Extension wiring issues

A friend of mine had a BT engineer visit and was apparently advised that their extension wiring was interfering 'slightly' with their FTTC. Is that actually possible? I know about the ring wire but I assumed that the filter in the faceplate meant that noise on the extension circuit is blocked out.

If the extensions are connected via the faceplate they should have no real effect.

If they are picking up a short wave radio station at full transmit power (which they should not) then might swamp the filtering.

The way to tell is remove the removable faceplate, restart the VDSL modem and see if speeds improve.

the filters aren't required to be 100% but just meet a dB attenuation requirement (or not, according to MHC's tests) so yes it is possible.

The optimal xDSL circuit terminates at the NTE/modem with no extensions.

Could also be things such as non-twisted pair wiring, or the use of wires from different pair, or a large noise source close to the extension cabling that breaks through the filter or is not attenuated enough by the filter.

Or may even be inter-mod products from two signals that are in the voice band!

If the extension wiring is taken off a junction box before the master socket it can interfere with FTTC more than slightly. My Infinity speed began at around my estimated 58Mbps and quickly dropped to 12, with a few days of 3. Disconnecting the (unused) extension let it climb (slowly) back to around 70Mbps.

The reason in my case was 25 year old GPO wiring: it did its original job fine and was hardly any issue on ADSL, but a disaster on Infinity.

in the above case one would hope that the BT guy would have fixed it and not just walk away from it !

in the above case one would hope that the BT guy would have fixed it and not just walk away from it !
Yes, you would hope so. In many cases they might well not notice unless they really thought of looking. In my case I had noticed the wiring was odd before installation and explicitly asked the installer about it. He assured me that it would not matter at all. I don't know if that was ignorance; or laziness/hurry not wanting to bother to fix it.

Helpful forum advice (I think both here and on BT Forum) enabled me to sort it later. Reading forums shows that it is not exactly a common problem, but not at all unusual either.

What you had is called a "Bridged Tap" and correct use of a JDSU should detect it and also suggest how long it actually is.

Dependent on the length of the "Bridged Tap" different tones will be affected by it. A long length will affect the lower tones and a short length the higher tones. There will be a notch in teh signal strength, reflected in SNR and also a notch in bits per tone. It can cause the central tone to have a 20dB drop in signal level and tones either side to have slightly less additional attenuation. You could also drop from a full loaded tone to just one or two bits in te centre of te notch slowing getting back to near fully loaded.

Interesting, thanks. Because of extensions to by 'bridged tap' extension I could well have been having reflections and loss at multiple frequency bands. I knew about that effect, though I didn't know the term 'bridged tap'. However, the effect on the initial sync was not too disastrous: 58 initial profile vs a later profile after everything sorted of 70.

I had assumed that the major effect which caused drops to 12 or even 3 was having an unfiltered phone on the extension, but I'd be interested in anyone else's more informed opinion. Not too concerned from the point of view of my own line as it is now fine; just generally interested.

Not just reflections but if it is "tuned" it can offer a very low impedance to a specific frequency and thus attenuate it quite a bit.