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Military Messaging over HF Radio: A comparison of ACP 127 and STANAG 4406 Annex E

Published on 14th February 2008

Overview

ACP 127 is the older NATO standard for formal military messaging that is being replaced by STANAG 4406. Both standards are used over HF Radio, and for STANAG 4406, this is specified in Annex E. This paper looks at how both standards work, and shows the benefits of the newer technologies.

ACP 127 over HF Radio

ACP 127 is a text based protocol for formal military messaging. Although there is not a standard specification for use over HF Radio, there have been many deployments using an architecture illustrated above.

A key feature of this architecture is that ACP 127 is closely integrated with the HF Modem. A message is sent over a Modem connection, and there is no protocol. The characters of the ACP 127 message are essentially sent down the line. Acknowledgements are handled at the ACP 127 level. When the operator at the other end of the link receives a message, an ACP 127 acknowledgement will be sent back (this is automated on some systems and manual chat between operators on others). There is no error detection or correction, and so errors at the modem level caused by radio noise will be reflected as errors in the message received. An example message as sent is shown below:

RR RCWNDB
DE RCCIC 134 02/0009Z
R 012345Z APR 00
FM CANAVHED
TO NAVAL RESERVE DIV WINNIPEG
BT
UNCLAS FOR SHIPPING DEPARTMENT FROM
PROCUREMENT LIAISON SECTION YOUR
291318Z MAR PD ADVISE WHEN MATERIAL
LISTED MY 160322Z MAR WILL BE READY
FOR SHIPMENT
BT

This might be received as:

RR RCWNDB
DE RCCIC 134 02/0009Z
R 012345Z APR 00
FM CANAVHED
TO NAVAL R$%34994 DIV WINN@#$#!"
BT
UNCLAS FOR SHIPPING DEPARTMENT FROM
PROCUREMENT LIAISON SECTION YOUR
291318Z MAR PD ADVISE WHEN MATERIAL
LISTED MY 160322Z MAR WILL BE READY
FOR SHIPMENT
BT

A skilled operator is likely to be able to correct “NAVAL R$%34994 DIV WINN@#$#!" to "NAVAL RESERVE DIV WINNIPEG" and pass on the message.

Limitations of ACP 127 over HF Radio

The core functionality of ACP 127 operating over HF Radio works well, but there are a number of limitations:

1. The manual error correction is undesirable,
2. It would be useful to have a system that works between end users without operator involvement.
3. The HF radio cannot be shared with other data applications while it is being used for ACP 127.
4. The approach does not extend to other applications and in particular those with binary formats.
5. The manual correction only works for older encryption systems. For modern encryption, an error at the radio level will change a much larger amount of data.

All of these limitations are addressed by STANAG 4406 Annex E and its associated standards.

STANAG 4406 Annex E over HF Radio

STANAG 4406 military messaging is used over HF Radio with a combination of standards:

• STANAG 4406 Annex E defines the overall approach, including message compression.
• ACP 142 handles multicast, and EMCON (Emission Control, or Radio Silence)
• STANAG 5066 handles integration with the HF Radio subsystem.

The first two protocols are described in detail in the Isode white paper Military Messaging over HF Radio and Satellite using STANAG 4406 Annex E. STANAG 5066 is described in the Isode white paper STANAG 5066: The Standard for Data Applications over HF Radio.

The diagram above shows the protocol stack used over HF Radio. STANAG 4406 provides end to end messaging, and this diagram shows protocol for transferring a message between two Message Transfer Agents.

A key capability of STANAG 5066 is that it enables multiple applications to share a single modem/radio. STANAG 5066 applications connect by a STANAG 5066 standard protocol to a single STANAG 5066 server associated with the modem/radio. This also allows the application to run on a different computer and connect to the STANAG 5066 server over TCP/IP. This is convenient for large deployments. For small deployments, all components may run on a single system. This is illustrated in the diagram above.

It can be seen that this architecture addresses the limitations of ACP 127.

Throughput

ACP 127 is a five bit protocol (it is a text format using the restricted ITA2 alphabet) that is mapped onto the modem in a simple manner. As a consequence it is data efficient.

STANAG 4406 uses an 8 bit format with higher functionality and there are several additional protocol layers. Although none of the protocols is unduly efficient, this does add data overhead. The compensating factors are:

• Newer modems have improved waveforms that give higher throughput.
• STANAG 4406 Annex E includes compression.
• Retransmission is more efficient in the event of errors,
• The system provides additional functionality, and does not need operator support for each message.
• The system works with modern encryption.

Conclusions

ACP 127 and STANAG 4406 can both be used over HF Radio. Use of ACP 127 over HF Radio has a number of disadvantages that are addressed by STANAG 4406 Annex E, in conjunction with ACP 142 and STANAG 5066.