Irrelevant to the OP but A and B supply an unfiltered ADSL signal - does that carry the voice frequencies also? If it does, is that A-B connection OK to use for a router without filtering off the voice frequencies?
Often wondered about that with the A-B connections at the back of filtered faceplates
The thing is, you have the opposite idea of what ADSL filters do to what is in fact the case. (Ignore the T3 ring wire as irrelevant for this discussion).
The ADSL/VDSL2 filter removes the broadband signals from the telephone connections. Not the phone signals from the broadband connection. If it didn't, you wouldn't be able to hear a darn thing on the phone. Ever heard a dialup modem connecting? Or picked up a ringing phone and just heard a high whistle? That is a modem (probably a fax) trying to connect.
Neither does a conventional filter split the two signals. It could only do that by containing two filters, one to remove broadband, feeding the phone socket, followed by another removing the phone stuff. So more than doubling the complexity of the bit of kit.
ADSL/VDSL2 sockets present everything that is on the line to whatever is plugged into them. Filtered phone sockets only get analogue audio frequencies. A modem or modem/router on a ADSL/VDSL2 filtered socket gets an unfiltered signal, including the phone frequencies which it ignores.
So the A/B connectors on the back of filtered faceplated get a completely raw signal, effectively direct from the main incoming Openreach line. Straight through - hidden main A/B >> test socket >> faceplate >> extension A/B supply.
Anything on the extension at the other end requires a filter.
In an installation where absolutely nothing
is connected to the phone line except a modem or modem/router, no filters are needed at all. (Though if physical extensions are present but not used, their ring wires may
need removing or filtering. I believe some early filtered faceplates did not filter it. It's the ring wire's capability of picking up and feeding back random electromagnetic noise that is the problem).
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Edited by RobertoS (Sun 12-Aug-12 00:05:59)