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

Question

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.

Answer 1

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.
And as set out in OFCOM's guidelines, all commodities under the constraints of BT wholesale's custom and practice with regard to end user services .... show no discrimination to any one supplier or end service provider.
My own opinions on the politics are here:
http://community.bt.com/t5/BT-Infinity/bt-wholesale-is-a-farcre/m-p/789354#M85436