Not Using ALE For Frequency Selection

The core function of ALE (Automatic Link Establishment) has historically been to select a frequency to use prior to exchanging data or voice.   An application will request to link to a peer identified by an ALE address and the ALE layer will select the frequency and establish a link.   This paper explores an architecture where the choice of frequency is made above the ALE level.  The ALE layer is still used to establish links using a frequency explicitly requested from the higher layer.

The paper starts by considering a number of downsides of ALE frequency selection, including:

• ALE waveforms are different to the ones used for data transfer
• For Wideband HF, ALE measurements use only 3kHz and not the full bandwidth
• Sounding times are too short to allow for HF Intermediate Term Variation
• Sounding overhead reduces time channel can be used and increases latency
• ALE units cannot generally take into account peer location
• ALE units often pick a poor frequency after an outage that affects many frequencies
• Poor handling of asymmetric links

The paper then looks at how all of these problems can be addressed by moving frequency selection above the ALE layer and in particular integrating the choice with the STANAG 5066 layer and using frequency pools to minimize frequency count.   It shows how frequency predication models and knowledge of mobile unit (peer) location can be used to more intelligently select the best frequency and improve overall performance.

It then considers the related problem of terminating ALE links and making full use of the ALE link management procedures specified in STANAG 5066 Edition 4^[1].  There are several trade-offs, making this a harder problem than initial link establishment.

Finally, the paper looks at experience with current systems implementation considerations for the approaches described in the paper.