The BT system does not support SRA
SRA is where the SNR margin basically remains where it is and the sync speed varies up and down on the fly without user intervention to match the conditions.
So during the night wheras now we have the SNR margin dropping and CRC's rising, under SRA the sync speed would be dropping but the important bit is that in the morning the sync speed would be rising as progressive re-syncs were done automatically as the real SNR margin started to increase.
Obviously it would help those lines which currently re-sync at night due to advserse drop in SNR margin or CRC's and hence never get back the speed the next day - so remain at a lower than possible speed soley due to the night time conditions which are now setting the daytime speed.
There was an experiment done on it near to the begining of ADSLmax (?possibly on a LLU system?) and as I vagually recall it did not work as well as might be expected, so was never implemented or progressed to a system wide rollout.
I can see plently of problems might occur over short term drops in SNR due to interference causing all sorts of feedback/instability loops in the control system meaning the sync speed being continually chased up and down by the SRA controller.