April 2008


Modern military communications are a key component of Network Centric Warfare. HF Radios are used extensively for military communications, and, although very slow, provide effective long distance communication in a wide range of situations. This paper looks at how HF Radio fits with Network Centric Warfare, and looks at approaches for integrating HF Radios to maximize their effectiveness.

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Network Centric Warfare

Network Centric Warfare, also commonly referred as Network Enhanced Capability, is widely documented and described. Key features include:

  • Everything is connected, to enable all players to communicate and share information.
  • A wide mix of technologies and components are involved.
  • High speed datalinks are utilized where possible.
  • Many applications are used, from core traditional components such as formal messaging and situational awareness, to new applications such as Video Teleconferencing, Voice over IP, Instant Messaging and Presence.
  • IP (Internet Protocol) is used everywhere. IP as the single network technology is a central technical approach.

HF Radio

HF Radio Communications

The key benefit of HF Radio is that it provides communication over very long distances (worldwide with a suitable aerial). It achieves this with "sky wave" communication, where the HF signal is reflected off the ionosphere (illustrated above) for long distance communication, or using Near Vertical Incidence Skywave (NVIS) for shorter distances. This benefit comes with a number of drawbacks:

  • Slow speed. Rates vary from 75 to 12,800 bits per second, with 1,200 bits per second typical. This is insufficient to support many desirable applications.
  • Cannot send and receive at the same time.
  • The equipment (radios, modems, batteries, antenna) is large and heavy. A small system with a whip antenna will weigh several kilograms.
  • Transmission can suffer from noise and errors.

Alternatives to HF Radio

There are many alternatives to HF Radio, and it is useful to understand the pros and cons of each. A clear conclusion is that there is no perfect mechanism.

Other Radio Frequencies

Transmissions at radio frequencies higher than HF (VHF, UHF, EHF, SHF) overcome all of the drawbacks of HF, and can provide high bandwidth data communications enabling key communication capabilities.

The main restriction of all of these frequencies is that they are limited to 'line of sight'. For some technologies, beyond line of sight (BLOS) can be achieved by use of a slightly curved communications path. This gives a little extra distance, but distance is still a key constraint.

Radio technologies provide communication over many miles, except in terrain where line of sight communication is more restricted. Higher frequency communication will be used in preference to HF, and so HF is primarily used for longer distance communication where other radio communication is not an option.


Satellite communication is becoming the preferred choice for long distance communication, offering relatively high bandwidth. However, there are a number of drawbacks:

  • Area coverage of a given satellite or satellite system is often constrained, and may not provide what is needed.
  • Satellites are expensive.
  • Military satellite terminals are quite a bit larger and heavier than HF radios. Some commercial terminals are smaller.
  • Satellite will not work in all terrain - for example it is not suitable in jungle.
  • National control may be an issue for countries that cannot afford their own satellite systems, and need to rely on commercial or foreign systems.
  • Satellite ground stations are subject to jamming and other threats.
  • The satellite is vulnerable single point of failure. China's January 2007 'demonstration' of it's anti-satellite capabilities is clear evidence of their vulnerability. Satellites are also potentially vulnerable to attack by laser or EMP (electromagnetic pulse).

UAVs and Kites

To overcome the line of site constraint of most radio technology, an alternative to satellite is to put a platform in a 'high' and visible position that can act as a relay between radios that do not have direct line of sight communication. This can be thought of as a low level satellite. The two main options for providing this are:

  1. UAVs (Unmanned Aerial Vehicles) There are still technical issues for both UAV and Radio and legal issues for planes without pilots to overcome before this approach is viable.
  2. Kites. A relatively low-tech approach.

A major issue for using either approach for long term communication (perhaps as an alternative to satellite) is the vulnerability to attack of the node.

The Role of HF Radio

In terms of planning an overall architecture the technology constraints lead to several possible views as to how HF Radio fits into the overall communications picture:

  1. A legacy component that will be replaced by newer technologies.
  2. A component for use in special and selected situations.
  3. A component that will be widely used.
  4. A strategic backup communication mechanism, in the event of satellite failure or destruction.

If options 2, 3 and/or 4 are chosen, it is critical that HF Radio is well integrated into the Network Centric Warfare architecture, and that effective application functionality is provided to support a full set of mission critical applications operating over HF Radio.

Why IP is Crucial

The core model of IP use is end to end communication with IP running over all of the networking components.

IP is a key element of the Network Centric Warfare architecture, as it allows a wide range of applications to be used over all of the varying network technologies that used. IP is the key standard that joins everything together.

The next sections look at how IP can be integrated with HF Radio. The first approach shown runs IP directly over the HF Radio, treating it like any other network component. While architecturally clean, the performance and management implications of this approach are very bad. A second approach is shown, which optimizes traffic over the HF link, and is the architecture recommended for HF Radio use.

Running IP over HF Radio

Running IP over HF Radio

With a direct application of this model to HF Radio, IP is operated over the HF link, leading to the sort of scenario shown above. This is an elegant architecture, and gives the key benefit that endpoints do not need to be aware of network topology, which can automatically adapt to changing situations and availability of communication links. This is a powerful and flexible model that works well for terrestrial networks, satellite and high speed radio links.

