Automatic link establishment
Automatic Link Establishment, commonly known as ALE, is the worldwide de facto standard for digitally initiating and sustaining HF radio communications.
Upon successful linking, the receiving station generally emits an audible alarm and shows a visual alert to the operator, thus indicating the incoming call.
The operator then un-mutes the radio and answers the call then can talk in a regular conversation or negotiates a data link using voice or the ALE built-in short text message format.
[2][3] An ALE radio system enables connection for voice conversation, alerting, data exchange, texting, instant messaging, email, file transfer, image, geo-position tracking, or telemetry.
With a radio operator initiating a call, the process normally takes a few minutes for the ALE to pick an HF frequency that is optimum for both sides of the communication link.
In this respect, the longstanding need in HF radio for repetitive calling on pre-determined time schedules or tedious monitoring static is eliminated.
By noting how much error-correction occurred in each received and decoded message, an ALE node can detect the "quality" of the path between the sending station and itself.
The redundancy-based scoring inherent in ALE thus allows for selecting the "best" available channel and (in more advanced ALE nodes) using all decoded traffic over some time window to sort channels into a list of decreasing probability-to-contact, significantly reducing co-channel interference to other users as well as dramatically decreasing the time needed to successfully link with the target node.
It combined existing channel-scanning selective calling concepts with microprocessors (enabling FEC decoding and quality scoring decisions), burst transmissions (minimizing co-channel interference), and transponding (allowing unattended operation and incoming-call signalling).
The primary application during the first 10 years of ALE use was government and military radio systems, and the limited customer base combined with the necessity to adhere to MILSPEC standards kept prices extremely high.
As the standards were adopted by other governments worldwide, more manufacturers produced competitively priced HF radios to meet this demand.
By the year 2000, there were enough civilian and government organizations worldwide using ALE that it became a de facto HF interoperability standard for situations where a priori channel and address coordination is possible.
Civilian and non-government adoption rates are much lower than 2G ALE due to the extreme cost as compared to surplus or entry-level 2G gear as well as the significantly increased system and planning complexity necessary to realize the benefits inherent in the 3G specification.
For many militaries, whose needs for maximized intra-organizational capability and capacity always strain existing systems, the additional cost and complexity of 3G are less problematic.
[8] This ability to enable tactical communication in conditions where dedicated trained operators and hardware are inappropriate is often considered to be the true improvement offered by ALE.
By enabling a station to participate nearly simultaneously in many different HF networks, ALE allows for convenient cross-organization message passing and monitoring without requiring dedicated separate equipment and operators for each partner organization.
This dramatically reduces staffing and equipment considerations, while enabling small mobile or portable stations to participate in multiple networks and subnetworks.
When combined with Near Vertical Incidence Skywave (NVIS) techniques and sufficient channels spread across the spectrum, an ALE node can provide greater than 95% success linking on the first call, nearly on par with SATCOM systems.
The more common 2G ALE signal waveform is designed to be compatible with standard 3 kHz SSB narrowband voice channel transceivers.
The over-the-air layers of the protocol involve the use of forward error correction, redundancy, and handshaking transponding similar to those used in ARQ techniques.
Largely due to the ubiquity of 2G ALE, it became the primary method for providing interoperability on HF between governmental and non-governmental disaster relief and emergency communications entities, and amateur radio volunteers.
Extraordinary response agencies and organizations use ALE to respond to situations in the world where conventional communications may have been temporarily overloaded or damaged.
Earthquakes, storms, volcanic eruptions, and power or communication infrastructure failures are typical situations in which organizations may deem ALE necessary to operations.
[11] After the event, hams developed more permanent ALE emergency/disaster relief networks, including internet connectivity, with a focus on interoperation between organizations.
[2] Amateur radio operators commonly provide local, regional, national, and international emergency / disaster relief communications.
[2] The lowest common denominator technique enables any manufacturer's ALE radios or software to be used for HF interoperability communications and networking.
ALE systems include Listen Before Transmit as a standard function, and in most cases this feature provides better busy channel detection of voice and data signals than the human ear.
