“The
fact of the matter is that the shorter the copper loop, the higher
the speed you will get. There is a new technology, a noise
cancellation technology called vectoring, which is now being
deployed, which will literally double the speed. So if you had a line
which was delivering 50 megabits per second now, with vectoring,
which is just a software solution, it's not, you know, it's not
expensive to deploy, you can then double that to 100 megabits per
second” – Malcolm Turnbull to ABC Radio National
(http://www.abc.net.au/radionational/programs/breakfast/malcolm-turnbull-on-changes-to-the-nbn/4557604
here).
“This is the folly of stipulating one
particular technology at the outset. FTTN is a much more powerful,
much cheaper, much better prospect now than it was four years ago.”
Damn: I have to try to look at
vectoring in a bit more detail, because if you don't understand it,
you won't see the pea under the thimble.
First, the line-by-line, then the
technology.
“The shorter the copper loop, the
higher the speed you will get” – Correct.
“There is a new technology, a
noise cancellation technology called vectoring, which is now being
deployed, which will literally double the speed. So if you had a line
which was delivering 50 megabits per second now, with vectoring,
which is just a software solution, it's not, you know, it's not
expensive to deploy, you can then double that to 100 megabits per
second” – here, Mr Turnbull
gets things a little bit scrambled.
It's
probably not his fault. The industry PR has done a neat job of
wrapping vectoring in a superhero's cape.
“Will literally double the speed”
– Vectoring will improve the performance to individual subscribers,
but the results they see in the real world are subject to a lot of
dependencies. "Can" double the speed would be more accurate.
“Just a software solution, not
expensive to deploy” – Only
if the equipment in place supports it. If you've rolled out a VDSL2
network, and it's new enough, then that might be true. You'll note in
the example I refer to below, Austria's A1 Telekom, the upgrade
involved hardware as well as software, which is much more espensive
than a software upgrade.
Tl;dr
starts here
I
realise that there are people queued up ready to fire their “ALP
stooge” guns at me. So here is a much more detailed explanation of
my position.
Here is an excellent – and not too hard-to-understand –
presentation from Ericsson. For the Tl;dr crowd, the main point is
that vectoring exists to take care of a particular kind of noise,
far-end crosstalk or FEXT.
If Bob and Alice are both DSL customers
– any kind of DSL will do – and they're served through the same
cable bundle, then each of them appears as “noise” to the other.
That is, Bob's signal will couple to Alice's twisted pair, and since
it's not Alice's signal, it's noise, and reduces the capacity
available to Alice for communication.
The same thing, of course, happens to
Bob: Alice's crosstalk cuts into his capacity.
However, the noise Alice generates is
different to random electrical noise: it has some predictable
characteristics, because you know it's a DSL signal that's generating
the noise. That means you know (for example) which frequency bands
will suffer noise, and that patterns that it will exhibit.
And that means – with good
mathematics (if you want employment in mathematics, network physical
layer work is a good place to start) – you can get rid of Bob's
noise on Alice's line, and vice-versa. This is vectoring.
Bob, Alice, and World+Dog
If there were only Bob and Alice to
worry about, vectoring wouldn't be a “new thing”. But every house
connected through a twisted pair is connected in a bundle of cables.
In a bundle of 48 cables, this would be unmanageable even with
vectoring, if every cable coupled its crosstalk to every other cable. Fortunately, that doesn't happen. Lines
will couple more strongly if they're close together. More
importantly, some lines – for example, where copper degradation or
damage spoils the line's impedance – will leak more noise than
others.
Ideally, all crosstalk should be
cancelled, but Ericsson's white paper notes that “close to optimal”
performance can be achieved by concentrating on eliminating the noise
from “dominant disturbers”.
And in the case of vectoring, “optimal
performance” means the speed VDSL2 can achieve on a single line.
“Vectoring” doesn't change the maximum
speed available on VDSL2 – it lets more users get closer to that
maximum speed in a crowded cable bundle.
In a
presentation to the 2013 Broadband World Forum, Gerald Clerckz of A1
Telekom Austria gave a presentation outlining that carrier's
experience with vectoring in its field trial in Korneuburg.
The
implementation involved:
- upgrading DSLAM line cards
- upgrading the DSLAM software
- upgrading modem firmware to support vectoring
- replacing modems that could not support vectoring.
A1
Telekom is pleased with vectoring, saying in the presentation that
vectoring is “fulfilling the expectations” while also noting that
“it cannot be seen as a replacement for FTTH”, but rather as a
bridging technology on the way.
With
that noted, there is a very wide spread of
real-world outcomes achieved in the field trial. At between 250 and
300 meters loop length, customers on the trial experienced download
speeds from around 120 Mbps down to around 35 Mbps; customers at
above 400 meters experienced speeds between about 22 Mbps up to 75
Mbps. The poor customer at 600 meters was only able to get around 30
Mbps.
I don't have permission to reproduce the original chart, but here are the extremes at different distances:
This, remember, is a real-world deployment, not a vendor's claim - and it's a set of data from a carrier that's happy with vectoring. It's a statistically valid sample, as well, since the test covered 500 homes, more than ten percent of the service area of the test.
What's clear, here, is that while vectoring can double speeds, there isn't much difference in the top-to-bottom envelope - and that diference evaporated at 600 meters (however, that may represent fewer data points). At 300 meters, the difference in best-case performance was 33 percent.