Unfortunately, this architecture does not work well for HF Radio. There are a number or reasons for this:

  • Use of IP over HF radio leads to very inefficient use of the HF link. The reasons for this are described in the Isode white paper Why IP over HF Radio should be avoided. Given the constrained bandwidth of the link, this reason on its own gives an overwhelming case.
  • It is a good approach to have applications that work with varying network performance and characteristics. However, IP over HF gives such unusual operational characteristics, that it will be hard to ensure that all applications adapt to it.
  • Application decision control is in the wrong place. Consider that a high resolution photo (several megabytes) is being sent to a platoon. Earlier, the platoon was connected with a UHF radio link, when this would have been easy to send. However, the platoon has moved out of range and switched to an HF Radio link. This is transparent to the sender, as the change has simply affected IP routing. The Message Transfer Agent will attempt to send the photo. This will almost certainly swamp the link and delivery will fail. The change of connectivity has significant implications on 'correct' application behavior.
  • HF Radio broadcast cannot be utilized.
  • If one end is in Radio Silence (or EMCON/Emission Control), it will be hard to get most applications to work.

IP with Application Relay

An alternate approach is now proposed as illustrated below:

IP with Application Relay

At the endpoints of the HF connection, this approach uses an application relay or proxy (the exact nature of this component will depend on the application). The key point of this architecture is that the server level components sending data over the HF link are close to the link and can use special protocols that are optimized for sending data over HF. This architecture optimizes performance over the HF link, in a way that is simply not possible with end to end IP.

The protocols used over the link will most likely be based on STANAG 5066. This is discussed in the Isode Whitepaper STANAG 5066: The Standard for Data Applications over HF Radio. From the overall architecture, the key point is that special protocols are used to optimize link usage, including use of broadcast and dealing with systems in EMCON.

A key aspect of this architecture is that the optimized protocols are at the server level, and that the relay/proxy will support standard protocols on the non-HF side. This means that the HF specific protocols are hidden from the non-HF components. In particular, standard clients and applications can be run on the end systems, so that the HF optimization is transparent to the end user and does not constrain the choice of end application product used.

The application relays provide a number of additional management advantages:

  • Where an application request is unsuitable for an HF link, it can be rejected (rather than blocking up the link). For example a very large email message can be non-delivered.
  • Multiple applications can communicate relative priority, to ensure that the most urgent applications get to use the link first.
  • Applications can co-ordinate to ensure that overall use of the link is efficient. STANAG 5066 is key to achieving this.

The key feature of this architecture is that it optimizes use of the HF Radio link, which is going to be the limiting performance factor whenever HF is used.

Mixing HF Radio with Faster Links

A benefit of the IP end to end architecture is that change of connectivity is transparent to the end user. This can still be achieved with the proposed architecture. To support use of HF Radio, the end applications need to be configured to make use of the Application Relays. Changing this configuration to use different links does not make sense, as it would force the end application to be aware of network connectivity. To make use of faster links, the application relays need to be aware of connectivity, and make appropriate decisions.

There will be three types of behavior, dependent on the application:

  1. Some applications, such as Video Teleconferencing, will not be supported over HF. These applications will only be supported over IP, and so will only work when a faster (IP) link is available.
  2. IP can be supported over HF, although it will generally be rather inefficient. STANAG 5066 mixes IP traffic with other applications that have direct application support. Some applications may be supported over IP, and for these the IP packets will just be routed over the best available link.
  3. For applications that have special support of HF, the application relay needs to determine the best route. If HF is the only option, the optimized protocol will be used over HF. If a high speed (IP) link is available, the application relay will be able to make use of it.

This combination will allow changeover between HF and faster links, in a manner that is reasonably transparent to the end applications.

Key Applications over HF Radio

A goal of Network Centric Warfare is to maximize information sharing. While not all applications are suitable for HF Radio, a number of applications are. This section sets out a list of applications that may be mission critical, are suitable for HF Radio, and could reasonably co-exist and share an HF link:

  • Voice. Voice is used over HF directly, and not over a data link. Data applications need to co-exist with voice usage, which will block all data transfers while voice is being used.
  • Situational awareness. Situational awareness is a key application for tactical military deployments. This will generally use product specific communication protocols.
  • Formal Messaging. Military formal messaging uses STANAG 4406, which has mappings for HF Radio specified in STANAG 4406 Annex E, using ACP 142 and STANAG 5066.
  • Internet Mail. Internet mail over HF is defined as a part of STANAG 5066, using the HMTP (HF Message Transfer Protocol).
  • Instant Messaging and Presence. XMPP (eXtensible Messaging and Presence Protocol) is the open standard for instant messaging and presence, which is being widely adopted for military applications. Presence status reporting and Instant Messaging can be useful applications for tactical deployments.
  • Directory. Replication of directory data over HF is important to support messaging applications (address book and security) and configuration for other applications.
  • Web. Limited Web browsing may be useful in some situations in support of mission critical operation. This is quite viable over faster HF links.

This is not intended as an exhaustive list, but to give a sense of what could be sensibly achieved over an HF link.


The key standards for optimized data transfer over HF Radio are in place. STANAG 5066 is the key HF integration standard, that is supported by many other specifications. There are specifications in place for both STANAG 4406 formal messaging and Internet mail to operate over STANAG 5066.

In order to support deployment of other applications in situations where the application relays/proxies are not provided by a single vendor, it is desirable to develop standards for other applications to operate over STANAG 5066.

What Isode Offers

Isode is a messaging and directory server vendor. Isode's strategy is to provide a range of application/relay and proxy servers for applications to operate efficiently over HF radio. Isode provides the necessary pieces to make the application/relay and proxy model work.

Isode currently offers support for STANAG 4406 formal messaging, with STANAG 5066 integration.


This paper has shown the key roles that HF Radio can play in support of Network Centric Warfare. It has shown that use of application relays offers substantially improved utilization of HF links and better application characteristics than a simple end to end IP approach. It also shows how Isode's products will support efficient application deployment over HF Radio.