|My main question is: when modem A is being used and modem B is switched off, is it necessary for the splitter for modem B to be removed and the filter put in its place? Or is the fact that modem B is switched off sufficient? |
It's sufficient to turn the modem off.
I suspect that it's necessary and that's why my friend gets very slow download speeds even when the other modem is off.
No it's not.
A filter is ONLY required if you're plugging a telephone in. If you're not plugging a phone in to that line, no filter is required.
If there's an issue with her download speeds she should google "bandwidth speed test" and try a couple from different sites and compare them to what she's supposed to be getting. There can be many different reasons for slow downloads and none of them have to do with filters.
Just FYI, not only am I a networking professional, I have ADSL at home and there are phone outlets in every single room in my house. The only ones I have filters in are the ones with an actual phone plugged into them. The rest sit empty. With the modem powered off, it's the equivalent of an empty outlet.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.