For the technical side of ADSL go here
http://www.iec.org/tutorials/by_tutorial_ascending.html

down near the bottom there is a report called
Spectral Compatibility... that will show that the effect of a large
number of ADSL ccts in a cable bundle has limited effect on
performance.

Cheers
Wayne

>>> "Daniel Omundsen" <d.omundsen@xtra.co.nz> 1/03/01 22:51:43 >>>

> :: it. I don't know the exact figures but I'd guess there must be
frequency
> :: components up to the 10Mhz range or more, and given how long
phone
lines
> :: are, they'd make rather nice antennas at those frequencies,
twisted
pair
> :: notwithstanding :) I sometimes wonder about the amount of
> :: crosstalk between
> :: close pairs that all have ADSL active - not so much a problem now
that
it
> :: is still few and far between, but perhaps a problem in the future
as it
> :: becomes more common.
>
> If I remember right, the frequency spectrum is from 32KHz to just
over
1MHz.
> The spectrum is split up into several bins (4KHz each) and the
equipment's
> supposed to be clever enough to drop bins that are in ranges with
> interference -- did I get that right, Daniel?

I don't know the exact frequencies, but I do know that there are
something
like 256 discrete carriers each of which takes part of the bit stream.
The
carriers start at frequencies just above voice band and go all the way
up
the the maximum usable frequency of your average several kms of copper.
Each
individual carrier is adjusted to compensate for noise in the cable,
attenuation, etc so that it carries the maximum amount of data possible
in
that frequency band. You have a couple of hundred modems stuck there in
that
box on your desk, each trying to go as fast as possible.

Trouble is that there is a whole bunch of other stuff that transmits
in
those same frequencies, such as AM radio and other ADSL lines. And
Simon is
right, a couple of kms of copper makes a pretty good antenna. The
modems
adjust for the specific interference levels in the copper when they
sync up,
and reduce their data rates on carriers that have noise. In general
the
longer the loop the more noise and attenuation at high frequencies,
that is
why the modem speed varies depending on how far you are from the
exchange.

My understanding is that ADSL can dynamically cope with gradual change
in
the characteristics of the copper. From what we are seeing, I suspect
the
modems aren't coping as well with rapid changes - if enough of the
carriers
are disrupted the modems decide to retrain.

Retraining is good, it optimises the modem for the new line
conditions.
Older high speed technologies just drop out until a contractor turns up
and
connects you to another cable pair.

> The ADSL technology used by Telecom is designed to withstand
environmental
> interference, IOW. Does an arc welder really drown out the whole
frequency
> spectrum used by ADSL?

I should have put a smiley after the arc welder comment. I would have
thought that the ADSL standards were designed with this in mind also.
But we
are seeing unexplained loss of synchronisation, so one possibility is
sudden
interference.

We have way way more more users on the network now than when we
started. And
more people are reporting this issue. This could mean the overall
noise
levels in the cables are rising. It could also mean that there is a
greater
pool of people able to observe and report the problem. Just that fact
that
we are talking about it on the list might be causing some people to
say
"yes, I have noticed that too" and report it.

Irrespective of what is causing it, we are trying to find out what we
can do
about it so we can fix it. Just bear with us.

> :: I would respectfully suggest that claiming the majority of users
use
> :: jetstream just to web browse, and therefore its not much of an
issue to
> :: them is flat wrong. At the moment Jetstream is expensive, and the
type
of
> :: people that are forking out for it are typically either businesses
or
> :: "power users" who are likely to be using it for VPN, ssh, and
other
> :: long-life connections just as much or more than web browsing.

> ::.I'm sure theres plenty of other people on this
> :: list in the same boat, and to us, micro-outages might as well be
> :: MAJOR-outages.
>
> Yes, I quite agree with the above. Jetstream isn't your typical
consumer
> grade product, aimed at casual Internet use. I use it the way Simon
> described, and would like to do so without these constant niggles.
>

Yes I totally agree with you that this is a very serious issue for
power
users. But might I respectfully suggest that the contributors to this
list
are not a representative sample of the overall Jetstream user base? We
are
propellorheads guys. There are many many more business and consumer
users
who use Jetstream just for fast web access than there are who need to
do
something clever like Internet VPNs. And the number of consultants who
set
up and remotely admin those VPNs is even smaller still.

There are over 10,000 lines out there now, and if I recall correctly
only
about 300 people on the list. If all 10,000 customers are spending
their
time sitting up to 3am remotely configuring IPSec on a FreeBSD machine
at
their mates house then NZ is a much sadder country that I thought :-)
:-)

I am not trying to undermine the seriousness of the issue, just putting
it
in perspective.

Would any of you guys prefer to go back to 64k DDS? Its definitely
very
reliable and well suited for mission critical office to office
connections.
But it is slow and will cost you.

Cheers,
Daniel.

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This message is part of the NZ ADSL mailing list.
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Received on Fri Mar 2 09:02:29 2001