20 May 2014

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.

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