STANAG 5066 is the NATO standard for link level over HF. STANAG 5070 is a draft specification for support of ALE with non-contiguous wideband HF. Current STANAG 5070 drafts also include a new link level standard derived in part from STANAG 5066. Having multiple standards for the same purpose (or overlapping purposes) is generally undesirable for users, nations and vendors. This paper considers how all of the HF link level requirements being considered can be addressed in an update to STANAG 5066.


STANAG 5066 Standard

STANAG 5066 Ed3 is a ratified NATO standard. An overview of STANAG 5066 is given in the Isode whitepaper [STANAG 5066: The Standard for Data Applications over HF Radio].  STANAG 5066 is widely deployed by many nations and implemented by multiple vendors. STANAG 5066 Ed4 has completed technical review and is expected to be widely adopted.

STANAG 5070 Draft

STANAG 5070 was allocated by NATO to specify use of ALE with non-contiguous wideband HF. This function fills a gap in the current set of HF specifications. It is desirable and important to have a specification for this. The current STANAG 5070 drafts, by Thales and in conjunction with French MoD, contain HF Link level functions which duplicate STANAG 5066 functions. Isode sees this duplication as highly undesirable. It is unclear if there is support from other nations for this work. There is no visible vendor activity beyond Thales and Thales partners.

Why One Standard is Desirable?

A key attraction of STANAG 5066 is that it provides a single interface to all HF infrastructure. Having separate interfaces, with some applications using STANAG 5066 and others using STANAG 5070 would be confusing and complex for deployment, create work for vendors, leading to delay and cost for end users.

This paper shows how all of the functions in the current STANAG 5070 drafts can be addressed in a straightforward manner in a STANAG 5066 update. Note that this excludes ALE for non-contiguous wideband HF; Isode recommends that STANAG 5070 is used for this purpose only.

Capabilities of STANAG 5066 Not in STANAG 5070

This section lists key capabilities in STANAG 5066 that are not provided in the current STANAG 5070 drafts.

  1. EMCON, multicast and broadcast support.
  2. Multi-node support using CSMA (Annex K).
  3. Multi-node support using Wireless Token Ring Protocol.
  4. Support for contiguous Wideband HF.
  5. Operation at low narrowband speeds down to 75bps (the STANAG 5070 TDD design will not support this).
  6. Operation without IP Crypto

This list makes clear the partial nature of STANAG 5070and that it is not suitable for general purpose use.

Duplicated Functions

This section looks at capabilities present in both the STANAG 5066 standard and the STANAG 5070 draft.

Link Level Functions

STANAG 5070 red side and black side are based on STANAG 5066 Ed3, with much of the text similar to STANAG 5066. This duplication is problematic for maintenance of specifications, as well as the problems caused by having two specifications of the same function.


It is desirable to provide a modern alternative to legacy stream crypto. STANAG 5070 provides an AES mechanism. STANAG 5066 Ed4 Annex T provides a mechanism that can be used with AES or sovereign crypto. Isode believes the Annex T mechanism to be superior, as it gives crypto algorithm flexibility and better resilience to HF errors.

STANAG 5070 Capabilities notcurrently provided by STANAG 5066

This section looks at the capabilities of STANAG 5070 draft that are not currently provided in STANAG 5066, and looks at how they could be provided as updates to STANAG 5066.


The STANAG 5070 draft uses a TDD (Time Division Duplex) approach, although reference is made to TDMA (Time Division Multiple Access).

STANAG 5066 has a Media Access Framework, and a placeholder for a TDMA protocol. It would be clean to add a TDD or TDMA MAC access mechanism to STANAG 5066 as a new Annex, probably using the Annex M placeholder.

Isode view on TDD/TDMA

STANAG 5066 provides CSMA and WTRP MAC mechanisms. Isode believes that these are good options and there is no reason to introduce another option. However, if the option is going to be introduced, it is best achieved as a new MAC layer option in STANAG 5066 and not as a new standard.

The Isode technical view is nicely presented in “IMPACT OF TURNAROUND TIME ON WIRELESS MAC PROTOCOLS” -Eric E. Johnson, Manikanden Balakrishna, and Zibin Tang’ which compares TDMA, CSMA and WTRP. It concludes that CSMA is best for lightly loaded networks and WTRP is best for heavily loaded networks.

Modem Level Data Rate Change

The STANAG 5070 draft handles data rate change at the modem level including a ARQ function. This contrasts with STANAG 5066 which handles data rate selection and ARQ at the link level.

It would be straightforward to incorporate modem level data rate change as a STANAG 5066 Annex. This would have been problematic with Ed3, but Ed4 moved to a data rate selection approach with rate control by sender. With Ed4, this approach could be integrated as an option.

Isode view on Modem Level Data Rate Change

Isode analysed the best level for data rate control, published in “Optimizing applications and data links for HF radio intermediate term variations: Can you ride the wave?” presented at Nordic HF in August 2016. This looked at how random changes in HF speed are, and concluded that short term variation after about five seconds is completely random. In order to adapt to short term changes to gain performance, you need switch in a second or two and the performance gain is much less than the overhead of switching. This concludes that this sort of mechanism will not improve performance.

Use of Non-Contiguous Wideband HF

STANAG 4539 Annex H specifies non-contiguous wideband. This could be supported in a straightforward manner by defining an annex to STANAG 5066 to enable it.  There does not seem to be a reason why a new standard is needed.

Isode view on non-contiguous Wideband HF

This paper makes clear Isode’s strong opposition to the STANAG 5070 drafts, apart from the non-contiguous ALE element. This section is to note that Isode is strongly supportive of non-contiguous wideband HF, as it addresses a number of operational considerations and has potential to provide useful operational characteristics.


Use of a TCP Performance Enhancing Proxy (PEP) is vital to operating TCP and Web applications over HF. The STANAG 5070 draft includes such a specification.

A TCP PEP is relatively independent of the HF Link Layer, and it makes sense to include as a STANAG 5066 Annex.

Isode has proposed and implemented a TCP PEP which gives better performance and resilience than the STANAG 5070 draft. This is described in the Isode white paper Measuring and Analysing HF-PEP for TCP communication and Web Browsing over HF .

IP Crypto

It can make sense for some nations to use IP Crypto to provide COMSEC. The STANAG 5070 draft is built around using IP Crypto and does not provide any alternative for COMSEC.

The Isode whitepaper Using Crypto over HF sets out an approach for using IP Crypto with STANAG 5066. This uses four open specifications:

  1. Providing STANAG 5066 services of IP (S5066-AP4)
  2. Implicit IP Client over STANAG 5066 (S5066-APP5)
  3. Providing Control Parameters for STANAG 5066 over UDP/IP through IP Crypto (S5066-APP6)
  4. XML Control Messages for STANAG 5066 over UDP/IP through a Data Diode (S5066-APP7)
The white paper explains in detail why this approach is significantly superior to the STANAG 5070 draft. It also includes performance measurements using Isode’s implementation.


It is clear from this analysis that a single converged HF link standard which is an update to STANAG 5066 could be provided to address all of the STANAG 5070 functions, apart from non-contiguous ALE, which needs to be a separate specification.

Isode’s strong recommendation is that STANAG 5070 reverts to providing only non-contiguous ALE, which is the purpose for which NATO allocated the STANAG number.

The merits of other functions should be reviewed in the NATO BLOS technical group and STANAG S066 updated to reflect agreed requirements.