28 February 2018

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

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