powerline communication devices?

Forget the whole thing - the power wiring is not suitable for such frequencies that are needed for the advertised transfer speeds. There are two main causes for it:

a) the wires are unsymmetrical and at places far from each other, building unexpected stubs at RF frequencies.

b) the insulation commonly used has plenty of losses at RF frequencies.

Besides, the PLC thingies are tremendous sources of radio interference and susceptible to all kinds of external radio fields.

Why is then the telephone wired xDSL better?

a) the wires are twisted together without the sudden stubs and impedance bumps caused by e.g. switch cabling and fluorescent light noise reduction capacitors (a dead short at RF).

b) the cable is intended for signal transmission with better insulation than power wires.

The whole idea of PLC is targeted to break the telecom monopolies in Germany and France.

Tauno Voipio tauno voipio (at) iki fi

I'd broadly agree, but you omit the important factor of expected operate distance. The two systems use very similar technologies, but the ADSL can operate over larger distances, for the reasons you mention. That simply means the usefull distance of PowerLine systems is not 'some km', by they appear quite practical for use within a dwelling. for an example, see

Where wider area networks are deployed by power companies, you will find they run fibre for that purpose, often with their main power cables.

-jg

The Swedes measured (IIRC, Technical University of Linkoping) the attenuation of their typical power cabling at PLC frequencies. It was up to 80 dB / 100 m, which would mean 400 dB for the targeted distance of 500 m.

For reference, it is about three times the attenuation of an EME (Earth-Moon-Earth) path.

IMHO, the communication which was obtained 'over' such cables did in fact go as radio past the cable.

There is an on-going PLC system in the Finnish city of Turku. To make the thing work at all, the PLC link was split to two legs: one at a higher frequency from the transformer to the house main electric central and another at a lower frequency from the electric central to the apartments. The distances are pretty far from the a couple of hundreds of meters target.