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[XRaySpeX]

It is well known that when switching from G.DMT to ADSL2+ that the reported attenuation often increases by about +3dB, but, obviously, the line length is unchanged. This is due, I understand, to the different methods of calculating attenuation between the 2 ADSL modes.

So, which attenuation, that of G.DMT or of ADSL2+, should you input to the speed calculator to obtain the most realistic ADSL2+ speed estimate?

If you were currently running on G.DMT and you wanted to estimate what your speed would be on ADSL2+:

• If you were to input your current G.DMT attenuation, you would get an ADSL2+ estimate that you will probably find, when you eventually switch to ADSL2+, does not tally with your eventual ADSL2+ attenuation.
• If you were to input your current G.DMT attenuation +3dB to allow for the expected increased ADSL2+ attenuation, you would get an slower ADSL2+ estimate but your line length would have magically increased. Clearly a nonsense!

[kitz]

If you read the notes underneath it should hopefully explain all your points.

As mentioned it in the notes it should only be used as a guide because all factors can't ever be taken into account even different routers on the same line can display different attenuation figures. 

[XRaySpeX]

Yes, I did read the notes but am not entirely clear.

Your general note 'The figures used are based on attenuation measured at 300Khz for adsl1' seems to imply that everything is based on the G.DMT attenuation.

Yes, I know it is only a guide, but I am only asking which is more likely to give the closer estimate. I would be grateful for a specific answer.

[Black Sheep]

Rather than bugger about trying to guess which attenuation changes may or may not apply, why not just visit a few ISP sites (BT, SKY, PlusNet etc), and follow the links to perform a speedtest. Most higher-end sites, will guesstimate what you are likely to get on all products available .......... ADSL1, ADSL2+ and/or VDSL.

[XRaySpeX]

Cuz a speedtest doesn't measure sync speed!

Let alone help predicting what you might get if you were to upgrade to ADSL2+.

[c6em]

Put in your ADSL1 G.DMT router quoted attenuation.

Here were/are my lines details from the past/current
ADSL1 attenuation 34, sync on the max limit of 8182 with SNR D/S was very high at 10.0
ADSL2 attenuation 36, sync (at 3dB target margin) = 14500

Kitz calculator for 34 attenuation
ADSL1 = 7600
ADSL2+= 12300
Real distance from the exchange = 2.2Km, Kitz calculated one 2.5Km

The method of calculating the attenuation figure supplied by the router is the same. It is just that ADSL2+ uses more/higher frequencies so the effect of including all these higher frequencies in the calculation summation formula means that the answer comes out usually 2 to 3 dB higher.
How much higher depends as the calculation is based on including in the calc' only those higher ADSL2 frequencies that the router actually uses rather than all those available to use up to the top limit.

[kitz]

There isnt a hard and fast rule because a lot depends on the router and how it reports the attenuation.
 

The calculator was originally written in early 2007 -  in the days when DSLmax was relatively new and Be was just staring to roll out adsl2+ hence the obvious  "The figures used are based on attenuation measured at 300Khz for adsl1." (Most older) adsl1 routers reported the attenuation at 300kHz.

On adsl2+ (most - but not all) routers tend to use an average figure across all the available tones.
The doubling of available tones and because the higher frequencies are naturally more attenuated, then the average for all the tones tends to increase by approx 3dB for a good line. 

The physical line condition hasn't changed, its just the way that routers report between adsl1 and adsl2+.  If an engineer tested your line then afaik they still tend to do so at 300kHz (BS could perhaps confirm).  To muddy the waters further I believe Be's MSANs measured at somewhere around tone 175. 1MHz is another test frequency that may be used.

So we cant really ever cover all circumstances... but since the estimator was based on statistics for 300kHz, thats why I say if you are already on adsl2+ then your router will likely be displaying attenuation based on the additional frequencies and you should bear this in mind.

So to recap.. all the estimates (both adsl1 and adls2+) are based on projections with attenuation measured at 300kHz, because these are what the technicians who do statistical projections for line attenuation* as opposed to signal attenuation which is what routers tend to report.


*think I have that the right way round.  If not - happy to be corrected

[kitz]

Cuz a speedtest doesn't measure sync speed!

Let alone help predicting what you might get if you were to upgrade to ADSL2+.

I think BS meant put your phone number into one of the ISP sites that has access to the BTw database to get the BT projected figures.

[Black Sheep]

That's EXACTLY what I meant.

"Let alone help predicting what you might get if you were to upgrade to ADSL2+."

It does predict what you will get if you upgrade. Of course, it's throughput, but this is the 'bottom line' that you would want to know, isn't it ?? That's what I would want to know if I was going to request an upgrade.

It's all guesswork, whether you try and work out the synch speed on a different modulation, or whether you use the throughput speed checker.

[XRaySpeX]

The BTw database is notoriously unreliable and its predictions are wildly inaccurate.

Even on my longstanding BTw based BB line it predicts only up to 13 Meg on a line that runs at 20 Meg @ 3dB NM  or probably 16-17 Meg @ 6dB NM.

I think to nail my original Q I will have to experiment with the various ADSL modes using a router where I can tweak the 6dB NM.

[XRaySpeX]

Of course, it's throughput

No, the ISP's checkers and the BT Availability Checker, i.e. the BT database, are predicting Downstream Line Rate (Sync Speed).

[kitz]

I think to nail my original Q I will have to experiment with the various ADSL modes using a router where I can tweak the 6dB NM.

I hope you're not subject to a (BTw/oR) DLM 

Whilst all the technical data is based on signal attenuation, you may also run into the issue of your router reporting the line attenuation..  which means that your findings may differ from someone at say 7dB.. which would vastly differ from someone with 60dB.  Hence the reason why laboratory data & stats will use the signal attenuation at a set frequency.

No trying to deter you in the slightest, just mentioning the caveats.  Have fun playing   I may even dig out some of my old stats sometime.

[kitz]

Even on my longstanding BTw based BB line it predicts only up to 13 Meg on a line that runs at 20 Meg @ 3dB NM  or probably 16-17 Meg @ 6dB NM.

Its well known that the BTw database is conservative.

The database said 16Mb for my line... despite being an ex-member of 'Be's 24Mb club'. 
I even got the full 24Mb down 2.6 Mb up at one time too - although admittedly I cheated and tweaked to 2.5dB SNRm to get it on annex_m.. but it would hold ok for quite a while. *

FTTC says 62Mb yet I get the full 80/20.

Likely for the reason that there are so many lines out there with bad internal wiring and non-optimised lines..  so - rightly or wrongly -they build in an allowance to cover their own back.


*Until crosstalk and a deteriorating line put paid to that

[Black Sheep]

I never knew it was synch speed they were predicting. 

I never use them, (tell a lie, I used it once to see when my FTTC cab became live), but in the same breath, I don't put that much onus on speed personally. All I want is reliability, as in the damned thing works when I want it to. That's just me though. 

[XRaySpeX]

which means that your findings may differ from someone at say 7dB.. which would vastly differ from someone with 60dB.  Hence the reason why laboratory data & stats will use the signal attenuation at a set frequency.

Yes, I realise it is not scientifically ideal, bur w/out recruiting an army of experimenters it's all I can do on my own.

Thanks