10 April 2017

a possible EMCON retransmission mode for Multicast Data Link protocol ?

This is an MDL-N (Multicast Data Link with NACKs) transfer received on 10958.0 KHz/USB. The transfer begins with an FLSU Point-to-MultiPoint call (BW5) followed by four forward transmissions consisting of LDL288 DATA PDUs (BW3); the ending BW4 burst is the LDL EOM PDU informing the receiving stations that the entire datagram has been transmitted. In MDL-N each forward transmission is followed by a pause, during which receivers that were not able to decode that transmission emit a very robust pseudonoise PSK symbol sequence to request retransmission: in this sample there is no NACK PDUs issued by the receiver stations.

At a first glance, one could say that the incoming datagram at the sender has been splitted in four LDL 288-byte data packets (unless the padding bytes in the last packet) which are then forwarded using four BW3 bursts ...but things are not properly in this way.
Indeed, looking at Figures 1-2,  it's easy to notice that packets #1 and #2 are exactly the same, as well as the packets #3 and #4
(275 bytes length)

Fig. 1 - packets #1, #2
Fig. 2 - packets #3, #4
This means that each packet is sent twice before to clear the TxFrame buffer, so that the receiver stations have two copy of the same message, each obtained by adding the packets #1, #3 and the packets #2, #4 (Fig. 4)

Fig. 4 - the received entire datagram
(note that the encryption is applied before the datagram is segmented).

According to these observations, I try to give an explanation that makes sense to that scenario.
For a reliable transmission of the message, each packet is always sent twice (at leats in this sample): a station that decodes the entire error-free message can deliver the message and discard the copy. Otherwise it must process the copies and attempt to correctly decode the message. Summarizing: each packet is transmitted n-times instead of n-retransmissions of the complete message.  This mechanism could contrast with the chance of use ACK/NACK PDUs, but - other than adds redundancy - it could be also a good solution in case of receiver stations which must remain in EMCON mode (radio silence) for extended periods and consequently can't send NACKs neither delayed acknowledgements.  
The same behavior has been observed also in some LDLn transfers, but here the retransmissions could be requested by the receiver station.
Most likely, to increase the reliability a sort of 'retransmission counter', or a 'configurable setting', indicates the number of times each packet shall be retransmitted (repeated) or not during a particular link , according to the established xDL PDU for that transmission and the conditions of the channel, no matter if the packets have been ACKed or not, to increase the probability that user data will be received correctly.

As pointed out, this is a my thought, a supposition based on what I see: I do not know if it's  just a coincidence or a sort of implementation of STANAG-4538 since I have not found confirmations in the documentation that is publicy available in the web. Further recordings are needed in order to better understand this kind of scenario. By the way, your comments are welcome.


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