Although Automatic Link Establishment (ALE) is not ubiquitous in Amateur Radio, it is seeing increased use among the agencies that Amateur Radio / ARES serves in an EMCOMM role. Having a baseline understanding of ALE can be useful.
Automatic Link Establishment (ALE) was developed to provide for establishing best quality high frequency communications without the need for consulting propagation forecast information, or the need of having training to interpret propagation forecast information. ALE uses digital calling, along with multiple schemes to ensure reliability (i.e. data interleaving, data redundancy, and Forward Error Correction), and bidirectional signal quality evaluation, to automatically choose the best frequency to be used between two stations. With ALE, it is not necessary to know the Maximum Usable Frequency (MUF), foF2 NVIS Critical Frequency, or the D-Layer -1dB Absorption Frequency. The radios automatically negotiate to determine the best frequency to be used.
ALE uses a collection of channels that are spread, ideally at even intervals, across a segment of radio spectrum. This collection of channels is referred to as a Scan Group. An ALE radio operates in scan mode, traversing the channels in the Scan Group with the speaker muted. Scan groups are usually limited to a maximum of 10-channels, and this is due to the need to limit the scan dwell time so that all radios scanning for a call will traverse the calling channel in the time period required to make a call. Scan groups of fewer channels are common where there is not sufficient spectrum allocation to achieve channel diversity.
In concert with the Scan Group, the radio maintains a Link Quality Analysis (LQA) table. The LQA table contains time-stamped bidirectional signal quality assessment information for each station in the ALE network. LQA data is used to determine the best frequency upon which a LINK can be established between stations. LQA data can be obtained or updated by issuing an LQA update call to a target station. A software algorithm is used to decay the LQA data over time, which serves to force an LQA table update to occur should the LQA data decay to the point where its use might result in unreliable communications.
Each radio on an ALE network is programmed with its own Self Address, and with the Self Address of other stations on the Network. For Amateur Radio ALE networks, the Amateur Radio call-sign is the Self Address. For other radio services, the Self Address is agency assigned.
Each Self Address also has an associated wait Slot number. The wait Slot number is used by a station in responding to a Group call, and is used to prevent data collisions by staggering responses to the Group Call. There are a limited number of wait slots, and from a practical perspective, a Group call would require applying limitations to the number of members supported by the Group. Under Group calling, it may be necessarily to hierarchically organize the membership and distribute messages down the membership hierarchy with strictly defined member responsibilities. For example, Idaho ARES might define a state-wide Group to include one primary and one alternate station self address per Idaho ARES District. Each District might define a Group call to include one primary and one alternate self address per Idaho county. Similarly, each Idaho county, depending on the number of ALE equipped stations, might then define one or more groups within the county. A message initiated statewide would then be addressed to the statewide group. The primary or secondary station in each Idaho ARES district would receive the message and then transmit the message to their District group. The primary or secondary station in the District Group would then retransmit the message to the County group. The primary or secondary station in the County group would the retransmit the message to each of the county members group. Slots are encoded in a 5-bit field, limiting the slot number to 32 values that span from 0 through 31, and this imposes the limit on the number of Self Addresses that can be subscribed to the same Group associated with a Group Call. Group calling has each ALE radio holding the station's Self Address and the Group Self Address in the radio configuration, with the Group Self Address having a unique Slot number on each radio. For an Amateur Radio application, where the Group Self Address is not the station's call sign, the Group Self Address should be viewed as a Tactical Call Sign, and the requirement to properly identify the station with the FCC call sign would fall directly upon the operator. Data collision avoidance, and its associated slot number, is not an issue for peer to peer calling.
To initiate a call to an ALE station, a CALL button is activated, and the operator then selects the target station Self Address from among a list of Self Addresses that are programmed into the selected Scan Group. If the LQA data is stale, or if an LQA call is specified, the calling radio will iterate over all channels in the Scan Group and bidirectionally exchange LQA data with the called station radio. Once the LQA data has been updated for all channels in the Scan Group, the calling radio then evaluates the LQA table data and determines which channel provides the best signal. The calling radio then returns to the best channel and then digitally calls the target station. When the called station answers the call, the stations are said to be LINKED, and both the calling station and called station speakers are unmuted.
If the LQA table contains current data (i.e. the data has not decayed to the point of being no use), the LQA exchange may be bypassed in the calling process and the calling radio may simply go to the best channel as a first attempt to call the called station.
If the called station does not answer, the calling station radio will assume that the LQA data is not viable and will move to the next channel in the Scan Group and re-attempt the call. For an unresponsive called radio, the calling radio may iterate over all channels in the Scan Group. If the radio is configured to implement a retry on a failed call, this process may be repeated.
When communications have been completed, either station can terminate the LINK by pressing the END CALL button. The radio initiating the termination of the LINK will digitally notify the other radio that the LINK is being terminated, and then both radios will re-enter scan mode.
ALE supports a limited digital message capability. This is referred to as Automatic Message Display (AMD). A call to another station can be performed as a Voice call or an AMD call. Further, AMD can be used once a LINK has been established, whether the LINK was established with a Voice Call or an AMD call. AMD is restricted in capability, only being able to convey a message of 90 characters in length. Further, ALE/AMD supports upper-case letters, numbers 0 through 9, ? and @ (referred to as a Basic 38 ASCII subset). This feature is woefully inadequate for most EMCOMM requirements, and because of this, it is common for ALE to be used to establish a LINK, and then another digital mode is used to exchange information. For example, an ALE call would be used to establish a LINK, then an ICS-213 might be exchanged using FLDIGI MT63, and then the LINK is terminated.
