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Author Topic: VDSL - tones used in each band & why  (Read 1279 times)

kitz

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Re: VDSL - tones used in each band & why
« Reply #15 on: December 23, 2017, 02:37:42 AM »

You can choose to ignore specific tones with said Asus devices. There's not much you can't override.

Which unfortunately then degrades neighbouring lines.    >:(
There's a damn good reason why things like spectral shaping & power management is so important. Its like someone turning up their radio, so the neighbour cant hear their own.   

In a way I can perhaps understand people who know what they are doing wanting to tweak target SNRM if the line is stable. Nor can I see too much harm in being able to ignore a specific tone - again as long as the EU knows what they are doing - as there may be cases where that could be useful if you live in an area that's getting RFI at a particular frequency.   
But I don't think being able to mess with power or changing band plans on EU whim is a good idea at all.  :no:
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Chrysalis

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Re: VDSL - tones used in each band & why
« Reply #16 on: December 23, 2017, 06:45:23 AM »

yeah I know its always been used, I remember in the adsl max days short lines not fully loading up the lower tones :), and yes the waterfill vs waterfall would probably not have much different results on lines unable to reach full sync, the increased ES would be just an impact on lines hitting the sync cap with a lot of spare margin.  But those lines would also be the most likely to be stable anyway so probably considered ok for them to run non optimally to help weaker lines.

I will read it when I get the time and motivation. :)

Now there is something the modem could do to perhaps over-ride the system.  What if it messed with the BER/bit allocation, could it have a side effect on the SNRM?    I don't know enough about the modem for it to give me much more than a pause for thought and make me go hmmm.

As I don't know much about the modem aspect when it comes to manufacturers specifications etc -  ejs is more into that and he may know a bit more.

   

I think it works something like this kitz.

The modem at the receiving end controls the actual bit allocation target SNRM etc. however the DSLAM can issue a request (instructions) on how the modem should behave, I expect in recent years modem vendors have (mostly) all cooperated and made the modem simply honour what the DSLAM requests, but for whatever reason the ASUS chipset vendor decided to not do this.  Of course the likes of broadcom make chipsets for both ends of the connection so its easier for them to make sure it all works in tandem.
« Last Edit: December 23, 2017, 07:02:51 AM by Chrysalis »
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kitz

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Re: VDSL - tones used in each band & why
« Reply #17 on: December 28, 2017, 08:51:24 PM »

As mentioned in another post, I started to respond to this a few days ago and had to stop.  I was going to attempt to look up some links to post but never got that far...  so rather than completely abandon the post, I'll just paste where I'd got up to before xmas, so here it is warts and all

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Quote
I think it works something like this kitz.


Yes I know that bit and standard DMT stuff which I'm au-fait with..  but I was referring to the more complex workings of PHY management, which is something seldom ever discussed, but is essential for DLM (& DSM) to work.   PHY is part of the modem firmware and thus something I don't know too much about, but I think ejs dabbles in this area when hacking firmware, which is why I said he may know more about what ASUS have done.

The ATU-R expects to receive certain configuration parameters from the DSLAM prior to initialisation be able to achieve a successful sync.
It's not anything to do with specific (BCM) chipsets, but is a defined ITU-T standard in its own right.
G.997.1 (Physical layer management for digital subscriber line transceivers/ PHY management) is quite an interesting document if you have time to sit down and digest it all.   

I attempted to several years ago purely because G.997.1 was central to the BT-v-ASSIA courtcase and BT/Openreach RAMBO boxes' use of something called ploam.   Ploam is what is referred to as the Q interface in G.997.1 

I'm not 100% certain, but I think (or at least thought)* it was impossible to over-ride some of these settings, which is why I always wondered how ASUS modems were managing to do what they do on the Openreach system.
 
However I believe it's possible that the modem could adjust GAIN and bitloading during show-time (ie after the modem has sync'd). It has to be able to do this because thats in part what bitswap process is about.  So my pause for thought moment yesterday, was that these modems could be messing with BER/GAIN/bitload perhaps during Showtime.  We know that part of the bitswap process, the modem can increase GAIN for specific tones.   Now suppose for example you forceably increased GAIN over the full range of tones, which would increase the dB power,  which in turn increases SNRM.


*Back to the 2 different types of DLM management I was talking about yesterday which I still cant recall the names of.   
Type 1 uses a target SNRm as a parameter, so it could be possible to get the modem to adjust the SNRM.   20CN/21CN uses this type of system.
Type 2 uses capping/banding rather than target SNRm as its primary parameter.  NGA uses this type of system and thus a straight forward over-ride of Target SNRM should be impossible on a DSM based system.

_______

As a side note, the courts threw out any breach of patent claims on the type 1 system.  ASSIA won for type 2, but from what I read it was more to do with the ILQ aspect. ILQ being the DLM status colour which I long ago explained and we knew about before the ASSIA couft case. 
Reading between the lines ASSIA's successful claim was based on the fact the NGA system monitored some data via RAMBO to get information for ILQ to decide when to change DLM parameters.   
 - ASSIA systems also use Type 2 DLM and DSM
 - Courts dismissed claims of patient breach on ILQ method used on BT's Type 1 DLM, because it was general practice
 - Since the court case Openreach DLM for removal of banding/capping is borked, yet ILQ continues to work for INP.


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GaryW

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Re: VDSL - tones used in each band & why
« Reply #18 on: December 29, 2017, 12:12:14 PM »

Just noticed this. Not looked at your stats.. but aren't those particular tones the usual guard bands or at least near to?  DSL uses stop bands between the different sub-channels.   Its not unusual for the DSLAM to add guard bands either side of the stop band.   Different DSLAM manufacturers may use slightly different guard and band plans.

Whilst the defined stop band between U0 and D1 may be tone 32,  all DSLAMs add further guard bands to either side of the stop band to prevent cross-talk between up and downstream.   

That's the weird thing, they aren't that close to the guard bands, certainly at the high end.  Looking at other people on ECI cabinets, their DS1 starts in the 40s rather than the high 60s and runs through to the upper limit of 857 rather than stopping at 785 as I'm seeing.  A good comparison (as we're both on 15k sync speeds) is probably scarab.
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