telephone extension interferance

There is an old telephone extension circuit already wired into 5 additional wall outlets. the broadband speed is 4.5 typically when it is connected directly into the master socket on its own. when the telephone circuit is connected this drops to 3.3 for 1 or 2 sockets and about 2+ when all are connected. All sockets have been checked and are wired correctly. Why should the circuit reduce the broadband speed?

In reply to a post by taxaceae:

Why should the circuit reduce the broadband speed?

Because the bell wires are connected possibly...

As per http://www.thinkbroadband.com/faq/sections/radsl.htm...

tried that. there is a little improvement but still a significant drop

A twisted pair balanced transmission circuit works well, but when you start to add bridge taps to it and unterminated legs after the receiving device the signal performance is impaired.

You're adding more wires with the ability to collect interference or imbalance the impedance.

Put the broadband router on a filtered faceplate or similar at the socket where the line enters, feed the rest through a filter and you'll get the best you can.

In which case consider using a faceplate filter like the one at http://www.coolwebhome.co.uk/faceplate/

and if the modem is not going to be at the master socket run a dedicated data extension using decent cable to where the modem will be.