MIL 188-141 2G-ALE

MIL-188-141A Standard, also known as Automatic Link Establishment (ALE), specified by the US Department of Defense in September 1988 and with two Change Notices in June 1992 and September 1993, is a procedure whereby radio stations are able to automatically set up their link thus eliminating the need for skilled operators - in fact the growing lack of trained and experienced staff was a driving force behind the idea of ALE.

In March 1999 MIL-188-141A was integrated into the new specification MIL-188-141B, Appendix A. The message protocol was thereby extended slightly.

A station will transmit a link quality burst which may or may not contain the address of another station on a series of pre-assigned frequencies. The listening station(s) will continuously scan through these frequencies. During scanning the receiving station will perform a link quality analysis and measure signal to noise ratio and bit error ratio. These measurements are used to set up a table in memory of link quality assessments for each station and frequency.

Based on the values of the table, the best frequency available is selected when the station wishes to transmit. When the ALE controller of a receiving station hears its own address (or the address of the group to which it belongs) it will stop the scanning and respond to the call. The stations will then either switch to a low speed data exchange mode or to a high speed data (FSK or PSK) modem or to voice mode.

The MIL-188-141A signal is an 8-tone MFSK signal in the range 750 - 2500 Hz spaced 250 Hz apart. Each tone (symbol) is 8 ms long corresponding to 125 Baud and represents three bits giving a bit rate of 375 bps.

The MIL-188-141A bit-stream is structured in 24-bit frame, which includes three bits preamble for the frame type and three 7-bit ASCII characters or just 21 bits unformatted binary data.

To increase robustness the 24-bit frame is Golay (24, 12) encoded, and then interleaved giving a total frame length of 48 bits + 1 stuff bit. Each 49-bit code word is transmitted three times one after another to combat burst interference.

In both specifications, especially according to MIL-188-141B Appendix B, the 21-bit ALE frame data can be encrypted before transmission. This feature is named Link Protection. The data may be encrypted according to different classified application levels: AL-1 to AL-4. Unencrypted data is transmitted with AL-0.

Only unencrypted data can be displayed correctly with the mode decoder. For protected application data – according to MIL-188-141B Appendix B (Link Protection) – the display may be meaningless, because a user specific key is necessary for data decryption.

MIL-STD-188-141 Alternate Quick Call ALE

Alternate Quick Call ALE (AQC-ALE) is derived from the MIL-STD-188-141B, Appendix A standard. 

When poor channel conditions exist, such as high noise levels, static crashes and weak signals that causes problems during a LINKing attempt where NORMAL-ALE fails to link the use of AQC-ALE can also be made to overcome such channel conditions.

AQC-ALE provides a link establishment technique that requires significantly less time to link than the baseline ALE system and is significantly better at establishing and maintaining a link on under poor channel conditions.
This is accomplished by some additional technology and trading-off some of the lesser used functions of the baseline ALE system for a faster linking process. The primary focus of AQC-ALE is to establish a link between two or more stations as rapidly as possible. Once linked, information can be exchanged in the most efficient manner as is common between stations. 

AQC-ALE is basically a tactical tool, in that although it supports Scanning/Sounding and most things that baseline (a.k.a. NORMAL ALE) supports, AQC-ALE is usually used in small tactical network operations rather than fixed point 24/7 ALE Network operations.

AQC-ALE allows for a maximum of a 6 character (2 ALE word) address for both the OWN, OTHER and NET address. In addition, AQC-ALE does NOT support AMD, DTM or DBM during link set up, or an AMD at link clear or an MOTD or initiating an QC-ALE link by use of a MIL-STD-188-110 modem associated link request.

AQC-ALE has a very fast calling handshake and all parties must be using FAST AGC and the Scanning/Sounding station MUST be operating at a 5 ch/sec scan rate for best results. A 2 ch/sec scan rate will work depending on the number of channels in the Scan Group and timing considerations as to when the calling station starts transmitting and when the Scanning station comes to that channel next, a 1 ch/sec. scan rate will not work reliably. An AQC-ALE call will reliably capture a station Scanning that is using a 6 channel or less Scan Group at a 2 ch/sec. scan rate and has “Listen Calls” checked and “Listen Time” set to 400.

Quick AMD Dictionary
AQC-ALE offers additional tactical features not found in baseline ALE, such as Quick AMD Dictionary operation. QAMD works from three dictionaries in a code book fashion where no actual text of the message is being sent out over the air. It is a table lookup value in the code book which are the dictionaries. There is no data encryption going on here. Using a combination of a fixed dictionary for most commonly needed words and mission specific expressions in the additional two dictionaries, which can be changed to fit the scenario of the Exercise or non-Exercise.

AMD Dictionary is a quick mouse double click system of selecting words and phrases to create an AMD when using AQC-ALE. The operator can not enter any words on the fly, the word or phrase must pre-exist in the dictionaries. If all the users are not using the same dictionaries then the receiving station will not print the proper message.

AQC-ALE MEET ME
AQC-ALE provides the “Meet Me” dialog for rapid frequency changes that can steer an entire net on the fly, the “Meet Me” Orderwire command capability.
The “Meet Me” capability requires that all stations in the link be under computer control and all are using the exact same GROUP/Channels configuration, then the NCS or any station can steer the group (or individual station) to a new channel instantly without the need for a voice announcement or even an AMD message.
The process simply requires entering the RX and TX frequency in Hz (channel based pull down from current GROUP is planned) and checking LSB if required and clicking OK, then an Ordewire message is sent and all the stations will QSY to the new frequency/mode pair if entered correctly and if existing in the current GROUP of all linked stations. Optionally stations may be added or deleted from the linked stations and an acknowledgement (Ack This) may be requested.