A bit confused...

[#1]

Got the following response to my ticket:
Thank you for getting back to us. I have tested your line and can see that you are currently only syncing at 5912Kbps and have an SNR of 12dB. You are indeed on the DLM trial with a traget SNR of 6dB - this is however as *name removed* advised a target and not a constant. If factors internally or on the line cause your SNR to increase and sync to drop then there is nothing the target can do to alter this. The trial will however ignore connection drops when determining the target SNR of your line as it remains fixed at 6dB. In light of this please try the following:
1) Ensure you are connected directly to the master socket and that all connections are solidly made and not loose.
2) Ensure that all devices are appropriately filtered and attempt to disconnect all devices other than the router - it is best to change the filter on the router when you do this.
Once completed you should notice a sync speed increase / SNR decrease - if problems continue please send this ticket back to us.
This implies that there is a fixed target SNR of 6dB on my line, however noise or other factors could cause my SNR to increase and sync rate to drop. The latter part of that sentence seems to imply that my SNR can change, whilst the former implies it is fixed.
So which is it? I'd assumed that fixed meant it wouldn't change and would always connect at 6dB no matter how many times it drops (my experience for rebooting the router is that this is what happens). However on some connection drops it connects at 12dB, hence the confusion (which the ticket response didn't clear up!).

[#2]

What it is is a bit unclear and confusing.
I will try to explain.
On a normal line the system in the exchange sets the default noise margin at 6dB when the line is first trained. This is the margin when the line synchronises.
If during the training period or subsequently the equipment in the exchange calculates that the line is unstable or potentially unstable it wil increase this default noise margin in steps of 3dB up to a maximum of 15dB to attempt to achieve stability.
On ADSL1 that was basically all it did but on ADSL2+ they introduced a further nasty called banding where the system would think that even with the high default noise margin the line wasn't stable and would then clamp the speed to within a range and in this case the noise margin can go very high indeed.
When the exchange looks for stability it looks at the number and frequency of disconnections and errors on the line. This is because the router can hold sync with the exchange but the number of errors can severely impact on the effective speed.
I believe that the ticket reply is referring to the no DLM trial where the automatic system completely ignores the above and will always try to connect at 6dB noise margin.
Note the word "TRY". If it can't achieve a connection at that noise margin it won't give up and effectively say "tough you have had it" it will try to connect at increasing noise margins until it succeeds. However when you resync the router it will try again at 6dB and if that succeeds then all well and good. If it doesn't then again it will progressively increase the noise margin until it does.
The suggestion made in the ticket is to ensure, as far as you can, that everything in your property is working correctly.
Although they refer to the master socket I would suggest trying the test socket if you are still seeing problems

[#3]

Hmm I thought that was always called the target SNR, a value that the connection would try to achieve and if it couldn't it would try the next best SNR level. I thought the idea of being removed from the DLM was that it would _always_ connect at 6dB no matter how stable the line was (I can't actually think it would be impossible for the line to connect at 6dB).

[#4]

Quote from: Snarf

(I can't actually think it would be impossible for the line to connect at 6dB).

It might connect but when it tried to authenticate the session it would fail