28 August 2017

a weird 3G-2G scenario or a likewise weird coincidence?

For interoperation with legacy systems, STANAG-4538 stations must be capable of accepting and establishing logical links using the second generation Automatic Link Establishment (2G-ALE) protocol specified in MIL-STD-188-141B Appendix A (188-141A). Indeed, STANAG-4538 #4.6.5 "Dual Demodulation" allows this scenario while stations are in the "scanning" state (Fig. 1)
Fig. 1
Looking at the waterfall in Figure 2, it seems that a calling station transmits its FLSU_Request PDU and then, after about 2.6 seconds, it sends a 188-141 2G-ALE call on that same channel.

Fig. 2
Assuming that the heard station can operates in 2G/3G way, and assuming that the FLSU_Confirm response was not sent or not received, this is a violation of S4538. Indeed, since the caller PU did not receive the FLSU_Confirm response, it must send up to N “xDL_Terminate” PDU’s (to abort the ARQ protocol) followed by the FLSU_Terminate PDU. If this were a Circuit Traffic scenario, the “xDL_Terminate” PDU’s would not be necessary, and the caller could send the FLSU_Terminate PDU immediately after the failed call response.

As shown in Figure 2, there is a FLSU_Request PDU only! So:

- or this is a weird way to operate (no Terminate PDUs before the 2G call) for a S4538/S5066 switch

Fig. 3

- or it's a likewise weird coincidence, ie two different stations (and different networks too?) putting their calls in the same channel. 
Anyway, no reply was heard.

BTW, the 2G-ALE Addresses in the play were: XPU (the caller station) and 1VR (the called station).

It's useful to note that you can forward S5066 data link protocols into a 2G-ALE or a 3G-ALE established logical link, BUT you can't do the same when using S4538 xDL data link protocols: in this case the logical link shall be establieshed using 3G ALE (LSU,FLSU).


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