STANAG 5066 is a NATO specification for running data applications over HF Radio. STANAG 5066 operates over an HF modem, and provides an interface for data applications to use and share an HF modem. It provides core data link services to enable applications to operate efficiently over HF radio, and specifies a protocol that enables a clean separation between applications and modem/radio level. This paper describes STANAG 5066, and shows why it is key to deploying applications over HF Radio.

What is STANAG 5066?

STANAG 5066 "Profile for High Frequency (HF) Radio Data Communication" is a NATO specification to enable applications to communicate efficiently over HF Radio. STANAG 5066 provides peer protocols that operate above an HF Modem and below the application level. STANAG 5066 includes the (mandatory) SIS (Subnet Interface Service) protocol that enables an application to connect to an HF Modem through a STANAG 5066 server over TCP/IP. This enables a clean separation between application and modem.

The diagram above shows a configuration of three sites communicating by HF Radio, using STANAG 5066 to provide end to end communication between a set of applications.

The site shown in detail illustrates how STANAG 5066 fits with applications and hardware. It comprises:

  • An HF Radio, which is an analogue device.
  • An HF Modem, which converts from analogue to digital.
  • Encryption (optional). Data Encryption will generally be used with HF Radio, and this will be achieved by an encryption unit (COMSEC) that sits between the HF Modem and STANAG 5066 Server.
  • A STANAG 5066 server. There will be one STANAG 5066 server associated with the modem. The STANAG 5066 Servers communicate with each other over the HF Modem, using protocols specified by STANAG 5066.
  • One or more data applications communicating with the STANAG 5066 server using the SIS protocol.

On a small system, the STANAG 5066 server and applications may run on the same hardware and connect locally. For a larger deployment, applications may be running on separate computers and connecting over a LAN.

The SIS protocol provides a clean separation between the radio subsystem (Radio/Modem/Encryption/STANAG 5066 Server) and the applications that can interface to this subsystem using the SIS protocol.

You can evaluate our STANAG 5066 server, fill in our contact form and let us know your requirements. We will contact you to arrange an evaluation.

Why STANAG 5066 is needed

It is useful to consider the key characteristics of HF Radio, which is sufficiently different to other systems that it becomes imperative to use a specially designed protocol. Key characteristics are:

  1. Low bandwidth. HF Radio is slow, with bandwidth ranging from 75 to 9600 bits per second, with a typical rate of 1200 bits per second.
  2. Noise. HF transmission is subject to varying levels and types of noise and interference.
  3. Variable bandwidth. A modern modem/radio system will respond to varying signal/noise ratio by adopting appropriate waveforms and forward error correction. This will result in varying bandwidth for the system using the modem.
  4. Simplex mode. An HF radio cannot detect incoming signals when it transmits, and so is not even half duplex. If more than one radio transmits at once, nothing gets through and none of the transmitting radios can detect the problem.
  5. Broadcast. HF Radio is a broadcast medium, and it is important to enable applications to use this in order to provide broadcast and multicast services.
  6. Receive only. Some military applications need to work where a radio is in EMCON (Emission Control) and not sending data.
  7. Long turnaround time. Turnaround time is the time taken for one radio to stop sending, and another radio to start. This can vary from a few seconds to a few tens of seconds. Interleaving is a technique commonly used to reduce the impact of burst noise, and this substantially increases turnaround time. To optimize throughput, a radio needs to transmit for a reasonably long period and then allow other radios to transmit. To get reasonable utilization of the bandwidth, the transmit time needs to be quite a lot longer than the turnaround time.
  8. Interface. An HF modem provides a quite basic interface; essentially send OR receive data.

This combination of requirements is quite unlike any other communications medium, and special protocols are needed to efficiently transmit data over HF.

The basic problems of HF apply at lower frequencies (LF and VLF), but at higher frequencies (VHF and above) support of data applications becomes much more straightforward and does not require a special protocol like STANAG 5066, although use of STANAG 5066 at these higher frequencies can still give performance advantages.

Unit Data: The Primary Application Service

STANAG 5066 provides a number of services to applications over the SIS protocol. The central service is called "Unit Data", where the application sends (or receives) a block of data, typically up to around 2 kBytes. There are two basic variants of Unit Data:

  1. Unreliable. Here the data is sent out, without any form of acknowledgement. This is used for broadcasting (i.e., to two or more remote radios) and for sending data to single stations in EMCON.
  2. Reliable. This is used for sending data to a single radio that is not in EMCON mode, and provides guaranteed delivery. Optional services associated with Reliable Unit Data are:
    • Acknowledgement of Unit Data delivery to the sending application.
    • Delivery of (multiple) Unit Data blocks to the receiving application in the order they were sent.

