[Premium-Rx] Power Line Carrier, Last Mile and xDSL

[Carcia, Frank A. HS]

George,
You guys are doing it right. The clowns here want to run the signal down a
single bare conductor. Next we will be running sewage down a open trench 1/2
the day and
fresh water the other half.    fc

-----Original Message-----
From: George Georgevits [mailto:georgg at bigpond.net.au]
Sent: Tuesday, August 19, 2003 5:24 PM
To: Premium Rx
Subject: [Premium-Rx] Power Line Carrier, Last Mile and xDSL


Guys,

I am a practicing amateur here in Australia. I am also a self-employed
communications engineer involved in this field. I work mainly in the test
and measurement area and have been involved extensively with the design and
testing of connection hardware and cables for high speed balanced line
applications, including "LOW END" applications such as xDSL. I say LOW END
because most of my work involves components used for high speed LAN
applications where bandwidths (and hence spectrum) reaches much higher
(typically I test to 250MHz). I am not all that familiar with Power Line
Carrier systems, but from what I have read, I tend to agree that it sounds
like a really dumb idea! No doubt if it becomes popular in the US, we will
be lucky enough to get it here too.

At this stage, I limit myself to xDSL applications for the following
comments. Firstly, the new ADSL2plus standard utilises spectrum a little
further than someone earlier had commented. It actually reaches to about
4.5MHz, and if anyone is really interested, I have the graph from the ITU
standard to prove it.

Regarding xDSL applications running on twisted pair in the street, I
initially had similar concerns to those some of you have raised in your
recent postings. I am note sure what you mean by Last Mile applications, but
I think they would fall into this category too.

Please note the following:

ADSL and other forms of DSL services are and have for some time been in use
extensively here in Australia. More particularly, VDSL has been in use
extensively on aerial cable in the Canberra region, and I have not heard of
a single complaint about interference from amateurs living there. As a
matter of interest, VDSL uses spectrum to 30MHz and the last 300 metres in
Canberra is delivered by aerial twisted pair Cat 5 cable.

These twisted pair technologies rely on cable pair balance to keep radiation
very low. Pair balance is typically 40dB or better at the frequencies in
use. It is only the common mode signal which can radiate, and in order to be
an effective radiator, the cable needs to be a significant fraction of a
wavelength long. What's more, if there was significant radiation, the first
thing it would interfere with is the adjacent pairs in the same cable
sheath. Such interference is known as crosstalk and it is one of the main
limiting factors for the use of such services in twisted pair cables.

Also, street cables are all fitted with either a grounded foil screen or (in
the case of older cables) a lead sheath. Although such screening serves
primarily as a moisture barrier, it also acts as an effective screen,
reducing any radiation out of the cable bundle significantly. The power
levels in cables containing these services sit typically around 0dBm or
lower. By the time you take into account the cable pair balance (40dB) and
the effectiveness of the cable screen (has to be at least another 20-30dB),
its no wonder there are no complaints about interference to HF.

I just thought the above might be useful for directing what to and what not
to worry about.

Regards,
George Georgevits
Power and Digital Instruments Pty Ltd
Sunny Sydney, Oz


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