Goodbye Telstra, Hullo ACTEW (FYI)
Fri, 10 Oct 1997 01:46:35 +1000
My remarks regarding the impossibility of achieving high data rates,
due to the radio-frequency interference problems of high frequencies
being radiated from power wires, were in the context of the signals
being injected at the local step-down transformer.
These are usually the size of a few washing machines, and live up
poles. They are like a long-handled lever which takes a three-phase
22,000 volt (for instance) high voltage, low current power feed down
to a three phase (120 degree shift between the sine waves) 240 volt
high current signal for the homes. (415 volts refers to the voltage
between any two of the three phases.)
I think there is a distance of typically half a kilometer - and
sometimes much more - from transformers to the home.
Whether or not the transformer site is the place of
injection/reception, these points need to be relatively few - no more
than one for ever few streets. Otherwise you are running fibre all
over the place, which means it is no longer so cost-effective.
Even if the injection point was at the power pole outside the home,
it is still an horrendous thought to couple RF energy back and
forth to the home, without it radiating into space.
If there was no such thing as AM radios, and the ACA to police
radio-frequency interference, then you could pump all sorts of
frequencies up to a few megahertz into the power lines at up to
a few hundred metres from the home. Some of it would get to the
home, despite the huge inductance of the cables - which are a metre
or two apart - and virtually all of it would be radiated as
In fact, the power lines are such good antennas that with enough
energy at each end, communication would still occur if a tree brought
down the cables in between . . .
But none of this is possible unless you are prepared to live with
massive radio inteference from injecting high frequency signals onto
It is possible to use various encoding schemes, including
spread-spectrum, and ADSL's discrtete multitone, on powerlines. The
information carrying capacity is still determined by the
signal-to-noise ratio multiplied by the bandwidth. I don't see much
of either on a power-line, which is a very low impedance, very noisy
thing in itself.
At lower frequencies, say below the AM band at 500 kHz, there is less
chance of trouble with radio interference, but then the amount of
bandwidth you have is relatively small, and the difficulties in
filtering these frequencies from the massive 50Hz signal which must go
straight to the load, or from the step-down transformer, get harder at
the lower frequencies.
I am sure they can get some data down power lines. My argument is
that there is no way they can get even a small fraction of what is
possible with a purpose-designed system like HFC. HFC has to be done
really carefully, with a lot of attention to ingress and egress, and
in particular to be sheilded from the crap which emanates from power
lines. To suggest that the power-lines themselves can compete with
HFC is ridiculous. For some reason the idea of the garbage truck
doubling as a food and medicine delivery service comes to mind.
If you find any web-based technical descriptions of this stuff, I
would like to check it out. Until I can be convinced that a whole set
of seemingly insurmountable fundamental problems are overcome, I will
say that anyone who believes in the practicality of serious data
communications or telephony, beyond low data-rate things like reading
meters and security devices, is making a big mistake.
By all means prove me wrong on this - with physics and real technical
information. I wish it was possible.
| Robin Whittle Consumer advocacy in telecommunications, |
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