The Unit Data service provides a number of things to the application:

  1. Multiplexing. It enables multiple applications to send and receive data at the same time.
  2. Flow Control. The STANAG 5066 server will provide flow control to the application, to control the amount of queued data that builds up.
  3. Precedence handling. Each Unit Data has a precedence value (Routine; Priority; Immediate; Flash) and higher precedence data is sent first. This is a vital feature for military applications.

Unit Data gives a simple building block that can be used by a wide variety of applications, and is the key capability provided by STANAG 5066 to applications using it.

Notes on How STANAG 5066 Works

STANAG 5066 is a complex and sophisticated specification. This section does not attempt to fully explain how it works, but describes a few key features to help better understand its value.

The operation of a STANAG 5066 server completely decouples control of the modem and sending/receiving data from the application communicating using the SIS protocol. This decoupling is a key feature and benefit of STANAG 5066.

STANAG 5066 controls which radio is transmitting and seeks to organize data to minimize the number of turnarounds. Where the maximum transmit time (127 seconds) can be used, this will give reasonable link utilization, for normal turnaround times.

At the modem level, STANAG 5066 uses packets (DPDUs) of a size appropriate to the modem speed. At 1200 bits/second 128 bytes will be used, which is much less than the typical Unit Data. Benefits of using smaller DPDUs include:

  • Acknowledgement is done for each DPDU, so if data loss occurs when transmitting a DPDU, only that DPDU (and not the whole Unit Data) need to be retransmitted.
  • When higher precedence Unit Data arrives it is sent for transmission by a STANAG 5066 server, sending can begin after transmission of the current DPDU (i.e., there is no need to wait for the full Unit Data transmission).

Acknowledgements, often referred to as ARQ (Acknowledgement ReQuest), are used for Reliable Unit Data and are made for each DPDU. In order to minimize the number of turnarounds, sending of acknowledgements is delayed. A typical sequence with two radios might be:

  1. Radio 1 transmits to Radio 2 for 127 seconds; it sends a number of DPDUs.
  2. Radio 2 transmits to Radio 1 for 127 seconds; it sends acknowledgments for the DPDUs received from Radio 1; then it sends some DPDUs.
  3. Radio 1 transmits to Radio 2 for 127 seconds; it sends acknowledgements for the DPDUs received from Radio 2; then it works out which DPDUs failed to transmit last time and resends them; then it sends more DPDUs.

As you can see this sequence makes efficient use of the link by minimizing turnarounds. You can also see that in the event of retransmissions, that there can be considerable delays in data getting through. These delays could be reduced, but at the cost of less efficient link utilization.

STANAG 5066 provides a number of management features, and one of the most interesting is remote modem control. The optimum modem parameters (e.g., Waveform choice; Forward Error Correction; and Interleaver) are best determined by the receiving system, based on signal/noise ratio. STANAG 5066 allows the receiving system to use this information to control the sending modem.

STANAG 5066 Editions 1, 2, 3 and 4

There are three published editions of STANAG 5066. The SIS protocol is the same for each edition, so that any can be used with a STANAG 5066 enabled application.

Edition 1, and Edition 2 (previously known as Edition 1, Amendment 1) radios do not attempt to transmit when it is known that another radio is transmitting. When there is silence, if two radios start transmitting together, they conflict with each other and all data is lost. This causes problems when there are more than a few radios and traffic is high. It becomes inefficient and chaotic. In practice, Edition 1 is only useful for very small numbers or radios, or where radios transmit for a low percentage of total time. Some STANAG 5066 vendors have added (proprietary) collision detection techniques to overcome these problems.

Edition 3 adds a mechanism call WTRP (Wireless Token Ring Protocol) which provides control over which Radio transmits and a CSMA (Carrier Sense Multiple Access) mechanism Edition 3 should be used for most deployments with medium and large numbers of radios.

Edition 4 is in preparation. We have published a copy of the draft specification. This addresses a wide range of defects in Edition 3 and adds support for Wideband HF up to 240 kbps.

Applications over STANAG 5066

STANAG 5066 is a layer protocol to support applications and defines a number of protocols that can be used over STANAG 5066, using the SIS protocol to connect to a STANAG 5066 Server. This includes X.400 formal messaging using STANAG 4406 Annex E, Email and ACP 127.

Other protocols can be specified to run over STANAG 5066, such as XMPP using the XEP-0365 “Server to Server communication over STANAG 5066 ARQ” specification.

Isode and STANAG 5066

Isode's view is that STANAG 5066 is key to deployment of applications over HF Radio. Isode provides a suite of products supporting STANAG 5066, based around the Icon-5066 product.


This paper has described STANAG 5066 and shown its critical role in supporting applications operating over HF Radio. The STANAG 5066 SIS protocol is the key integration point between HF Applications and the underlying HF Modem and Radio systems.