19 February 2017

a 3G Multicast Data Link w/ NAKs?

few days ago, 16 Feb, I copied a 3G-HF transmission on 10958.0 KHz/USB consisting of an initial FLSU PDU (BW5 burst) followed by 13 not-ACKed LDL PDUs (BW3 bursts) and ending with a single BW4 burst: since LDL is a stop-and-wait ARQ protocol, ACK burst were expected. This scenario recalls the 3G Multicast Data Link with NAKs (MDLN) protocol previously copied and reported here (Fig. 1).

Fig. 1
I could not find official NATO/Stanag documentations about the 3G Multicast but only some "proposed" papers; Multicast MDL Protocol is still cited as "still in development" in STANG-4538 Amendment 2 Draft 0.3, the one at my disposal, thus the following are my suppositions based on the clues which I can see, so, comments are welcome (after all, this blog is just a collection of notes and experiences of a digital signals enthusiast and amateur analyst and does not claim to be a scientific blog).

Multicast Data Link protocol (MDL/MDLN) shares many of the characteristics of the other 3G data link protocols but unlike the 3G ARQ protocols, MDL links employ the one-way link setup: each transmission begins with an TM,  FTM or FLSU PDU that indicates the MDL mode that will be used in the remainder of the transmission.
The MDL_Data PDUs use the BW2 and BW3 burst waveforms used in HDL and LDL: in this case the MDL-288, a stream of 288-byte bursts, is used (as known the BW3 data section can be any multiple of 32 bytes, from 32 up to 512 bytes).
Fig. 2 - BW3 frame
The transfer ends with a BW4 burst, Fig. 3, most likely acting as MDL_EOM PDU (any PDU sent using BW4 in the forward direction is an EOM PDU).

Fig. 3
Sending an FLSU_Terminate would impose a triple demodulation requirement [1] on the receiving stations (they do not expect BW5 bursts) thus, as it happens in LDL transfers, the calling station sends an MDL_EOM PDU (BW4 burst) to signal the receiving stations that the datagram has been transferred, and hence will send no more MDL_Data PDUs for the current datagram.  MDL_EOM PDU would use BW1 burst in case of MDL-5k (HDL BW2 used for MDL_Data)?

Once demodulated, the received datagram transports Harris Citadel off-line encrypted messages (Fig. 4) 

Fig. 4

[1] Dual Demodulation clarifying example:
PU1 issues a FLSU_Request to PU2, requesting LDL ARQ traffic. PU2 issues a FLSU_Confirm that is not received by PU1 due to poor propagation. Since PU2 must only look for at most two waveforms, it looks for the LDL Forward Packet waveform (BW3) and the LDL_EOM PDU (BW4). Thus, in order to terminate the link due to missing the FLSU_Confirm, PU1 must send a LDL_EOM followed by a FLSU_Terminate.

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