3 May 2017

STANAG-5066, ARQ & non-ARQ PDUs in a real-world HF radio link

The STANAG-5066 standard, and its second generation Data Link Protocol, provides data transfer using ARQ as well as non-ARQ point-to-point, broadcast or multicast data transfer. The Data Transfer Sublayer (DTS) is responsible for the efficient data transfer across the radio link and use the D_PDU types displayed in Figure 1 to support both ARQ and Non-ARQ services:

Fig. 1 - D_PDU types used by STANAG-5066 DTS
For what concerns the I-frame D_PDUs: 

- the NRQ (No Repeat-Request or non-ARQ) Protocol, commonly known as broadcast mode, only operates in a simplex mode since the local node, after sending I-frames, does not wait for an indication from the remote node as to whether or not the I-frames were correctly received. Multiple repetitions of I-frames can be transmitted in order to increase the likelihood of reception under poor channel conditions, in accordance with the requested service characteristics.

- the SRQ (Selective Repeat-Request) Protocol operates in a half or full duplex mode since the local node, after sending I-frames, waits for an indication in the form of a selective acknowledgement from the remote node as to whether the I-frames were correctly received or not. The local node then either sends the next I-frames, if all the previous I-frames were correctly received, or retransmits copies of the previous I-frames that were not. The local node will retransmit copies of the previous I-frames if no indication is received after a predetermined time interval.

Pinpointing D_PDUs in a real-world HF radio link is not difficult since, regardless of type, they all begin with the same Maury-Styles 16-bit sync sequence 0xEB90, with the least significant bit (LSB) transmitted first
 (MSB) 1 1 1 0 1 0 1 1 1 0 0 1 0 0 0 0 (LSB)
The D_PDU type field occupies the 4 most significant bits of the 3rd byte (Figure 2).

Fig. 2 - Generic D_PDU Frame Structure
The chosen example is a S-5066 data transfer that uses MIL 188-110A Serial as HF waveform (Fig. 3):

Fig. 3- 188-110A over-the-air symbols
Once removed the overhead bits added by 188-110A, the D_PDUs can be isolated by syncing the resulting bitstream with the sequence 0xEB90 (the DS_PDU SYNC sequence): the result is displayed in Figure 4.

Fig. 4
The NON-ARQ DATA (type 7) and EXPEDITED NON-ARQ DATA (type 8) D_PDUs are used to send segmented data when the transmitting node needs no explicit confirmation the data was received (NRQ mode).

Fig. 5 - type 8 D_PDU
The DATA-ONLY (type 0) D_PDU is used to send segmented data when the transmitting node needs an explicit confirmation the data was received. The DATA-ONLY D_PDU is used in conjunction with a basic selective automatic repeat request type of protocol.

Fig. 6 - type 0 D_PDU

The ACK-ONLY (type 1) D_PDU is used to selectively acknowledge received DATAONLY or DATA-ACK D_PDUs when the receiving station has no segmented C_PDUs of its own to send.

Fig. 7 - type 1 D_PDU

As indicated in Figure 2, the header of every D_PDU includes an end-of-transmission (EOT) field. This 8-bit field specifies how much of the current transmission remains, in units of one-half second. This elegantly eliminates the end-of-transmission ambiguity that arises during an extended channel fade. If even a single header is received error-free, the receiver knows when it will be safe to send an ACK. Note that this field bounds the duration of STANAG 5066 transmissions at just over two minutes. This field is also used in case of non-ARQ (type 8) D_PDUS, as displayed in Figure 8

Fig. 8 - the (decreasing value) EOT field

in some cases, the shown results could suffer of the lack of error-frames which have not been correctly demodulated or discarded.

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