[AusNOG] G.Fast and NBN VDSL coexistence

John Edwards jaedwards at gmail.com
Fri Nov 18 17:32:20 AEDT 2022

I think it will be fine.

In the scenario where a building has 3 xDSL providers I think they will be
more worried about being commercially viable than what happens in the
matrix maths of an underlying layer 1 protocol.

If there's enough user density in a modern apartment building to cause
crosstalk that can overwhelm cat5e cabling, then the residents already have
a bigger problem with WiFi interference which will be blamed for any packet

Vectoring competing for the same DMT bins will manifest itself as line
errors, which most consumer devices don't even measure. The DSLAM profile
will almost certainly have forward error correction turned on which will
smooth out crosstalk bumps and TCP will correct the rest. The entire
building's speeds will slow a few mbps, but ISP tech support will have a
script saying that this is normal for a service with "up to" 300mbps so
they won't send out a tech to investigate unless it's under 12mbps.

The G.Fast users with vectoring competition will still get a more
consistent high speed service than the apartments who chose a fixed 5G
broadband product.

At the end of the day, G.fast will open up an additional 65Mhz of
point-to-point spectrum, which has to be a net positive.


On Fri, 18 Nov 2022 at 13:36, David Hughes <david at hughes.id> wrote:

> Hi John
> So I'm guess if Carrier ABC turns up and installs G.Fast into a building
> it could mask out the VDSL frequencies and they may happily co-exist with
> NBN services.  But, if down the track Carrier XYZ wants to sell G.Fast into
> that same building they're stuffed because you couldn't have more than 1 x
> G.Fast DSLAM on the same bundle.
> David
> ...
> On 18 Nov 2022, at 12:31 pm, John Edwards <jaedwards at gmail.com> wrote:
> Hi David,
> CommsAlliance G658 is the document for this, here's one that does mention
> G.Fast:
> https://www.commsalliance.com.au/__data/assets/pdf_file/0014/62321/C658_1_2019.pdf
> The working group for this is CommsAlliance W93 which mentions an
> expectation that it will align with ITU-T G.9700.
> My take, based on some dusty knowledge of VDSL2 vs ADSL2, rather than any
> recent field experience:
> Because G.Fast uses a very different spacing for its subcarriers, I
> imagine that it can't easily interoperate with VDSL2 vectoring.
> The smaller carrier size of VDSL2 is going to make it less susceptible to
> errors from interference, the same way old 20Mhz WiFi beats 40Mhz and 80Mhz
> channels on newer hardware.
> VDSL2 goes up to 35Mhz, whereas G.Fast can use 106MHZ of cable bandwidth,
> so I suspect it will only be able to fully realise the vectoring benefit
> for the spectrum above 35Mhz. Below that the DSLAM is going to have to cut
> 48Khz chunks (about 400kbps) out of VDSL2 in order to give right-of-way to
> G.fast.
> This might be a viable strategy if VDSL2 is only used to deliver "up to"
> 100mbps, but the current power-spectral-density masking approach doesn't
> specifically allow for this.
> My understanding of top-level Vectoring is that it only works if all of
> the services in the bundle are connected to the same DSLAM anyway, which
> rules out multiple carriers on the same bundle. I believe it would be
> possible to use lower-level vectoring to "bully" more compliant systems out
> of the way, but good luck getting your vendor to add that as a feature.
> John
> On Fri, 18 Nov 2022 at 10:52, <david at hughes.id> wrote:
>> Morning all
>> What's the current situation with coexistence of V.Fast on the same cable
>> bundle as NBN provisioned VDSL2?  Is there still a problem with VDSL
>> Vectoring  and V.Fast's "crosstalk cancellation" stomping on each other?
>> What about multiple V.Fast installations (i.e. multiple carriers with
>> G.Fast kit)  in addition to VDSL signalling from an NBN node or FTTB?  Any
>> insights would be appreciated.
>> Thanks
>> David
>> ...
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