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[William Grimsley]

Hi,

Does anyone know what a typical maximum data rate drop per FTTC connection is? Is it about 200 - 500 kbps?

Thanks.

[roseway]

What do you mean? Are you talking about crosstalk?

[Chrysalis]

if you mean crosstalk over the life of the connection it can be a whole lot more than 500kbps, but like eric said you need to explain what you mean.

[William Grimsley]

Yes, sorry I am talking about crosstalk.

[Chrysalis]

only BT will have the figures for their copper network, but in published trial results, average speed drop can be measured in 10s of mbit/sec rather than a few hundred kbit/sec.

[William Grimsley]

only BT will have the figures for their copper network, but in published trial results, average speed drop can be measured in 10s of mbit/sec rather than a few hundred kbit/sec.

For one customer? If that was for one customer, I'd be back on ADSL speeds by now!

[WWWombat]

There's a document from the Broadband Forum (that puts together standards for how DSL systems work) that describes vectoring.

Inside, there's a great graph that shows just how bad crosstalk can get, in the absolute worst case. And how good things can be if vectoring is implemented.

The downside is that it doesn't show FTTC in quite the same way it works in the UK - it doesn't include the power masking we use to keep ADSL working, and it is based on 0.4mm copper, where we are more likely to be on 0.5mm. However, it gives a good overview...

https://www.broadband-forum.org/marketing/download/mktgdocs/MR-257.pdf

Figures 6 and 7 are most appropriate.

I can't explain it in more detail right now, but ask questions for when I'm back again...

only BT will have the figures for their copper network, but in published trial results, average speed drop can be measured in 10s of mbit/sec rather than a few hundred kbit/sec.

For one customer? If that was for one customer, I'd be back on ADSL speeds by now!

In crosstalk, the pairs that run closest to yours are always going to be the biggest disturber. If those don't carry VDSL, then you'll see little. If they do, you'll see a lot. Other pairs sitting a few mm further away in the bundle will affect your line less.

You can find heatmaps from lab results, showing a 2D map of how line X affects line Y.

[Chrysalis]

only BT will have the figures for their copper network, but in published trial results, average speed drop can be measured in 10s of mbit/sec rather than a few hundred kbit/sec.

For one customer? If that was for one customer, I'd be back on ADSL speeds by now!

yeah but vdsl still tonks adsl.

would rather have a 110mbit line down to 70mbit from crosstalk than a 6mbit line

[Chrysalis]

There's a document from the Broadband Forum (that puts together standards for how DSL systems work) that describes vectoring.

Inside, there's a great graph that shows just how bad crosstalk can get, in the absolute worst case. And how good things can be if vectoring is implemented.

The downside is that it doesn't show FTTC in quite the same way it works in the UK - it doesn't include the power masking we use to keep ADSL working, and it is based on 0.4mm copper, where we are more likely to be on 0.5mm. However, it gives a good overview...

https://www.broadband-forum.org/marketing/download/mktgdocs/MR-257.pdf

Figures 6 and 7 are most appropriate.

I can't explain it in more detail right now, but ask questions for when I'm back again...

only BT will have the figures for their copper network, but in published trial results, average speed drop can be measured in 10s of mbit/sec rather than a few hundred kbit/sec.

For one customer? If that was for one customer, I'd be back on ADSL speeds by now!

In crosstalk, the pairs that run closest to yours are always going to be the biggest disturber. If those don't carry VDSL, then you'll see little. If they do, you'll see a lot. Other pairs sitting a few mm further away in the bundle will affect your line less.

You can find heatmaps from lab results, showing a 2D map of how line X affects line Y.

at the same time was no ali tho which the uk has a lot off,  leics is a mix of sub 0.3 copper and ali

[William Grimsley]

There's a document from the Broadband Forum (that puts together standards for how DSL systems work) that describes vectoring.

Inside, there's a great graph that shows just how bad crosstalk can get, in the absolute worst case. And how good things can be if vectoring is implemented.

The downside is that it doesn't show FTTC in quite the same way it works in the UK - it doesn't include the power masking we use to keep ADSL working, and it is based on 0.4mm copper, where we are more likely to be on 0.5mm. However, it gives a good overview...

https://www.broadband-forum.org/marketing/download/mktgdocs/MR-257.pdf

Figures 6 and 7 are most appropriate.

I can't explain it in more detail right now, but ask questions for when I'm back again...

only BT will have the figures for their copper network, but in published trial results, average speed drop can be measured in 10s of mbit/sec rather than a few hundred kbit/sec.

For one customer? If that was for one customer, I'd be back on ADSL speeds by now!

In crosstalk, the pairs that run closest to yours are always going to be the biggest disturber. If those don't carry VDSL, then you'll see little. If they do, you'll see a lot. Other pairs sitting a few mm further away in the bundle will affect your line less.

You can find heatmaps from lab results, showing a 2D map of how line X affects line Y.

That's really interesting! Just shows that it all depends on the type of connection that other pairs are running on which makes a lot of sense as if they're running VDSL like I am then I'm going to see the biggest drop in speed.

I didn't see a graph showing how bad crosstalk can get. Is that in the linked document or is that somewhere else?

[WWWombat]

Yes - figure 6 in that document.

Let me see if I can annotate it a bit...

[ejs]

There is a newer edition of that document:
https://www.broadband-forum.org/marketing/download/mktgdocs/MR-257_Issue-2.pdf

[WWWombat]

Here's a copy of the figure, annotated.

Remember that it isn't very indicative of UK behaviour - but is indicative of a flavour of VDSL2 somewhere, and gives a reasonable picture of how badly crosstalk *can* impact things.

The figure depicts 80 VDSL2 users in a cable with 100 pairs. That is very high usage.

The upper blue triangles represent perfect behaviour. You might see this if you were the only VDSL line on the cab.
The red circles represent the speeds that could be achieved for vectored lines.
The blue x-shaped crosses represent the range of speeds for the non-vectored lines.
The lowest blue triangle represent a theoretical worst-case speed.

In The UK, we run without vectoring, so could expect speeds to be somewhat like the blue x-shaped crosses.
Of course, take-up probably hasn't reached 80% on many cables yet, so things might not be as bad as that.

However, the "tie pairs" that carry VDSL2 signal back from the FTTC cabinet into the PCP, before distribution around the neighbourhood *will* end up completely full of VDSL2 users. We will see full crosstalk there - but thankfully only for a relatively short distance.

I've added 2 explanations to the figure
- Blue text for behaviour at 200m
- Purple text for behaviour at 500m

[WWWombat]

Thanks @ejs. I'll take a look later.

[ejs]

I think that particular diagram is a strange one, because it's the performance with some of the lines using vectoring, and some not, at the same time.