Last update 26 Feb 2010.
A semi-myth. All this does is set one figure from which a second is calculated.
The first is the Maximum Stable Rate (MSR). This is the lowest connection speed held during the 10 days. Do not confuse connection speed (also known as sync speed or data rate) with download speed.
The second is the Fault Threshold Rate (FTR). This is 80% of the MSR.
If after the 10-day period you start to get connection speeds frequently below the FTR for your line then BT will consider this as possibly needing investigation. Recently, since the introduction of WBC the emphasis seems to be on “consider” and “possibly” rather than on “needing” investigation.
The DLM is also running during the 10 days, but no long-term settings (other than perhaps the Interleaving setting) are made by it and it continues as normal afterwards.
Note that LLU operators do not seem to have a 10-day period on initial connection, though there can be a few days of line optimisation. O2 for example (these days) normally turn off their DLM after two or three days.