, works with "static" framing windows of the UART, which are suited to the standard formats. So, facing this stream it most likely can synchronize using the 5N1 format because in case of 5N2 the UART does not see an integer number of stop bits.

About the graphic representatiosn of the bits in Fig. 7, it is worth saying that the bit editors work with an integer number of bits (they can't represent half bit) and that anyway the bit stream is the result as parsed by the decoder(!) and not the raw stream before the parsing. The 5N1.5 bit view is possible by aggregating two consecutive frames and then get an integer number of 15 consecutive bits (ie 7.5 x 2): as shown in Figure 7, the 15-bit period stream just exhibits 2 start bits and 3 stop bits... as expected for two aggregate 5N1.5 frames. Quoting Utility Planet "ITA2 bit states are based on timing, and they do not correspond to the base-2 places used in binary numerical notation" [3].

I noted the same behavior in a recorded transmission from Fort de France Martinique FUF (downloaded somewhere from the web): in this case I used multiPSK (Fig. 8).

Fig.8

As a final note, the ending message "INT ZBZ" (interrogative ZBZ) in military terms stands as "What is the printing acceptability of my signals ?", or simply "everything is ok with my transmission?" [4].

[1]  https://www.youtube.com/watch?v=8YBJ05aJcnA   

https://www.youtube.com/watch?v=47N4v4LuUec&t=19s

[2]  https://sourceforge.net/projects/realterm/
[3]  http://mt-utility.blogspot.it/2008/04/.../ita2.html

[4]  https://www.pwcares.org/doc/Z-Signals.pdf

https://yadi.sk/d/r8wjIqdt3SjFgN

Baudot encoding with 1.5 stop bits

Although a single 7.5 bits length frame is quite easy to see in the time-domain,  its bit-oriented view is impossible at least in the tools at my disposal (unless to aggregate two consecutive frames and then get an integer number of 15 consecutive bits).

This difficulty about the representation of 7.5 bits frames may lead to misinterpretation and erros, as I did it (!), when using SA/Bee to analyze what seemed a 150bps/500 Hz FSK burst transmission copied on 16155.0 Khz (cf). Speed was misured using the AM detector tool (Fig. 1)

Fig. 1

and once demodulated, each burst exhibits a 15-bit period (Fig. 2)

Fig. 2

But the ACF tells a different story: the signal  is structured in 7.5 bits lenght frames each consisting of 1 start bit, 5 data bit and 1.5 stop bits and duration of ~100 msec (Fig. 3). This means ITA-2 coding, commonly referred to as Baudot, and a speed of 75 bps!

Fig. 3

As said, the impossibility to represent an "half bit" makes that two consecutive frames are considered and then either the speed and the period are misrepresented!

The transmission can be decoded using a common Baudot decoder and setting the right speed (75bps) and shift (500Hz): it consists of ten groups of 5 figures per burst, as shown in Fig. 4. Removing the five extra-bits (start e stop bits) from the 15-bit period stream we just get the 61 ITA2 5-bit characters of each burst.

Fig. 4

It's worth noting that things are worse trying to force the speed to 75 bps: one bit is lost  (Figs 5,6)

Fig. 5

Fig.6

By the way, the 1.5 bit lenght of the stop signal is derived from the design of early teletype machines, which was designed this way because the electromechanical technology of its day was not precise enough for synchronous operation: thus the systems needed to be re-synchronized at the start of each character while the stop duration gave the system time to recover before the next start signal.

Baudot FSK 50Bd/100

.
This FSK transmission was copied on 6330.8 (cf) at 1128z: shift is clearly 100Hz while some problems arise when measuring the speed. Indeed, the measurement of the speed based on FFT may fail in case of non-integer number of bits as in Baudot/ITA2 code where the stop bit lasts 1.5 bit: in this case SA assumes an integer number of bits, so  it prints out a value of 53.47 Baud (Fig. 1)

Fig. 1

In such cases the speed shall be measured using the "raster" tool of SA (Fig. 2): the structure of the frame is 7.5 bit (1 bit start, 5 bit of data and 1.5 stop bit) and the time line is 299.4 msec for 15 bit that makes a speed of 50 Baud.

Fig. 2

Baudod decoders work fine and print out the content of the message after the RYRY sequence, in this case: "ZHGD ZHGD ZHGD DE N4O4 N4O4 N4O4 QRK ? +?". The user is not identified, probably Russian Military.

Fig. 4

update
my friend KarapuZ catch a similar transmission on 5565.0/USB,  callsigns are very similar to the once I had: "ZBNV ZBNV DE 7X6R 7X6R 7X6R QRK ? +?". 

So far, these are the heard callsigns:
ZBNV, ZHGD
N4O4, 7X6R

https://yadi.sk/d/K_eEO7cT3QFLLU