When DSL has a speed less than the promised one there can be any of the three problems. The copper wiring at your home is loose and has wear and tears or cable from the ISP side is loose or outside wiring has some issue. How to boost DSL speed
How To Boost DSL Speed To Increase The Speed Of Data Transferring.
When the issue is present on the ISP side or outside wiring, then it is the duty of ISP workers to solve it. But when the issue is present in the copper wiring in our house, we need to take steps. For this, you need to look at the following steps.
1) Use micro filters on the line from the wall jack to your phone and PC, to reduce the unwanted noise on the telephone line. Noise might interfere with DSL or voice connection which reduces the speed.
2) Find the jack which is closest to your phone line and plug your modem into it.
3) Electronic devices generate electromagnetic noise that interferes with DSL connection reducing the speed. If your modem is connected near, then it is recommended to move your modem from large electronic devices.
4) If you see the DSL light blinking again and again with time, then it indicates an unstable DSL line configuration. In this situation, you need to contact ISP.