I wonder whether DLM takes FECs into account in deciding whether to restore fastpath after it's applied interleaving? That would have some logic, because FECs are an indication of how much error correction is needed to maintain the low ES level.
Contrary to other people's opinions, I *do* think DLM pays attention to FEC counts in some way. I recently commented on someone's line (probably on TBB), where the line had an uptime of over 70 days, and their ES count in that time was still below 2800. That ought to have been a recipe for DLM de-intervention, right?
However, their FEC count was increasing at a hell of a rate - I don't recall exactly, but it might have been multiple-tens per second, possibly low-hundreds per second. This was enough to show that the current DLM settings were doing the perfect job.
I recall that SIN 498 requires that modems be able to report the FECS counter (FEC-seconds), which is something we never see in telnet or GUI output. I assume it works in the same way that ES's accumulate WRT the CRC counter. The fact it was required makes me suspicious that DLM does indeed use it for some purpose.