Crosstalk speculation

Over on the Plusnet forums I've suggested that the 40 -> 55 package boost by BT might (and I've tried to be tentative) cause a noticeable increase in crosstalk affects across the country. Do you think it might? I'm not talking about a sudden drop in speeds across the boards. Merely that if someone were analysing sync speeds over time they might notice a slight 'bump' in the graphs as the package is rolled out.

It's just an idle curiosity/musing thought not an attempt to warn people of the possibility

It might, but then someone moving into the area with a short wave interfering device could also. The joy of mights.

Is that a real possibility? Increasing sync speed doesn't involve increasing the line power. All it does is increase the bit loading and eat into the SNR margin. I suppose the modulated waveform will be that bit more complex too, but would that cause a measurable level of increase cross-talk?

If it does, it is probably only likely to affect and be caused by those who currently have an achievable in excess of around 35Mbps.

The higher speeds will use the higher tones and maybe slightly change the bit loading on some of the tones just below. So, if anyone is to notice any effect it could be those on an 80Mbps service.

Your post is actually quite timely as I had been thinking about this over the past few days. I suffer with a major cross talk issue from on neighbour four houses away - when their service is OFF, I have an achievable and sync about 4Mbps higher than when it is ON. Then again in the past few days I have suffered a further 1Mbps drop. I have a feeling they may have recently moved to the 55Mbps option.

I'd not expect it to drop peoples sync speed (but it might for some on the up to 76 Mbps as might see more competition in their part of the spectrum).

Without spending a few months in a lab and lots of complex mathematics that may be shown to be wrong in real world, such is the complexity of the RF world impossible to say for sure.

In reply to a post by MrSaffron:

I'd not expect it to drop peoples sync speed (but it might for some on the up to 76 Mbps as might see more competition in their part of the spectrum).

Without spending a few months in a lab and lots of complex mathematics that may be shown to be wrong in real world, such is the complexity of the RF world impossible to say for sure.

Exactly ... as I suggest in my other post.

I thought that all the VDSL2 services used the same frequency range and what actually changed was the bit loading. A 40mbps link can still be putting bits into the highest frequency slots. I suppose it might be that on some medium length lines there was no bit loading in the top frequency bins at all and going to 55mbps might mean some so far unused bins now have some bit loading.

This strikes me as all a bit marginal. A few previously unused bins in the higher range due to the 40-55 move on some lines doesn't seem to me to be likely to be more than very marginal on its impact. I would have thought the visibility of any such impact will be swamped by the general increase in FTTC take-up.

In reply to a post by TheEulerID:

This strikes me as all a bit marginal. A few previously unused bins in the higher range due to the 40-55 move on some lines doesn't seem to me to be likely to be more than very marginal on its impact. I would have thought the visibility of any such impact will be swamped by the general increase in FTTC take-up.

You're probably right. Certainly taken over the longer term the effect is irrelevant. But in the short term it just struck me that there might be a blip in affects.

To be clear where I've said there might be some effect I am not saying that it will be apocalyptic, but given crosstalk is so unpredictable, unless you run massive processing power over it (how vectoring works) then hard to say.

Unused bins mean the frequency is not used, so using more bins can mean more power and spectrum used by a specific line. In theory yes take-up is likely to swamp the small change.

Its one of those scenarios where you cannot say zero effect and quantifying effect is difficult, i.e. people love 100% type answers but you cannot always given them.

In reply to a post by TheEulerID:

I thought that all the VDSL2 services used the same frequency range and what actually changed was the bit loading. A 40mbps link can still be putting bits into the highest frequency slots. I suppose it might be that on some medium length lines there was no bit loading in the top frequency bins at all and going to 55mbps might mean some so far unused bins now have some bit loading.

For downstream:
When the modem has tones available for use on, say, a service capped at 40Mbps but the line is capable of an awful lot more, I'm pretty sure the filling algorithm will choose to load all tones to a low bit-loading, rather than highly-loading some and leaving others empty.

That means any line that gains from 40Mbps to 55Mbps will do so by extra bitloading on tones that are already transmitting. In digital terms, this gives the receiver a harder job (it has to distinguish between more/closer amplitude and phase options). But in analogue terms, the transmitted signal will still be of the same order: still a modified sine wave, still the same power.

I think the end result will be similar crosstalk.

I've just checked a couple of lines on MDWS, which are currently 40Mbps but have capabilities of 80Mbps. They all use the full set of tones, but a lot of D3 only uses 2 bits/tone.

For Upstream:
The filling algorithm gets affected by upstream power back off, so U1 can get left with unloaded tones, while tones in U2 are more heavily used.

If upstream speed caps were being increased, then you might well see more crosstalk there.

