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Author Topic: Generic Ethernet Access Fibre to the Cabinet (GEA-FTTC) Service and Interface  (Read 10661 times)

Chrysalis

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I think they will be using adsl2 rather than adsl2+ capped at 12mbit, the 12mbit coincides with adsl2's own limit.

Adsl2 is better than adsl2+ on long lines, some say it doesnt matter, but it did on my ukonline line, when I synced with adsl2+ I had a lower sync speed and worse stability than with adsl2.
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kitz

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Quote
Adsl2 is better than adsl2+ on long lines , some say it doesnt matter,

I recall having a telephone conversation with Azzaka, many years ago (He'd rung after reading my theory on here & it was also partially a courtesy type call as I'd just got out of hospital so it must have been one of the times in 2008).  BT had just introduced ADSL2+ and we discussed the theory why adsl2 worked better on long lines.  Zen had customers with long lines just having been moved to adsl2+ that were performing quite poorly and they'd seen an increase in fault reports from these customers.   I still stick to the same theory I had back then, and have since repeated it countless times.

ADSL2 has improved modulation over ADSL1. It has much more efficient algorithms than ADSL1 in particular when it comes to overheads for error correction.  The RS codeword is far more efficient and is how '8Mb' was extended to '12Mb' over the same frequency band to 1.1 MHz.

ADSL2+ uses ADSL2 technology, but with one major difference - it doubled the amount of bins available for use and utilises frequencies up to 2.2MHz

Its highly unlikely any line over 50db atten (its probably closer to 45dB) is ever going to be able to make use of the additional frequencies, but making all these additional tones available seemed to cause more background type noise.*  despite the line not being able to make use of them.
So why not give the line the best of both worlds?  Better modulation & error correction algorithms of adsl2 yet not open the line to tones that they were never going to use. 

Anyhow back to the call with Azzaka and he said that as an experiment, Zen were trying adsl2 on these longer lines (rather than taking them back to adsl1 which is what the ISPs previously did) and it did indeed seem to be proving successful. 
afaik Zen were the first [BTw] ISP to try this, but since then Ive seen other ISPs follow suite and its a far more accepted these days that adsl2 may be best for some longer lines. 


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1. I don't profess to know exactly why this would be, other than it does appear to cause some sort of 'seepage' to other tones.  This similar theory is why complete tones may be blocked totally in the cases of say radio ham interference at the DSLAM or why certain rf filters are used. Blocking them completely is more effective than allowing them to remain in use and cause seepage to other frequencies.
2. We've all seen the increase in attenuation of appx 3dB that happens when a line is moved from adsl1 & adsl2 to adsl2+.  Ive seen some state this is the reason why.. but I'm not totally convinced about this being a major factor because atten is just a measurement and line conditions are still the same... plus there are more tones for loop loss to be calculated over.
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kitz

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Thanks wombat, this has turned out to be an interesting discussion.

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but the phrase "fall back" from Zyxel is interesting

Im not sure either as the details are scant, but the Calix mention of PTM to me is the key.
ADSL2+ will run over PTM. PTM also has a big advantage of far less (framing) overheads than ATM, so this will help throughput speeds a bit too.
   

There's no need for different modems either - take for example my Zyxel can be used on adsl1/2/2+ or VDSL from the one DSL socket and its not the only one that can do this Billion and TPlink to name a few. 

Yes theres currently 2 different (software) interfaces:-
   [1]An ATM interface for ADSL and
   [2] a PTM interface for VDSL.

But if you ran ADSL2+ over PTM then surely the router should be able to auto-sense VDSL/VDSL2/ADSL2+ in exactly the same way that a router can auto-sense ADSL1/ADSL2/ADSL2+ over ATM? 
Obviously there would still need to be a configuration change on the DSLAM but that happens anyhow now with adsl1/adsl2/adsl2+ etc.

Certain US ISPs are already upgrading their ATM networks to PTM (such as AT&T) and switching their standard ADSL users away from ATM and over to PTM framing.

With BT, on FTTC enabled exchanges a PTM network is already there. There's line cards available to support VDSL & ADSL2+. They'd perhaps have to rethink the PSDmasks but I really dont see that being too complicated.

The only fly in the ointment that I can think of, is LLU... and if it would cause the likes of Sky to spit out their dummies.  :baby: 
They are already unhappy about no direct access to the DSLAMs in the cab for VDSL so how much of a stink will they kick up when it comes to ADSL too?
There's already one line of thought that the LLU providers demands to provide access is what could be causing in part some delay to the roll out of vectoring.
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pluto

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...but making all these additional tones available seemed to cause more background type noise.*  despite the line not being able to make use of them....

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1. I don't profess to know exactly why this would be, other than it does appear to cause some sort of 'seepage' to other tones

This does make sense as the noise within a channel is more or less proportional to its bandwidth. Assuming the filtering to be correctly implemented, the extra bandwidth required for ADSL2+ entails the quid pro quo of higher noise figures. An ADSL2+ channel without the advantage of the additional ADSL2+ signal present would quite probably be less successful than the more bandwidth-restricted option of ADSL2 ordinaire.

Think of the obvious reductio ad absurdum: an early transatlantic cable carrying just one tone, detected by the human ear, could function adequately because most of the noise could be filtered out (or, in those days, the signal tuned-in – but much the same thing in practice).

"The wider you open the window, the more muck flies in"
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boost

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Im not sure either as the details are scant, but the Calix mention of PTM to me is the key.

I would guess any VDSL2 enabled kit will run PTM over xDSL? I think Adtran, for one, does it.

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They'd perhaps have to rethink the PSDmasks but I really dont see that being too complicated.

What's your thinking? I seem to remember reading the street side cabs were technically unregulated with regard to PSD masking; it's only the exchange which is 'regulated' if that's the right word?
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