My solution for ADSL frequent dropped connections

Some time ago I had problems with my ADSL connection. It was stable when my "speed" was limited by the provider to 2 Mbps. After a service upgrade I had frequent dropped connections particularly in the evenings. With the new service upgrade the "speed" was limited only by the characteristics of my phone line (around to 6 Mbps at that time).

This post describes the solution that I found to reduce  the number of connections dropouts due to the modem sync loss.

I checked through my modem that the SNR (ration signal/noise) decreased considerably throughout the day, starting around 6 dB in the morning, 3 dB at afternoon, 0 dB at end of the day and consequently a loss of synchronization.

I thought that if I could increase the SNR via a connection "speed" reduction then I can improve the situation. As my modem does not allow the parameterization of the maximum "speed" I tried to reduce my "speed" through an external circuit.

The trick is to "mislead" the modem making it "think" that my phone line is worse than it really is. Once the modem is synchronized, I can put the phone line back to the "normal" state. In this way I could reduce my speed to about 5 Mbps and have a SNR around 9 dB in the morning and keep a SNR around 3 dB at night that gives some margin to avoid dropouts.

The working principle of the used circuit is the following:

When the modem is powered ON with the switch "S1" in the open position, the modem will be connected to phone line with the inductor "L1" connected in series. This inductor causes the modem to synchronize at a lower "speed" (compared with the phone line without the circuit). It reduces the bandwidth of the phone line and consequently the synchronization "speed".  At this stage the synchronization "speed" of my modem is approximately 5 Mbps and the SNR around 6 dB.

When the modem is already synchronized then we close the switch S1. This switch "eliminates" the inductor "L1" from the circuit and put back the phone line to its normal state. At this stage the synchronization "speed" of my modem is approximately 5 Mbps (as before) but the SNR is around 9 dB.

When we close the switch "S1" and if the "L1" inductance is relatively high, the transition will be "abrupt" for the modem and we will have a synchronization loss. To make this transition more smooth I used the following circuit:

L1 = 2 uH

L2 = 2 uH

Note: The inductance values are approximate and obtained experimentally for my environment. If you want to try this circuit please check what is the best values of inductance for your environment (modem and phone line). You can start with a high inductance value and after removes turn by turn of the coil.

I added the switch "S3". It allows disconnect/connect the phone line instead of the modem power.

The operation sequence is the following:

1 - All switches OFF

The modem is disconnected from the phone line. We have a connection loss.

2 - S3 ON

Wait for the modem synchronization.

3 - S1 ON

The inductor "L1" is "eliminated" from the circuit.

4 - S2 ON

The inductor "L2" is "eliminated" from the circuit. The sequence is finished.

We need to repeat this process whenever the modem is restarted (for example, after a power failure).

The prototype of the circuit was built with material that already had at home.

Conclusion

With this circuit I reduced the number of dropouts from around 3 to 5 by day to about 1 or 2 per week. Currently I have another provider that uses better lines/equipment and my connection is stable at 7 Mbps without the use of my external circuit.