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I've recently switched ISP and I'm using a router they supplied. After much trial and error with micro filters and cables I've found that my reduced ADSL broadband speed was happening because I'd left my old router plugged into a phone socket, but powered off. Unplugging it from the phone socket and my speed returns.

I then swapped to use my router to connect and left the ISP supplied one connected but turned off and again the speed dropped, but then also improved again once it was unplugged from the phone socket.

So it doesn't seem to be specific to my particular router but a more general issue with two routers.

So essentially I'm asking: How can a 2nd router plugged into the phone socket but turned off cause about a 30% drop in speed?

In case anyone is wondering I'd only left it connected as I was planning on switching back to using my router instead of the ISP supplied one once my connection was up and running.

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How can a 2nd router plugged into the phone socket but turned off cause about a 30% drop in speed?

ADSL can usually adapt to the line electrical characteristics, and ADSL modem/routers usually have passive stages that alter those characteristics.

So, probably having both routers in parallel degraded e.g. the resistance seen by the active router, making it choose a different (lower) modulation and carrier frequency - just as if the copper cable was longer than normal.

You'll usually see that being farther from the cabinet results in slower ADSL negotiation speeds (where I live, this is very common, even if they're now moving to FTTC). Just as well, replacing the wire from the home to the cabinet with a high quality, new wire may result in a speed increase (or a reduction of decrease).

You can easily check whether this is the case by plugging a cable in the powered off router and measuring the resistance/impedance between the wires. In theory, a perfect device should exhibit infinite resistance. In practice I bet it won't.

Another (not very likely) possibility is that the routers had actually little to do with the matter - the culprit might have been the second power plug. Whatever was plugged in the plug where the powered-off router was connected was incorrectly earthed and injected noise on the line. While possible, this is unlikely because the noise from a power line is 50/60 Hz, well below ADSL low-pass which I think is around 4 kHz or above, and the router would have probably malfunctioned in other ways (up to and including releasing the magic smoke). Even so, you can check it by swapping the power plugs while keeping all other connections the same, and only one router connected. If the quality drops even with a single router plugged in, it has to be the plug.

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  • It's not the 2nd possibility you suggested as the same happened with either router unplugged from the power socket but just the phone cable connected. First suggestion sounds more likely explanation.
    – Chris R
    Feb 20, 2017 at 22:51

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