ADSL2+ has a lower power state available to it, which has the potential to save some 2.9 GWh/year for every million ADSL2+ lines in operation, and also make the task of running MSAN's in exchange buildings within their operating temperature range easier.
BT Wholesale ran a trial testing this low power (L2 mode) which is known as BT Cool Broadband on some 29 different types of ADSL2+ modem that ended in December 2011, and should at the end March 2012 have started a six week trial, prior to any launch of the mode across the network.
The low power mode should not result in the lose of any data with seamless transfers into and out of the mode, and will only be used if the amount of data across an ADSL2+ or ADSL2 line is below 128 Kbps. Once data transfer above this level is requested, the modem and exchange hardware will switch back to the full power mode.
In the low power mode the modem and MSAN can negotiate a lower rate, that for those who monitor their noise margin may result in higher than expected noise margins being reported when in the low power mode, compared to when working in full power mode.
To avoid lines that are switching in and out of the low power mode causing unneccessary crosstalk the low power mode is only entered after a period of time. The level of power reduction is also limited to a maximum drop of 10dB.
For those geeks amongst you there is the TR-202 document on Broadband Forum that goes into depth on how this mode works.
The power saving from an individual line seems insignificant, but when added up the potential is there for BT Wholesale to save several million pounds in electricity costs. Those addicted to monitoring the state of their broadband connection, just need to be aware of the effect this may have on router stats.