Automatic Link Establishment (ALE) An Overview

(HFIA users note: If you have a better way of explaining ALE to potential users, please feel free to send it in.)

(HF Radio users, it you would like a more detailed description of ALE functions or operations please drop a note (e-mail) to the HFIA)

The Concept: The basic concept is that the radio, not the radio operator, does the hard work of maintaining information on propagation conditions, the stations that are in the current network and the best frequency to use to establish connectivity to any station in the network.

The Implementation: (The basic rules)

1. Each station in an ALE network is assigned an "address" as a call-sign (1 to 15 characters).
2. Each station in the ALE network monitors,"scans", a set number of frequencies (2-100) at a specified scan rate (2-10 frequencies per second).
3. (Optional) Each station in the ALE network "sounds" (broadcasts its callsign on each frequency) at a specified interval.
4. (Optional) If a stations hears it's callsign, while scanning, broadcast by another station, it will respond. No matter if it responds or not, it will record the "Link Quality Analysis" (LQA) value.
5. If a station is required to make a "call", (contact another stations) it will select the "" frequency to the station as determine from the LQA value for that station or by calling on each frequency in the scan list.
6. If a station needs to contact a single station, or group of stations, or any station, or all stations in the network, or the entire network, or a special subset of the network either by callsign or but search (all members with call signs with 'AA' in the callsign), it will broadcast a message specifying call-sign of the desired station(s), or special call signs indicating all, or any station, on one of the assigned network frequencies.
7. If a station receives a call or it if places a call and the appropriate station(s) responds the station will notify the operator of the link.

Definitions: Descriptions of terms and functions of ALE or HF radio. Those keywords or acronyms that may require a more detailed description are highlighted and point to the acronyms and technical terms description page.

The how ALE works:

An HF radio system equipped with either an embedded ALE modem or an external ALE controller is basically controlled by that ALE modem or controller. The ALE controller tells the radio to tune to a specified frequency, key and transmit, or monitor (receive on a selected frequency) for a specified period. The ALE controller basically tells the radio what to do and then the controller broadcasts or monitors for ALE tones. The ALE tone set is a set of basic single frequency tones that when decoded provide information to the receiving ALE controller. Each group of three tones (tribit) broadcast and received has a equivalent meaning to the ALE controller. Each tribit is interrupted as a command word or a portion of an ASCII character. As the ALE tones are received the controller interrupts the control words produced and determines if the message or call is for its station either as an individual or a member of a net, group, or speckle call. If the call does not address the receiving station then the control instructs the radio to continue scanning.

The second step of the process is determine who is calling and what action is required to respond to the call. Depending on the type of call, the receiving station(s) will respond as required and record any specific data as required.

An ALE call broken down to its basic components is nothing more then a radio call made by the ALE controller.

• Calling another Station
• The operator or ALE controller selects the callsign of the station being called, determines the best frequency or channel to call on, then processes the call.
• The ALE protocol for an individual call is broken down into three portions
• The "TO" portion of the call, containing the callsign of the station being called (TO ABC) and the calling stations callsign (TIS DEF)
• The acknowledgment of the call from the called stations (TO DEF, TIS ABC).
• The acknowledgment of link by the calling station (TO ABC, TIS DEF). At this point both stations are considered linked.
• Once "Linked" the human operators may communication by voice, data or AMD messages.
• The link may be terminated by either party or may terminate by lack of activity by one party.
• The link is terminated by sending a "THIS WAS" (TWAS) message to the other station.

The basic ALE process is similar to a voice call made by a human operator, except it is accomplished by the ALE controllers using a series of tones, with meaning to the receiving station(s). Since the ALE controller is designed to listen for these tones and can recognize the individual tones in the present of noise (man-made or natural), the link can be established under conditions were voice communications may be impaired or impossible.

One point to remember is that ALE is based on several points:

• Each station in the ALE network is scans a predefined set of frequencies. For the purpose of this discussion, we will use 10 frequencies (channels). These frequencies are called the "Scan List".
• Each station will monitor each frequency for a set period of time. Lets say that each frequency will be monitored for 500 ms each. This is 2 channels per second. This is a selectable parameter when programming an ALE controller.
• Call duration (the amount of time used to send the "TO" portion of the call) and sound duration are based on the number of channels in the scan list. You have to call on a channel long enough for another station to scan to that channel (remember the called station is monitoring each channel in the scan list for 500 ms and going to the next channel and repeating the monitoring) and listen for its callsign. The call duration is based on the number of frequencies in the scan list, the number of characters in the callsign, plus the time required to transmitter the sending callsign.
• To determine the best frequency to call another station, each station monitors each ALE call heard on each frequency of the scan list. This information is recorded by noting the calling stations callsign, the time, the channel, and the LQA value for that transmission.
• To provide a means of maintaining the network, each station may "Sound". This process is based on the previous established need to let other stations know what channels a station can be contacted. To accomplish this a station can sounds. This is a call, where a station broadcasts its callsign on each of the assigned channels in the scan list. The format, TO ABC - TIS ABC, provides the receiving stations a determine who is in the network, the channels that they can be contacted on, and the ability to rank each channel (LQA) as to the quality of the link. A station can run silent (not transmit), but still know what channels a station may be contacted on, if communications are required. Sounding is a programmable feature of the ALE controller, it may be turned on or off, the duration of sound in seconds, the sounding interval, and the ability to sound now may be programmed.
• A station (in most cases) has the ability to listen and not transmit, even to calls that are addressed to that stations. The station will record the LQA data for the call, but still provides for the ability to not transmit if the situation requires.
• If a station is required to contact another station which does not have valid or current LQA data (channel, time, and LQA value) then the calling station will call that station on each channel in the scan list until contact is made. This function is again programmable in the ALE controller.
• If calling another station and contact (link) is not established on the best frequency as determined by the current LQA data, the call will be attempted on the second best channel, etc. or until the number of tries (call attempts) has been exceeded. This is again a programmable feature of the ALE controller.
• The basics are:
• Record the LQA data for each station in the network.
• If programmed sound at fixed intervals, so the other stations in the network can contact your station.
• If required to call a station, the ALE controller will determine the best channel to call that station.
• Upon termination of a link, each station will resume scanning.

ALE Networks:

An ALE network can be based on a number of needs or requirements of the user(s). Within an ALE network each station is assigned a unique callsign. This provides for the ability for a single station to belong to a number of different ALE networks, with different call signs. ALE networks are based on the scan list (list of frequencies) for that network. Again since a single station may be a member of a number of different networks or may be required to communicate with a number of networks, networks membership is based on the use of the stations callsign and the scan list used. A programmable function of each ALE controller provides for a list of call signs of network members and "Self Address" (addresses that the station may be known by (AKA) in other networks). So to switch networks, the station need only change scan lists, since each scan list is based on a specific ALE network.

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