With the use of an ADSL-loop-extender the original SNR of the ADSL lines seems to go to margins that might interfere with the data communications. (For example SNR for a regular ADSL Lines we tested was originally 6db but when we use the ADSL-loop-extender the SNR goes to 4db) * for this case the ADSL-loop-extender increases the customer bandwidth from 512K up to 2MB

You can think of SNR margin as the measure of quality of the service; it defines the ability of the service to work error free during noise bursts. SNR Margin of 6 is considered to be normal quality data circuit; 8 or 10 is needed for ipTV. SNR margin and bandwidth settings in the DSLAM may often work at opposing purposes. The higher the SNR Margin you select, the lower the bandwidth will be. Most DSLAMs allow you to specify min, max and target SNR Margin and min, max and target bandwidth. For most circuits, the DSLAM and modem will sync at the target for both parameters, but if you are stretching the circuit to the limits of its capacity or if you have a line with bursty errors, you may see one of these parameters working at something other than the target rate. My guess is that your circuit will not sync at the values that you specified for target values but will sync if it relaxes the settings to the minimum. If the DSLAM/modem can't get the target bit rate (2Mbits?) at the target SNR Margin (6?), then it will try to adjust things. You may get 2Mbits at SNR 4 or 1.8Mbits at SNR 6 depending on how the modem and DSLAM decide to make it work. I suggest that you set the minimum sync rate to be 0 and let the service sync to a rate that will meet your SNR Margin target. This will prevent the customer from going out of service on hot days, when they could work at a degraded rate. This issue isn't particularly unique to the use of the extender; you could make this happen on a shorter circuit with the same settings without an extender. The extender has no influence on this behaviour.