With the recent release of the nbn corporate plan, we’ve learnt that nbn is starting to indicate ‘potential’ upgrade paths for the list of technologies they’re rolling out.
The particular technology that I want to cover today is FTTN (Fibre To The Node).
I tweeted this morning after reading some initial pages of the corporate report, on how nbn is planning to deliver any meaningful upgrade to those who actually need it, when talking about the upgrade here, I must mention that the lucky (Those who’s nodes are within ~500m of their home) will be covered by some sort of potential upgrade in the future the users I want to focus on are those who have a longer line length anything > 500m
We know that nbn is working with vendors in regards to two technologies that affect copper based xDSL services, these are
When digging into the technical papers and statements released regarding these technologies I find it hard to see any actual evidence that we can upgrade FTTN speeds across long lines, none of these technologies are being designed for longer copper loops.
We know that some users are on exponentially long lines that can range upwards of 1,000m*
Reading the press releases from Nokia in particular we see that
>200 m: In deployments with longer loops, VDSL2 vectoring is, and will remain, the technology of choice. G.fast is simply not optimized for these loop lengths. The G.fast standard targets 150 Mb/s aggregate speed at 250 m on a 0.5 mm line. VDSL2 vectoring can deliver 140 Mb/s to 150 Mb/s aggregate speed at up to 400 m.
The line is in the sand, the vendor is saying that >200m+ G.FAST is simply not optimized for.
This also brings other issues when looking at how the nbn works today, as most of you know when the nbn is activated it runs in a ‘co-existance’ period which means that the nbn runs VDSL2 at a lower frequency to ensure that ADSL services are not affected.
If nbn plan to use G.FAST, nbn will be pushing frequencies upto 212MHz, currently VDSL2 runs at upto 30MHz if I am correct, this would mean that VDSL2 (17a) would suffer some sort of interference when G.FAST is running alongside it.
I quoted this as the law of diminishing returns, lets be frank what these technologies do is push up the frequency that the data is transmitted through the copper, because of this basic physics tells us that faster frequency has to equal less length for this kind of plan to work.
There is a way to fix this issue, which is getting the fibre closer to the premises, we know that the problem with fibre deployments is that usually the last mile deployment is expensive & slow, (although results around the world are starting to challenge this assumption)
So the question is how do we get the fibre closer to the home so that we have the least amount copper as possible, unfortunately this problem cant be easily fixed now that so much of the network has already been rolled out, nbn already has designed the way they want to rollout their network. If we look at vendor recommendations we can see that the reality of this technology is getting it so close to the home that we can run the copper at these extremely high frequencies.
So we can pretty much ruleout the method of G.FAST for those who are on long line lengths unless nbn is prepared to rollout closer to homes, and for existing homes due to the way that nbn seems to have designed the network its going to be costly to extend the fibre as unfortunately we cant use a node as a sort of ‘passthrough’ that we can run fibre from the fibre will have to run from an nbn TFAN.
This means increased costs to upgrade from the node to get closer to homes, this is in my opinion a bad design choice, it should have been designed to allow some sort of passthrough which would mean that FOD would eventually be cheaper and that upgrades wouldn’t mean some sort of new node instead of potentially ripping out nodes.
The news isnt much better for XG.FAST either, in reality the same basic physics issue exists, to get more speed out of copper we need to run on higher frequencies this eventually means that line length becomes a huge issue in determining the success or viability of any upgrade.
nbn’s own documents start to reveal the eventual problem, the line length is the issue especially when copper is involved**
So the point here is that nbn cannot reasonably say they have an upgrade path for the entire FTTN network that doesnt involve more costs due to the way the network has been designed.
The eventuallity here is the basic statement that higher frequencies = less length.
The problem I see arising is that users who are on long line lengths aren’t included in these upgrade paths not unless they’re willing to pay for FOD.
*: Link here
**: Link here
Update: 2/09/2017: I have updated the frequency / bandwidth that VDSL2 runs at after reading the latest vendor documents it does seem VDSL2 can now run upto 30MHz. Thanks to /u/tektektektektek for pointing this out to me.