1 October 2020

3G-ALE synchronous Fast Link Setup (FLSU) failures

Monitoring some US Army MARS frequencies such as 6910 KHz/USB, it happens to see 3G-ALE bursts pairs which are not the usual two-way LQA exchanges, as it happens in 2G-ALE (118-141A), but rather synchronous fast link setup (FLSU) failures: indeed, the bursts are sent by the same station, since the levels (5.64 and 5.28 dB).

Fig. 1
The scenario in figure 1 depicts the case in which an xDL ARQ protocol is unsuccessfully invoked via the original FLSU_Call. Quoting STANAG-4538 "Since the calling PU did not receive the FLSU_Confirm response, it must assume that the response did not propagate properly and that the called PU is prepared for the xDL packet transfer protocol. As such, the called PU is set up to receive either the first xDL forward packet PDU, or an xDL_Terminate PDU. Sending a FLSU_Terminate (ie a BW5 burst) would impose a triple demodulation requirement on the receiving PU! (1).
Thus, the calling PU must send up to “N” bursts carrying the xDL_EOM PDU to abort the ARQ protocol (under the xDL protocol specification, “N” is defined by the number of xDL_EOM PDUs that would fit within the time slot of a forward packet PDU)".

Fig. 2 - synchronous LSU failure scenario (from STANAG-4538 Ed.1)
In this sample PU1 (caller) issues a BW5 FLSU_Request to PU2 (called), requesting LDL ARQ traffic, but it does not detect any response from the called station. PU1 shall assume that PU2 issued a FLSU_Confirm but it's not received by PU1. Since PU2 must only look for at most two waveforms (1), 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 sends a LDL_EOM (BW4 PDU).
In that same frequency it's possible to hear also 2G-ALE [T-WAS] calls (figure 3), heard callsigns: 3QH, FHUCLR, NC6CLR, M19, STR,.... Who knows, perhaps they use 3G-ALE in that same way.

Fig. 3 - 3G-ALE FLSU (failed) followed by 2G-ALE call

(6910 KHz could be noised by US-based and Latin America-based freebanders/outbanders chatting in LSB)
(1)  STANAG-4538 #4.6.5 "Dual Demodulation": under no circumstances shall PUs be required to simultaneously demodulate more than two waveforms. Any scenario requiring more than dual-demodulation is either an error in the specification or an error in interpretation. The table below defines the dual-demodulation requirements:
STANAG-4538: TABLE 4.6.5-1 Dual Demodulation States

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