From my experience on other similar modulation schemes they do tend to fully load the tones - it probably loads the processor less in that, for example, there is only has one fully loaded tone to demod rather than two half loaded.

I think the bit loading algorithm will just try and equalise the SNR margin across the frequency range. Inevitably that means the higher frequency tones will have lower loadings. However, I wonder if there are any lines actually synced at 40mbps which fail to use the highest frequency tones? Clearly any lines which sync at less that 40mbps will not suddenly start using higher tones for the 55mbps service. I suspect that any lines actually able to hit 40mbps are already using those tones.

I think it is probably a little more subtle than that, but my reading is that this isn't going to be a real issue.

I've looked at bit loading patterns on VDSL & ADSL modems (where you can get the stats - there are also plenty online as well) and I've yet to see a single one that works as you've described. That is fully load bins to the margin limit and leave others unused. It would be a distinctly sub-optimal thing to do in that equalising the SNR margin on each bin will make for a more resilient connection. Also, the argument that it saves processing cycles is surely spurious. These chipsets are designed to work at the highest speed that the VDSL2 supports, and it's all done using dedicated DSP (digital signal processing) circuits, not using a general purpose CPU (which wouldn't stand a chance of dealing with the task). I doubt it makes the slightest difference to the efficiency of the circuits where all the heavy lifting is done whether all the bins are used or not.

This set of graphs is typical

http://vdsl2.altervista.org/

no affect, the signal is still pumped out at the same power.

Bit loading isnt the cause of crosstalk.

In reply to a post by MHC:

From my experience on other similar modulation schemes they do tend to fully load the tones - it probably loads the processor less in that, for example, there is only has one fully loaded tone to demod rather than two half loaded.

I think there is value to be gained from doing the opposite. A policy of deliberately under-using as many tones as possible, you make the whole communication channel less susceptible to noise. Every under-used tone is easier for the receiver to identify in the QAM constellation.

If the modem reported to me that actual SNRM was 12dB, I think I'd prefer that the "extra" 6dB came from every tone, rather than as 0dB (no extra) on half the tones and 12 dB on the other half (because they were unused).

I suspect that "processor load" is a strange quantity, as the part affected most will be a DSP, which will be designed for the heavy parallel load of 4000+ tones, unlike a CPU, with one (or just a few) threads.

In reply to a post by TheEulerID:

I think the bit loading algorithm will just try and equalise the SNR margin across the frequency range.

It makes sense for that to happen, yes.

In reply to a post by TheEulerID:

However, I wonder if there are any lines actually synced at 40mbps which fail to use the highest frequency tones?

Yes, I could find some lines like this. The analysis I did on some lines suggested that, even if the modem were given the extra 15Mbps of speed, it still wouldn't use the tones that weren't in use.

However, if what you suggest in the first part of the quote (about SNRM balancing) is true, then there probably *are* some lines which would have used some extra tones.

I'll have to check some more later

I think it is probably a little more subtle than that, but my reading is that this isn't going to be a real issue.

That's my thinking, currently.

I suspect the number of lines that have the potential to sync over 40mbps yet have no bit loading on the highest tones is, if it exists at all, a very small number. Given the way the SNR tails off with frequency is fairly predictable for a well behaved line it might be able to model it.

That said, given the way the ANFP changes the PSD according to the cabinet's distance (electrically speaking) from the exchange, this might not be so simple.

In any event, I'm gradually convincing myself that the 40-55mbps upgrade on BT Infinity 1 lines is not going to have any measurable impact (although I await the inevitable postings from people saying they are seeing such an impact through a loss of speed on their particular non-BT consumer service).

Shouldnt make hardly any difference - DMT uses whats called a waterfill algorithm for bitloading.
Waterfill means that it does not fill all the lower tones before moving on to the next. It means it loads all available tones with 1 bit, before starting over the whole frequency to fill up with 2 bits and so on and so on.

Therefore its highly unlikely that the EU will be utilising any new frequencies that they couldn't previously use. They will just load more bits per bin across the frequencies available up to the maximum bits that tone can load depending upon the SNR.

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The waterfill method is much clearer to see in on an adsl line which isnt subject to as much PCB and differing PSD masks that VDSL is. A prime example is here

This line is capable of much higher speeds than the technology limitations. See how none of the tones get full bit loading (despite there being sufficient SNR to fully load a bin). There's sufficient bitloading at the higher end frequencies to be able to allow the line to sync at full speed without the need for the lower tones to need to load the full 15 bits.

If DMT didnt use the waterfill method, then that line would show fully loaded 15bits for the lower tones and no loading of the higher frequencies.

The inverted U shaping up to tone 50 is the result of PSD mask.