The recommended SNRM is 6dB. quite often lines need a lot more. afaik theres only Be that allow the 3dB Target SNR, and you need to be on a short and stable good line for that to work.
Most routers will start to get a bit shakey once the SNRM drops below 6dB... but the ones we recommend as being good will cope with speeds lower. As broadstairs says.. theres not many that will cope at 1dB.. some may remain connected - theres also a few that will go into -ve figures.. but the line quality at this level is likely to be very bad. You will be racking up lots of errors and packets that have to be retransmitted.
The re-requesting of packets is what makes the connection slow.
Therefore its just not worth running the SNRM very low because any increase in sync speed will be negated by re-transmits which may actually cause a slower throughput speed. The end result could well be a slower realtime speed and certainly latency problems, than if you hadnt bothered tweaking the SNRM down that low.
The SNR Margin is there for a purpose - to absorb any line fluctuations and allow for normal SNR fluctuations.
As regards to how much increase it will give you - I wouldnt really like to say, some some see 400kbps per dB.. some say 500kbps per profile stage (3dB).. it all depends on your line really. As always the shorter lines tend to fair best.
>> With DMT, all you do is force the router to ignore the DSLAM/MSAN target snr and sync with its own (i think, correct if I am wrong)
Depends on the router.. its basically called over-riding the one set on the DSLAM, by getting it to negotiate at a % OR at dBs lower than set.
IIRC You'll notice that some versions of DMT are different and some set at a % and some at dBs.. hence why you often have to play with the slider to find the right level.
