31 May 2017

3G link + S5066: example of Circuit Mode (HF2000, Swedish Army)


Nice example of how to send STANAG-5066 data on a 3G link, using the Circuit Mode service provided by the 3G-HF STANAG-4538 profile.
The link is established with the 1-way FLSU procedure then 188-110A Serial at 1200 bps is used for the data transfer. After the transfer is completed, an FLSU_Term PDU is sent by the caller and the link is terminated (Fig. 1).

Fig. 1
The most interesting aspect is the use of STANAG-5066, which has been detected thanks to the lack of the encryption before the MS110 modem: indeed, STANAG-5066 allows to indentify the Authority/Country by the addresses coded into the Data PDU (D_PDU), unless dummy addresses are used:

Once removed the overhead bits added by MS110, the D_PDUs can be isolated by syncing the resulting bitstream with the sequence 0xEB90 (regardless of type, all the D_PDUs begin with the same sync sequence): the result is displayed in Figure 2.

Fig. 2
The Size-of-Address Field specifies the number of bytes in which the source and destination address are encoded, the address field may be from 1 to 7 bytes in length (as in this case), with the source and destination address of equal length.The first half is the destination address and the second half is the source address:

In this case:
source address: 006.046.000.028
destination address: 006.046.001.010
both belonging to the block 6.46.x.y allocated to Sweden (Table N-6 European National Addressing Schema):

Fig. 3
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