[AusNOG] V.Fast and NBN VDSL coexistence

[John Edwards]

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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