>> So overall the Netgear DG834 models are usually good on long lines,am I correct ?
Well in the past many users have had sucess with them. However we cant be blase and ignore what has recently come out about the AR7 chipsets. I dont think zen would have released the informationation on a whim. There possibly is something but just what the full contributing factors are yet we really dont know - hence the hesitation right now over the whole situation and I dont think anyone would put their neck on the line and say for sure one way or the other.
>> I believe that your noise margin will usually come down every two weeks if your line is stable to a default of 6db.
This has been mentioned before - some users reported that it does. Ive seen someones do it after 4 weeks. Ive also seen cases reported where it doesnt.
Sorry but this is one area where you will find there is little information about what is supposed to happen and no-one seems to have been able to find out exactly just what the triggers are for the reduction of target SNR.
>> Can your ISP ask them to lower it from say 12db or 9db to 6.
Yes its one of the configurable attributes.. earlier this year Mr_Saffron on TBB and James from Plusnet used to have success in getting this done. However over the past few months BTw seem to be a bit more "no can do" when it came to these requests. Even many good ISPs can play ticket tennis with BTw of late to try and get this done :/
>> Or would the customer be able to override this with say a Netgear or a Speedtouch and tweak it down themselves to a 6db noise margin
The netgear dg834g v1-3 doesnt have this facility.. The DG834gt does as do most speedtouches.
If you use DMT can can tweak your target SNR yourself.
However its important to bear in mind that you are tweaking a percentage.. therefore if your target SNR is profiled at 9dB then you should be able to get it down to 6dB. However if your target SNR is 15dB, then although you could bring it down to say 12dB, I dont think 6dB is possible.