There is a dampening sub-function to the re-profiling component of the DLM algorithm - to prevent oscillation or "churn". Churn was a deficiency of earlier DLM algorithms. It is where a DSL repeatedly re-grades upwards and then downwards, due to minor variations in line conditions. The churn occurs in a never-ending cycle, and introduces instability of its own (unnecessary re-syncs).
This dampening effect is more pronounced when moving up to a more "aggressive" profile (i.e. to higher data rate and/or lower interleaving/INP), than when re-profiling down to a less aggressive one (i.e. a lower data rate and/or higher interleaving/INP).
In other words, the DLM takes somewhat longer to re-profile upwards than it does to re-profile down.
BT has a number of patents describe its DLM algorithms, some dating to 2007, if not earlier. There is a smattering of pseudo-code in the patents that document the dampening effect.
In later patents the inventors describe a variable sample rate. Seriously defective lines are identified separately, according to error counts and retrains per sample period. These lines are then sampled at greater frequency than ones showing stability in the past. Eventually if downwards re-profiling still doesn't resolve the instability, a poor line is flagged up for engineer's inspection.
The typical DLM sample rate is reportedly every 15 minutes (a DLM "timeslot"). And there are 96 of those in a 24 hour period.
Various efforts are made to identify "area-wide events". These events cause a batch of line re-syncs at the same aggregation transceiver device, or geographically-neighbouring devices. An obvious example is a thunderstorm. These transient events should be excluded from the DLMs dataset used for re-profiling.
Attempts are also made to identify user-initiated re-syncs, so that they too do not cause unnecessary re-profiling. Some devices, e.g. those based on the Broadcom BCM63xx chipset support a "Dying Gasp" when power is pulled. This is a final G993.2 message to inform the DSLAM that there has been a power failure at the CPE. This is used to distinguish the re-train from an automatic or forced re-train due to deteriorating line conditions.
P.S. Patents for BT's DLM algorithms include EP2342902B1, EP2169980A1, US2012/0099424
Edited by asbokid (Fri 24-May-13 17:20:48)