How is the ADSL Downstream Speed chosen and what c...

I changed to BT Broadband last October and had problems with the ADSL Downstream connection speed (which affects the BT Profile ) . Contacting offshore was a brick wall and eventually I emailed the Head of BT. I received excellent help from the local Technical Help and the speed appeared to settle down.
However, at least once a week, the speed still drops from around 2656 Kbps to 2144 Kps. From the daily btspeedtester results I have recorded since October, the change is directly related to a reduction in ADSL Downstream Output Power (in this instance, from 4.1 to 3.3 dbm).
The Line Attenuation Down and Up has remained within 0.5db over the period (58.6 and 31.5 db respectively) and the Noise Margin Downstream (for an output power of 4.1 dbm), between 7 and 8.3 db.
This raises questions regarding control of the Output Power Downstream when the connection is being set up :-
a) Does BT,at initial set up, test the line and decide on a value (eg 4.1dbm), depending on the line parameters, which is input to the modem at the exchange? I notice in some forums, figures of 14dbm quoted.
b) What parameter(s) control the power, in this instance, reducing it by 20% on occasions, for no apparent reason and taking three days to revert to the original figure, which is ridiculous.
Nobody in BT Technical Help knows the answer and I wonder if anyone has the relevant information on the above.
As far as I am aware the cable from the exchange is underground and there has been no significant rain in the Glasgow area for two months, so the old excuse about the weather does not hold !

Thanks for info. Am aware of the Noise margin being at least 6db.
When the the power has dropped on several occasions to 3.3dbm, the noise margin has increased to around 10db, therefore does not appear to tie in with maintaining the margin to around 6db.
Due to a reduction in ADSL speed, the net result is the IP Profile is reduced to 1750 and sometimes 1500, with the obvious effect in thruput
Near end crosstalk is usually the major problem in cable systems, however, in this case, the receive leg at the exchange is bandwidth limited (only 448 Kbs) and probably has a low pass filter at the input to the ADSL modem.
Have noticed in other forums outpowers being quoted at around 15dbm, considerably higher than mine !

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Re: How is the ADSL Downstream Speed chosen and wh...

I have a similar problem of the link being 'reset' most nights, usually between 2 and 4 AM. The DSLAM is reset and looses the target setting for the Noise Margin. It should be around 6-9 dB, but mine constantly gets reset to 16.5 dB and hence a very slow throughput.
Openreach engineers have twice had it remotely reset to 6 dB but after a few weeks it returns to 16.5 dB which is unacceptable. In my view it should be permanently coded into the DSLAM so it can't be 'fiddled' and therefore give a better experience to users.
The experience shows that any claims of 'maximum line speed capability' are false as they are biased by the noise margin target set in the DSLAM.
I also understand from the Engineers that there are numerous resets performed during the 'quiet' periods at night when most users are asleep. That gives rise to many of the queries people have about why it changes. As far as I can make out there is no logical technical reason for all this change (I am a retired electronics engineer with 50 years practical experience) so it seems to me to be some attempt at limiting the load on the 'backbone' so they don't have to invest in equipment that can handle greater thoughput properly.
They will eventually have to invest as the fibre scheme rolls out further. However, many rural customers, like us, will have a very long wait before seeing any benefits from fibre.

DSLAMS only drop line connections for the purpose of line training, and implementing line running methods during the training period.
It's the EU's router or modem that drops connections under normal conditions, often through excessive error rate breach caused either by line noise or impulse noise on the DSL path.
You're correct in assuming your noise margin should be 6-9db under "normal circumstances" ... this is considered target margin on which BT dslams and msans will attempt to negotiate a DSL connection to an EU modem.
If the noise margin is increasing that is an indication that the line characteristics are under a profound effect by external influences, impulse noise or line crackle, and thus the margin is increased by the line management in order for the dslam to continue to bitload the datastream, at the sacrifice of a chunk of linespeed. All normal, and part of line management's role in both defending the network against corrupted data, and maintaining a connection with the end user, so far as it can.
Dslam line control does benefit both user and network, but the network comes first. The foundation for DLM , dynamic line management is not to permit a good end user experience, it's to defend thousands of network nodes against data corruption, on a network that is the backbone of the country's economy, as well as a vehicle for residential communications.
No resets are performed so far as DLM is concerned during night time hours, merely the network is dynamic and ever changing so far as bandwidth usage is concerned, and runs in a totally automatic and indiscriminating manner.
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http://community.bt.com/t5/BT-Infinity/bt-wholesale-is-a-farcre/m-p/789354#M85436

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