Just any notes about the datalink protocol utilized by Israeli Navy (IDF) and their proprietary PSK8/2400Bd modem.
The datalink protocol transfers data utilizing "Tx-Frames" formats consisting of 6, 12 and 18 fixed dimension packets of 55 bytes (440 bits) for a full area of 330, 660, and 990 bytes per Tx-Frame. The choice of the Tx-Frame to usage (TF6, TF12, TF18) depends on the size of the data to be transmitted, taking into account the minimum possible waste of bytes; indeed, if the size of data is of a shorter length, a series of null bytes (of value zero) is appended to the data so as to extend it to the legth of the chosen Tx-Frame (Figs 1,2). Thus, a generic Tx-Frame may be seen as a n-position array, in which n is simply a multiple of 6 and each position can accommodate a single 55-byte packet:
- TF6 accomodates six 55-byte packets
- TF12 accomodates 12 55-byte packets
- TF18 accomodates eighteen 55-byte packets
probably besides a TF24 exists, but so far I have not seen a specified format during my monitorings.
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| Fig. 1 - TF6 kind TX-Frames (7 bursts) |
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| Fig. 2 - TF12 kind TX-Frames (7 bursts) |
The first packet of a Tx-Frame contains a 56-bit/7-byte synchronization series (please note the curious progression):
10001111010011111100111100101111101011110110111110000000 (0xF1F2F3F4F5F601)
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| Fig. 3 |
Each Tx-Frame is transmitted utilizing a single burst waveform, so a transmission is made up of as many bursts as the number of the data messages to be sent. All the types of Tx-Frames can coexist within the same transmission (Figure 4): since null value bytes filling is used, this could mean that fragmentation is not foreseen, i.e. if the size of the data to be sent is greater than the TF(n) then TF(n+1) it's used.
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| Fig. 4 - 3 types of Tx-Frames utilized in a same transmission |
The protocol does not appear to be an ARQ sysem but alternatively a FEC broadcast with the add of redundancy; for example, sometimes is possible to see that multiple copy of the same datagrams are sent within the same transmission (Figure 5). Obviously, the receiver shall discard the repeated Tx-Frames.
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| Fig. 5 - a six bursts transmission bearing 3-times repeated Tx-Frames |
Burst Waveform
The HF modem generates a burst waveform consisting of 2 signal formats: parallel and serial, that's why the designation "hybrid/mixed" modem. The first part of the waveforms consists of 4 or 7 tones where the 3rd and 4th respectively indicate the 1800-Hz of the audio subcarrier being modulated. Tones are likely utilized in the receive modem for the detection of the signal present, correction of doppler and recognition of the beginning of the preamble (Figure 6).
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| Fig. 6 - first tones |
The section following the 4/7 tones is modulated at the rate of 75 Bd utilizing a parallel QPSK waveform (Figure 7); although the bursts may have different lengths, it has always the same duration. The analysis of a single channel' bitstream shows a well-defined 96-bit/82-byte framing with an first sync sequence: this section is most likely a "preamble" aimed to supply synchronization and parameters related to the following data section (autobaud waveform?).
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| Fig. 7 - analysis of a single QPSK/75Bd channel |
The following section has the same ACF (66.6ms) and parameters (PSK8 2400 Bd) of a MIL 188-110A serial modem, although a circumstantial MS-110A demodulator doesn't not admit the waveform as such. The 160-symbol ACF pattern indicates a data rate from 150 up to 1200 bps (1). This is the data section of the burst waveform.
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| Fig. 8 - analysis of the serial segment |
In order to prove that the QPSK parallel section is actually a preamble, and not about the data, I randomly cut off a condition of that section obtaining a "reduced burst": resulting bitstreams stay unaffected by the shorter duration of the section (Figure 9).
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| Fig. 9 |
In my opinion, extra bytes (such as : CRC string, number of packets in the Tx-Frame, position of the packet within the Tx-Frame, first/last packet,...) are appended to each data packet during the formation of the burst waveform: these extra bytes are omitted in the bitstreams.
(1) In case of MS-110A low data rates (from 150 up to 1200 bps) the data frames are structured as a 40-symbol pattern: each frame consisting of a data block consisting of 20 data symbols, followed by a probe consisting of 20 symbols of known data. The expected ACF value is then 16.67 ms, but the actual 1 is 66.67 ms i.e. 4 times greather (Figure 5). The reason is that 4 groups of the pairs data + probe number 160 symbols (4×40) and they are just "in sync" with the scrambler dimension (160 symbols) causing the strong 66.67 ms ACF spikes.
