3 February 2018

STANAG-4538, LDL32 transfer bearing a clear-text message

The transmission was heard at 0904z on 11426.5 KHz USB (01 Feb 2018) and it's the first time I meet a clear-text message sent using a 3G-HF xDL protocol, in this case the LDL (Low-Latency Data Link) protocol.

In this transfer each LDL_DATA PDU carries a single data packet composed of 32 bytes of user data (LDL32 mode) followed by a 17-bit sequence number and an 8-bit control field (presently unused) added by the LDL protocol: this way the LDL32 PDUs are composed of 256+25 = 281 bits. The Burst Waveform 3 (BW3) is used for transfers of traffic data by the LDL protocol; BW3 computes and adds a 32-bit Cyclic Redundancy Check (CRC) to the LDL data packet plus 7 flush bits having the value 0, so the total length of an LDL32 BW3 is 281+32+7 = 320 bits (Figure 1).

Fig. 1 - LDL32 BW3 structure
As indicated in Figure 1, in order to get the user-data stream we have first to remove the first packet since it is sent twice, then remove the 64-bit LDL32 and BW3 overhead from each of the remaining 3 packets.  The result, 3 x 32-byte user data segments, is shown in Figure 2.

Fig. 2 - 3x32 byte user data segments
As you see the stream exhibits a 8-bit period and consists of a clear-text (not encrypted!) message composed using the  "Arabic chat alphabet" also known as Arabish:


"The Arabic chat alphabet is used to communicate in Arabic over the Internet or for sending messages via cellular phones. It is a character encoding of Arabic to the Latin script and the Arabic numerals.  Regional variations in the pronunciation of an Arabic letter can also produce some variation in its transliteration (e.g. ﺝ might be transliterated as j by a speaker of the Levantine dialect, or as g by a speaker of the Egyptian dialect)." [1] 
Since there are different transliterations depending on the region, and considering possible receive errors, the right English translation of the message is difficult. I tried to translate the received text directly into Arabic using the on-line google translator but got a bit poor meaning text ("So that you may go down with me") so I twitted a brief analysis hoping to have some useful comments... and I had them from my friends Guido (aka Decode_Signals) and - naturally - from KarapuZ.
Guido confirmed my opinion about the Arabish and advised the above link to wikipedia, while KarapuZ suggested another transliteration that probably makes a better sense: "(so that) he will come to Gaza with him" (Figure 3).

Fig.3 - the transliteration proposed by KarapuZ
I do not know who run the 11426.5 KHz frequency, AFAIK S-4538 is used in military environment, neither if the received text is a test message o a real message: maybe other messages (useful to understand the context) were sent just before and I did not hear them, anyway that frequency deserves to be monitored. 
I post the analysis for the same reason: anyone have some other comments or can shed a light on the meaning of the message?

[1] https://en.wikipedia.org/wiki/Arabic_chat_alphabet

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