Hints and Tips - Electrical Noise
I had a customer who called me frequently on weekend evenings saying his DSL was unusable but every time I visited during the day everything was working OK. After banning the use of EDonkey there was still a problem so I installed a monitor to see what was going on
After monitoring his router for a period of time it became apparent that the electrical noise his router experienced did indeed increase during the evening, to a point where the ADSL connection became almost unusable.
As the customer had opted for the fastest possible connection theoretically usable for his distance from the France Telecom central office, but was experiencing greater noise and attenuation than predicted the final solution was to downgrade the connection to a slower speed, using a modulation scheme that was more robust in the presence of noise and which gave a perceptually faster connection than previously because it maintained a solid connection and did not keep stopping to retrain in an attempt to maintain an unrealistically high speed.
The cause of the noise? The applied fix made the customer happy so he had no interest in investigating further. The prime suspect was a cordless baby alarm which transmitted voice down the power lines and was often in use at the times when the DSL line was at its worst.
I later learned of issues with the AR7 chipset used in many popular ADSL routers at that time. This chipset was used in all the alternative routers I tested at this location. A British Internet Services Provider discovered performance problems with this chipset when used with long lines under certain circumstances, particularly fluctuating Noise Margin. This almost certainly exacerbated the situation.
This chart 
shows how the noise margin (which was already very low for the speed of the line) would decrease yet more during the evening. During the day the noise margin was just good enough to give extremely good network performance. Unfortunately the evening was the time the line was most needed and connection problems were least tolerated.
