> :: 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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Received on Thu Mar 1 22:47:17 2001