Thanks for replying. This is all very interesting and expanding my knowledge. Another thought, how could the power be transmitted in two directions ?? IE- 50Vdc (approx. 100Vac when ring current is applied), going into the EU's premises, and the required power for the mini-slams coming back from the EU's premises ??
Please don't think I'm trying to be awkward here, far from it, I'm genuinely interested. 
BS, the ETSI documents are not exactly bed-time reading, but I think the sort of nominal line power levels they are talking about are 60vDC @ 300mA, total power < 20W, and each EU pair provides its own power. It
does indeed cause problems for POTS - both ringing current and off-hook indications, so,
while the line is back-powered, the FTTdp/'back-powered' faceplate combination (AC supplied) in co-operation intercept those signals at their respective ends, and pass them down the line signalling in supervisory signals instead. If/when the AC power to the back-powering faceplate fails, then everything reverts to POTS signalling. But note, this is only a conceptual plug-in faceplate; I doubt the final design(s) are actually worked out yet.
One of the design challenges is to operate at such lower power levels; but there have already been sucessful demonstrations I believe. There are graphs in the documents showing power fall-off with drop-wire length, but they have in mind generally average short drops.
HTH
