1 May 2017

STANAG-4538, point-to-point FLSU_Terminate PDU

Contrary to what one would think, an established 3G-HF PTP (Point-To-Point) packet link is terminated by the called PU rather than the caller PU; in S-4538 terminology, the akronym PU refers to a generic Partecipating Unit, typically a radio station. The link termination is a capability of the FLSU protocol by means of the FLSU_Term Protocol Data Unit (PDU) which terminates the sender’s participation in the current link, analogous to the  use of the 2G-ALE TWAS word.
A good example of a 3G-HF PTP link termination is shown in Figure 1: the different strengths of the received signals help to spot the two PUs.

Fig. 1
The caller PU issues a 2-way FLSU_Request Protocol Data Unit (PDU) which conveys the caller PU address, called PU address, priority, and the desired traffic service (xDL ARQ mode). The called PDU responds with a FLSU_Confirm PDU, indicating the ability to continue with the requested traffic service.
Both the caller and called alternate sending xDL PDUs, with the caller sending data using the xDL Data Send PDU, and the called responding with the xDL ACK/NAK PDU. This process continues until all data has been transferred error-free, as indicated by the caller sending redundant xDL End Of Message (EOM) PDU’s. Immediately after the xDL transfer is complete, both stations iunitiate the Fast Traffic Management (FTM) protocol to negotiate further traffic. This gives the called PU an opportunity to send data packets in the reverse direction (see this post).
After a the so-called Reversal Timeout has occurred (1), the last station to receive an xDL transfer shall terminate the link with an FLSU_Term PDU (2). It's worth noting that after the EOM PDUs, both PUs remain linked (!) and since the caller has already completed its data transfer, it is up to the user process at the called PU to deliver any packet traffic it has, or to terminate the link if it doesn't have traffic to send.
Note that:
- an xDL_EOM PDU indicates to the receiving station(s) that the data transfer will be terminated;
- an FLSU_Term PDU indicates to the receiving station(s) the sender’s departure from the current link.
Obviously, in MDL or EMCON scenarios the link termination is up to the caller PU (Fig. 2):

Fig. 2 - PTM scenario
The initial FLSU PDU establishes the point-to-multipoint link, then a series of three LDL-412 PDUs carry a multicast message. After the datagram transfer is completed, also using retransmissions of packets, the caller PU terminates the link with an FLSU_Term PDU. 
Note that the receiving PUs are set up to receive either an MDL forward packet PDU (BW3 burst waveform in this case) or an EOM PDU (BW4 burst waveform in this case): sending a FLSU_Term (BW5 burst waveform) at time (1) would impose a (not allowed) triple demodulation requirement on the receiving PUs.

All FLSU PDUs use the BW5 burst waveform and support a 50-bit structure, or BW6 and a 51-bit structure in conjunction with HDL+; the 51st bit is an additional CRC bit. The PDU Type field indicates the role of the PDU in the potocol: type = 2 is used for the Term PDU (5.3 Protocol data units, Annex C to STANAG-4538 Amendment 2).

As a final note, sometimes MIL 188-141C is referred to STANAG-4538 but this is not totally correct: yes, the specifications contained in the Appendix C have been replaced with reference to the essentially identical NATO STANAG-4538, but MIL 188-141C does not provide the FLSU Fast Link Setup protocol (BW5 burst waveform) neither HDL+ protocol (BW6 and BW7 burst waveforms).



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