This document specifies an approach to STANAG 5066 Data Rate Selection, to address some significant problems with Data Rate Change in the current standard. This document is part of the STANAG 5066 Extension Protocol (S5066-EP) series. The complete set of documents in this series are:

  1. STANAG 5066 Extension Protocol Index (S5066-EP1)
  2. STANAG 5066 Padding DPDU (S5066-EP2)
  3. Pipelining the CAS 1 Linking Protocol (S5066-EP3)
  4. Data Rate Selection in STANAG 5066 for Autobaud Waveforms (S5066-EP4)
  5. STANAG 5066 Large Windows Support (S5066-EP5)
  6. Slotted Option for STANAG 5066 Annex K (S5066-EP6)
  7. Advertising Extended Capabilities (S5066-EP7)
  8. Block Based EOTs (S5066-EP8)
  9. Compact Acknowledgement (S5066-EP9)
  10. Extension DPDU (S5066-EP10)
  11. Variable C_PDU Segment Size (S5066-EP11)
  12. HF Wireless Token Ring Protocol (S5066-EP12)
  13. STANAG 5066 Routing Sublayer (S5066-EP13)
  14. STANAG 5066 TRANSEC Crypto Layer using AES and other Protocols (S5066-EP14)
  15. AES Key Distribution for TRANSEC and Half Loop (S5066-EP15)

1. Problems to be Addressed

There are several problems with the current STANAG 5066 standard that this specification looks to address.

  1. STANAG 5066 was developed when the primary HF waveforms in use did not support auto-baud.
  2. There is no support for the new WBHF Waveforms.
  3. The approach is problematic for duplex communication.
  4. There is no support for non-ARQ multicast protocols such as ACP142 over UDOP.
  5. Start at 300bps
    • Slow to get to fast speeds (lots of single steps)
    • Problematic if 75bps propagates and 300bps does not

C.6.4.2 does give the following note on autobaud waveforms: "If the waveform and modems used support automatic recognition of coding and interleaving parameters by the receiver, the data rate change procedures defined here may not be necessary. Following receipt of an EOW DRC advisory message, a system using such a waveform and modem can simply change the transmit data rate." This enables reasonable operation using advisory EOWs. However, there is insufficient clarity and the new WBHF waveforms are not included.

The new approach here focuses on auto-baud waverforms, providing clarity of operation and capabilities not available with the current specification.

2. Overview of Approach

The approach specified here is based on Isode research set out in the Isode whitepaper Optimizing STANAG 5066 Parameter Settings for HF & WBHF and on analysis of measurements Measurements of Skywave HF Radio Intermediate Term Variation and Implications for Optimizing Link Performance.

The basic model of operation is that Sender decides on parameters. This contrasts with the current standard which has Receiver control with the Sender allowed to decline requests to go faster. At the start of each transmission, the sender will make a Data Rate Selection. This contrasts with the negotiated Data Rate Change in the current standard.

The Sender may use any parameters at its disposal to make final choice on parameters. The amount of data to send and QoS requirements of the application will generally be central to the choice made.

The Receiver will send advisory information to the Sender using EOWs. As EOWs are single bytes of information, this specification has designed a compact approach to sending what is most useful. These EOWs contain information to enable the sender to make "best choice". They do not control sender choice.

This specification also defines an EOW for the Sender to send to the Receiver, which indicates the approach being taken by the Sender. This information will help the Receiver to prioritize information that it sends to the Sender.
Other parameters such as transmission time and duplication strategy will be decided by the sender based on local information and the data to be transmitted.

3. EOW Specifications

This section defines a number of EOW single byte format messages. EOWs types 0-7 are specified in the core standard. EOW 8-15 are not used.

These EOWs should be sent and repeated where possible in DPDUs being sent. Every DPDU should contain either one of these EOWs or another EOW communicating useful information.

3.1 Recommended Speed for Maximum Throughput

EOW Type: 8

This EOW contains the receiver’s recommendation for transmission speed to be used to obtain maximum throughput. It is likely that transmission at this speed will lead to significant data loss followed by retransmission. The EOW is encoded as follows.


The Speed field specified transmission speed. This encodes the actual speed for standard HF, and the waveform for WBHF (with the speed dependent on channel size).

