most important HF milcomms standards

the NATO HF House

 Modern systems utilize the HF modem as a building block and integrate ARQ protocols, adaptive data rate control, ALE control, security architecture, and application protocols to provide a complete, and often interoperable, HF data system. Data is sent to support a wide variety of applications, including chat, email, situational awareness,and others. Some of these can be satisfied simply by broadcasting data using an HF modem. However, many applications require error-free data. Therefore, most data systems utilize a reliable ARQ based data link to ensure that the data gets to the destination(s) properly.

There are two emerging de facto standards for military and government HF data communications: STANAG 5066 and STANAG 4538. Both these standards send error-free data and adapt to the channel conditions to push through as much data as the channel will allow. Both systems can be used to send a variety of data including files, email, and generic Internet Protocol (IP) traffic. How they do it is quite different. Both systems will be discussed here.

STANAG-5066

A STANAG 5066 system typically consists of several components including a PC, a modem, a crypto, and a radio with ALE. Often several of these components are integrated in the radio, but they function as individual units as shown in Figure 1. STANAG 5066 defines a protocol stack for exchanging digital information over HF radio. A simplified conceptual diagram is shown in Figure 1. Normally, the STANAG 5066 protocols and overall system control is implemented in software that runs on the PC. The standard permits certain options, but the normal (interoperable) configuration is a MIL-STD-188-110B modem including the 75 to 9600 bps waveforms, second generation MIL-STD-188-141B ALE, and a crypto.

Fig. 1 - Typical STANAG 5066 System Block Diagram

STANAG 5066 has become the choice of many military organizations for data communications. This is due to a number of factors, not the least of which is the ability for users to use their existing radio and modem technology, providing a cost effective way to add data capabilities to existing networks. Another important factor that cannot be overlooked is the plentiful number of implementations, and the levels of interoperability they have obtained between various vendors. Implementations such as the Harris “Wireless” data products include STANAG 5066, and associated e-mail standards: Compressed File Transfer Protocol (CFTP), and HF Mail Protocol (HMTP), as well as a socket-based IP client.

STANAG-4538

STANAG 4538 is a NATO Standardization Agreement (STANAG) defining a unified family of HF waveforms and protocols for linking and data transfer. THE STANAG 4538 waveforms and protocols are Third Generation (3G) HF techniques developed in the late 1990s, which provide improved voice and data communications for the tactical user. They include efficient serial tone burst waveforms, faster and more robust automated linking, and advanced ARQ data link protocols. This has resulted in significant advances in system performance including faster linking, operation at lower signal-to-noise (SNRs), and improved network capacity.
A typical block diagram of a STANAG 4538 based system is shown in Figure 2. The components typically consist of a PC and an integrated radio with embedded ALE, modem, crypto, and data link. Normally, only the higher level e-mail protocols run in the PC. The data link protocols are embedded into the radio.

Fig. 2 - STANAG 4538 Functional Block Diagram

STANAG 4538 systems are well suited to a wide variety of applications such as secure voice, secure e-mail, position reporting/situational awareness, chat messaging, file transfer, and IP data over HF. The STANAG 4538 protocols are used to provide data and voice services in plaintext and ciphertext modes, with COMSEC provided by the radio’s embedded crypto module. For data services, there are three STANAG 4538 data link protocols: the High Throughput Data Link protocol (HDL), the Low Latency Data Link protocol (LDL), and the higher throughput HDL+ data link protocol. The combination of the three protocols is often referred to as “xDL.” For each transfer, one of these protocols is selected based on the amount and nature of the data to be sent and the current channel conditions. Note that STANAG 4538 can also setup links for other types of data traffic (i.e. STANAG 5066).

The series of actions used to send voice or data in 3G systems is similar to that of 2G systems; however, the system implementation in 3G is more integrated. The architectural differences between typical STANAG 5066 systems and STANAG 4538 systems are illustrated in Figure 3.

Fig.3 - 2G and 3G Data Systems

Several factors contribute to the significant performance improvements realized by 3G over the previous generation:

Synchronization: The radios are synchronized for both ALE and data transmissions. This allows a radio to eliminate the time consuming and congestion creating “stop scan” preamble of 2G systems and retransmit timeouts are reduced for forward and reverse data transmissions.

Robustness: Each of the signaling packets, known as Protocol Data Units (PDUs), is specialized for specific purposes; e.g., calls, acknowledgements. Modulation, coding, Interleaver and data length are optimized for the information they convey.

Integrated Modem: The ALE and data waveforms are designed as a family of burst waveforms. This makes it practical for a single modem to implement all functions. In 2G systems, the modem (typically MILSTD-188-110B or STANAG 4539) and the ALE system (typically MILSTD-188-141B) are separate implementations and often separate units.
The resulting handoff delays between the modem and ALE are a significant source of lost throughput.

