Automatic Link Establishment (ALE)

Harris RF-7210A Autolink II ALE Controller (NSN 5895-01-357-1966)

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General Overview
Automatic Link Establishment (ALE) is the principle where a specialized radio modem, known as an ALE adaptive controller, is assigned the task of automatically controlling an HF receiver and transmitter in order to establish the highest quality communication link with 1 or multiple HF radio stations.

ALE controllers can be found as external devices (such as pictured above, the Harris RF-7210A) or as imbedded options in modern HF radio equipment.

Link establishment is dependent on many factors and is fully optimized by the use of a micro computer in the ALE controller.  What follows is a basic description of how ALE functions as the in-depth workings of ALE can be elaborate and complicated to illustrate.

ALE controllers function on the basic principles of LQA (Link Quality Analysis) and SOUNDing. These tasks are accomplished using the following common elements.

1. Each controller has a pre-determined set of frequencies programmed into memory channels.
2. Channels are continuously scanned (typically at a rate of 2 channels per second).
3. Each controller has a pre-determined set of call signs programmed into memory which includes its own station call sign (SELF), NET work call signs, GROUP call signs and INDIVIdual call signs.
4. ALE controllers transmit LQAs which SOUND the programmed frequencies for best link quality factors on a regular, automated or operator initiated basis.
5. When listening, ALE units log station call signs, frequencies heard on, and assign a RANKing score relevant to the quality of the link on a per channel basis.

When a station desires to place a call, the ALE controller attempts to LINK to the outstation using the data collected during ALE and SOUNDing activities. If the outstation's data has not been collected by the sending ALE, the controller will seek the station and attempt to LINK using all programmed channels.

Upon a successful LINK, the ALE controllers will cease the channel scanning process and alert the operators that the system has establish a connection and that stations should now exchange traffic. Upon completion of a LINK session, the ALE controllers will send a LINK TERMINATION command and the ALE units will return to the scanning mode awaiting further traffic. Built-in safeguards also insure that ALE controllers will return to the SCAN mode in the event of loss-of-contact.

As well as being capable of LQA and Optimum Working Frequency channel selection, modern ALE controllers are also capable of sending short (87 ASCII characters long) orderwire digital messages known as Automatic Message Display (AMD) to ANY or ALL members on the network. ALE controllers can contact individual stations by their call sign, ALL stations, or ANY stations on the NETwork or GROUP. ALL call and ANY call make use of wildcard characters in substitution for individual call sings such as @?@ (ALL) and @@? (ANY). NULL Address Calls are used for system maintenance and are sent as @@@.

Waveform
ALE controllers conform to MIL-STD-188-141A link establishment functions and perform encoding and decoding of data by the use of an 8-ARY binary FSK modulator/demodulator.

Compatibility
Not all ALE controllers are compatible. Further technological developments have now produced 3 generations of link controllers mainly referred to AUTOLINK I (1G ALE (first generation)), AUTOLINK II (2G ALE (second generation)) and most recently 3G ALE (3rd generation). The second generation controller is both downwards and upwards compatible with the first generation controllers as long as the compatibility features are enabled in the system software.

The basic difference between the first and second generation units is in the type of addressing utilized. Call signs in the generation 1 controllers are limited to numerical digits (i.e. 112358) and call signs in the generation 2 controllers can employ up to 15 alphanumerical identifiers (i.e. UUT or UNIT1 etc...).

Reliability
Research indicates that equipment employing ALE technology over a set of 10 to 20 HF frequencies dispersed across the spectrum can raise the probability of communication (above 95%) to a distant station several thousand kilometres away to nearly that of SATCOM systems.

Standard Configurations

Sample Audio (2G ALE)
ALE modem performing a SOUNDing transmission.

ALE modem performing an ANY CALL transmission.

ALE modem sending an AMD transmission to 1 outstation.

Spectrum View of the ALE Waveform

3G ALE is an improved version of 2G ALE and is defined by STANAG 4538 and MIL-STD-188-141B Appendix C. It supports enhanced data throughput and linking performance over noisy HF skywave channels and is tailored and currently only found in strategic communication systems.

Improvements over 2G ALE
- Faster link establishment
- Linking at lower signal-to-noise ratio (SNR)
- Improved channel efficiency
- ALE and data traffic use the same family of waveforms
- Higher throughput for short and long data messages
- Better support for Internet protocols and application

New features in 3G ALE
- Fast Link Setup (FLSU) for fast link set up times
- Burst PSK waveforms
- Synchronous scanning of calling channels
- Partitioning of stations into dwell groups
- Multi-slot channel access using call priorities
- Carrier sense multiple access with collision avoidance (CSMA/CA) channel access procedure
- Linking Protection (LP): LP key tables (56-bit) and key select function
  > Automatic key management (Time of day based key selection)
  > Time-of-day distribution by HF means, as a backup to GPS time-of-day distribution

Revised: February 14, 2024
(C) 1999-2024, Richard Lacroix. All rights reserved.