Value Speed (bps)
0001-1101 WBHF. Waveform is the recommended Annex D Waveform number (1-13)
10001 75
10010 150
10011 300
10100 600
10101 1200
10110 2400
10111 3200
11000 4800
11001 6400
11010 8000
11011 9600
11100 12800

The Interleaver field specifies the minimum Interleaver recommended for use with this transmission speed. Longer interleavers give better performance for data, and so in general the longest interleaver option should be selected.  However, longer interleaver means longer block size. In some situations, particularly when transmitting smaller amounts of data, it is desirable to reduce the block size. This field gives the minimum interleaver recommended.  

Only one value of this EOW will be currently active, and so one value at most is sent with in a given transmission. This value should be repeated if possible.

The Interleaver field is encoded as follows:

Value Interleaver
000 No recommendation
001 Ultra Short
010 Very Short
011 Short
100 Medium
101 Long
110 Very Long
111 Reserved

3.2 Recommended Speed for Low Latency Data

EOW Type: 9

This is the speed recommended by the receiver for sending small volumes of data where latency is important. This is the fastest speed for which the receiver believes that data loss will be very low.

Only one value of this EOW will be currently active, and so one value at most is sent with in a given transmission. This value should be repeated if possible It uses the same encoding as the previous EOW.

3.3 Recommended Maximum CPDU Segment Size

EOW Type: 10

This EOW gives guidance on setting the CPDU segment size to optimize performance. For variable speed networks, this applies to the recommended maximum throughput size. For fixed speed, this applies to the fixed speed, where this parameter is likely to be particularly useful.

The EOW is treated as an integer value, multiplied by 4 to get the recommended maxmimum CPDU segment size.

3.4 Sender Approach

EOW Type: 11

The other EOWs are sent from receiver to sender, to communicate information to help the sender choose parameter settings. This EOW is sent by the sender to communicate the approach being taken. This will help the receiver to prioritize which EOWs to send. Prioritization may mean only sending one type, or repeating transmission of some EOW types more. For example if bulk data is being transferred, the "recommended speed for maximum throughput" is likely to be the most important EOW. This will be most important at low speeds or short transmissions where it may only be possible to send a limited number of EOWs. At fixed speed, only the recommended DPDU size is needed and the receiver needs to calculate this on the basis of fixed speed. When choosing the settings, the sender should consider which EOWs it is most interested to receive. This setting may also guide use of EOWs which will be defined in a future update of this specification.

The following encoding is used:

6 5 4 3 2 1 LSB
EOW 12 Needed Strategy Fixed
  • If bit 0 is set to 1, fixed speed is used. Otherwise variable
  • Bits 1 and 2 have the following meanings:
    • 00: Bulk data being transferred (to be optimized for throughput).
    • 01: Low latency data being transferred.
    • 10: A mix of bulk and low latency data being transferred.
    • 11: Bulk data with low latency requirements (e.g., Web browsing).
  • If Bit 3 is set, this indicates that the local system cannot determine modem parameters on received data and so EOW 12 needs to be sent. Note that this communication is from receiver to sender and so applies to the opposite direction to bits 0-2.
  • Bits 4-7 are reserved for future use and must be set to 0.

The following table shows a recommended receiver interpretation of these values. Priority indicates choice of use when slots are limited and for repeats.

Approach Setting EOW 8 (Throughput) EOW 9 (Low Latency) EOW 10 (CPDU Segment Size)
Fixed Speed Use for Interleaver Only Do Not Use Use
Variable + Bulk (00) Use (top priority) Do Not Use Use
Variable + Low Latency (01) Do Not Use Use (top priority) Use
Variable + Mixed (10) Use (top priority) Use (top priority) Use
Variable + Intermediate (11)_ Use (top priority) Use Use

3.5 Speed Used

EOW Type: 12

For some modems, the receiver can determine the speed and interleaver used from the modem. However, this might not be possible in all configurations. This EOW allows a sender to specify the speed and interleaver being used. It is recommended to use this EOW when the sender does not know if the receiver can determine this information locally.

This uses the same encoding as EOW types 8 and 9.

4. Changes to STANAG 5066

This specification defines how to perform DRC for auto-baud waveforms. It is strongly recommended that any deployment needing DRC should use auto-baud and follow this procedure.

The STANAG 5066 standard defines a complex DRC procedure. This is unlikely to be useful in a modern deployment.

An implementation following this specification may implement the standardized DRC procedure and it is recommended to clearly state whether or not this is supported.

5. Backwards Compatibility

This specification uses EOW types that are not assigned by the standard so an implementation compliant to the base standards will ignore them. As this specification causes EOWs to flow in both directions and implementation can quickly detect if a peer supports this specification.