This has most of the answers
http://www.solwise.co.uk/downloads/files/adsl-around-the-home.pdf
On page 3 is the key statement
There are other manufacturers
http://www.solwise.co.uk/downloads/files/adsl-around-the-home.pdf
On page 3 is the key statement
The following paragraph is the key to this optimisation.
There should be only one path for the ADSL signal - from your master socket to your modem. There should be no blind alleys caused by t-junctions or branches on the way. All of these blind alleys should be closed off with a filter so that the ADSL signal cannot pass down them.
There are other manufacturers