ALE also supports SOUNDING, but caution must be used here. Sounding announces the calling radio Self Address onto the ALE network, and causes the receiving radio to insert the calling radio Self Address into the list of available Self Addresses that can be called. Although this might sound like a great feature, limited storage for Self Addresses in some ALE equipment implementations might cause another more critical Self Address to be pushed out of the list. For this reason, some ALE networks prohibit ALE Sounding. The Sounding capability should only be used on a network where Sounding is not prohibited. Sounding can occur automatically at a repeated time interval, or manually. Note that on a network where Sounding is supported, there may be time constraints placed on when Sounding can occur. If a time constraint is imposed on a network that allows sounding, only Manual sounding should be used. If no time constrain is imposed on a network that allows sounding, automatic Sounding can be used.
Timers are implemented to return a radio to scan mode should the LINK be unintentionally left in the active mode or the UN-LINK request not be received. This is critical in ensuring that the radio is available to receive a call after the communications exchange has been completed.
In order for an ALE network to provide the best performance possible, the 10-channel scan group should use frequencies that are evenly spaced across a segment of spectrum. For example, 10-channels that span from 2 MHz to 10 MHz might be chosen for night time or NVIS communications, where the channels occur at approximately 0.8 MHz intervals. A daytime scan group might use 10-channels that span from 5 MHz to 20 MHz. Unfortunately, the HF allocations for Amateur Radio do not provide for establishing best practices in implementing an ALE network because Amateur Radio allocations occur in clumps (and a linear spread of channels across a segment of spectrum is simply not possible), and this imposes degraded performance of the Amateur Radio ALE network relative to what is possible with establishing day and night/NVIS scan groups over a more linear channel spacing that occurs in other radio services.
There are two variants of the ALE specification, including a Federal standard and a Mil standard. ALE equipment that supports the Mil standard usually supports the Fed standard. ALE equipment that supports the Fed standard may or may not support the Mil Std. For best interoperability, ALE equipment that supports both the Mil standard and the Fed standard are desired.
The ALE specification also indicates a generation version level for the ALE implementation. Although the current specification supports 3G (which uses a GPS to derive timing in establishing calling sequences), the older 2G implementation is the most prevalent and is what is used by the agencies that ARES interoperates with.
Automatic Link Establishment (ALE) equipment comes in two basic varieties:
Embedded ALE Radio
Software ALE (using a radio sound card interface & rig control)
Embedded ALE has the ALE controller, and all ALE configuration support (i.e. Self Addresses, Channels, Scan Groups, Call Retries, etc.), built in to the radio. Common radios that implement embedded ALE includes a number of Military surplus radios (i.e. Harris, Datron, Sunair, etc.), Embedded ALE is also available in the Motorola Micom Mobat 2 and 3 series radios, and in the Icom IC-F8101-31 and IC-F8101-33 radios (caution: IC-F8101 model prior to -31 and -33 implement Fed standard ALE only. The -31 and -33 radios implement both Fed standard and Mil standard ALE, with the -33 model having an FCC LMR Part 90 type acceptance).
Software ALE can be implemented by interfacing radio control and sound to a Windows computer using either PC-ALE or MARS-ALE software. PC-ALE is license free but is not on a maintained software source path. MARS-ALE is on a maintained software source path, but requires an Air Force MARS, Army MARS or USCG AUX call sign and validation in order to obtain a license file. Note that some interoperability incompatibility has been observed with both PC-ALE and MARS-ALE. Among these are that terminating a LINK after an AMD call to an ALE radio that has embedded ALE may not result in terminating the LINK. Without the use of timers, the embedded ALE radio will become unreachable. Even with timers, the embedded ALE radio will be unreachable temporarily (until the timer times out).
Use of ALE requires an antenna system that supports the rapid changing of frequencies. Typical ALE Scan Group scan rates have the radio scan either 2-channels or 5-channels per second. Options for station implementation are:
Multi-band Antenna
Broadband Antenna
Very fast memory auto-tuner
A multi-band antenna presents inter-dependencies in the tuning of each band, and can be very difficult to set up.
A broad-band antenna, such as a Balanced Terminated Folded Dipole (BTFD), requires no configuration and is easy to set up. There are formulae that specify the minimum length for a BTFD and the minimum spreader spacing relative to the lowest frequency of operation. These antennas have an un-fair bad reputation in Amateur Radio. Further investigation will often show no first hand experience with the antenna or show that the complaints refer to an implementation that has the antenna physically shorter than can be supported on the frequency of operation. The United States Navy studied these antennas in the 1930's and found them to perform quite well. The BTFD is in common use among the Federal and State agencies who ARES serve, and they work quite well in an ALE environment. The BTFD is probably the least expensive antenna system solution for ALE, and especially so if self-built.
A memory auto-tuner is a viable option, but the tuning must be fast enough to occur in the ALE calling sequence such that the radio does not move to the next calling channel before tuning completes. The SGC auto-tuners and the Elecraft KAT500 auto-tuner are known to be sufficiently fast enough to support ALE operations. Note that when configuring the auto-tuner, a trial tuning should be performed on every channel in every scan group so that subsequent memory tuning need not perform the tuning sequence. Note that some auto-tuners do not specify the maximum time to tune for a tuning solution that is in memory. If the maximum time to tune is not specified, it may not be possible to determine if a given auto-tuner is a good fit for ALE operation. When an auto-tuner is used with either an embedded ALE radio or a software ALE implementation, the configuration must include an indication that the tuner is in the circuit.
ALE 2G networks have been established on Amateur Radio. These include one network that is focused on supporting digital communications (HFN) and a second network focused on supporting voice communications (HFL). Additional information can be found on the HFLINK web-site. As with all things digital, Upper-Side-Band is used for ALE.