Advanced Data Link Protocol Techniques: The STANAG 4538 ‘XDL’ data link protocols are incremental redundancy Hybrid-ARQ protocols using a technique commonly known as “code combining.” When a packet is received containing errors, the receiving radio retains the
received data symbols from the packet and requests retransmission of the packet. If the packet retransmission also contains errors, the receiving radio combines its symbols with those of the prior packet transmission and is usually able to recover the packet without errors, yielding a significant improvement in throughput under difficult channel conditions.

3G HF Protocol suite

A summary of some of the most important HF radio communication standards includes:

+ STANAG 4285 — Characteristics of 75-3600 bps Single-Tone Modulators/Demodulators for HF Links. Defines the parameters that insure interoperability between single-tone modems designed for communicating via HF radio links. This standard is often used for NATO HF Maritime Broadcast. This standard is largely obsolete because of STANAG 4539.

+ STANAG 4529 — Modification of NATO STANAG 4285 to deliver data and voice in 1240 Hz bandwidth at rates up to 1200 bps.

+ STANAG 5066 — is a NATO standard data communications protocol that defines 2nd generation system-system communications over variable speed data links. It provides a general, open, and interoperable system for data communications over HF radio. It also defines protocols to efficiently exchange e-mail over HF circuits.

+ STANAG 4538 — Technical Standards for an Automatic Radio Control System for HF Communication Links. It is a 3rd generation HF automation standard. This standard defines a 3rd generation ALE system that has two different modes: Fast Link Setup (FLSU) and Robust Link Setup (RLSU). It also has a highly efficient ARQ data link protocol which is combined with robust waveforms.

+ MIL-STD-188-110 — (physical layer) Interoperability and Performance Standards for Data Modems. Establishes requirements that ensure specified 80 levels of performance of voice-frequency data modems used in communications systems. This standard includes the 75-2400 bps Serial Tone PSK-8, 3600-9600 bps QAM, FSK, and 39 tone waveforms. It is compatible with the equivalent waveforms in STANAG 4539. In details:

MIL-STD-188-110A

• APPENDIX A - describes the 16-tone differential phase-shift keying (DPSK) mode
• APPENDIX B - describes the 39-tone parallel mode

MIL-STD-188-110B

• APPENDIX A - describes the 16-tone differential phase-shift keying (DPSK) mode
• APPENDIX B - describes the 39-tone parallel mode
• APPENDIX C - describes HF data modem waveforms for data rates above 2400 bps
• APPENDIX D - describes the optional subnetwork interface to be provided by data modems
• APPENDIX F - describes HF data modem operation over multiple discrete channels (including independent sidebands of a single carrier). In addition, a waveform is described that supports data rates of 9600 to 19,200 bps over two-independent-sideband (2-ISB) radios using the waveforms from APPENDIX C.

MIL-STD-188-110C

• APPENDIX A - describes the requirements for an optional local area network (LAN) interface for radio data modems
• APPENDIX B - describes the 39-tone parallel mode (not recommended for new systems)
• APPENDIX C - describes HF data modem waveforms for data rates above 2400 bps in 3 kHz channels. These may be termed narrowband, medium data rate (MDR) waveforms
• APPENDIX D - describes the HF data modem waveforms for use with single contiguous bandwidths from 3 KHz to 24 KHz 

+ MIL-STD-188-141 — (data link layer) Interoperability and Performance Standards for edium and High-Frequency Radio Equipment. Establishes requirements for interoperability and performance for HF radio equipment. The main body of standard covers HF radio specifications. The optional appendices contain details about implementing ALE systems, waveforms, signal structures, protocols, and LQA. It also includes Linking Protection. In details:

MIL-STD-188-141B

• APPENDIX A - Second-Generation (2G) HF ALE
• APPENDIX B - Linking Protection
• APPENDIX C - Third-Generation (3G) HF ALE (+ burst waveforms)
• APPENDIX D - HF Radio Networking
• APPENDIX E - Application Protocols for HF Radio Networks
• APPENDIX F - Anti-Jam and Anti-Interference Techniques
• APPENDIX G - HF Data Link Protocol

MIL-STD-188-141C

• APPENDIX A - Automatic Link Establishment System
• APPENDIX B - Linking Protection
• APPENDIX C - (moved to STANAG-4538)
• APPENDIX D - HF Radio Networking
• APPENDIX E - Application Protocols for HF Radio Networks
• APPENDIX F - Radio Requirements for Co-Located Installation