Hi and welcome
>> My noise margin has NOT altered since removing the bell wire
>> What has changed is my sync rate
That is excellent news
Just to clarify the higher sync rate will have been as a result of a better SNR. Your router will always try and sync up giving you a set dB of SNR Margin - this is your Target SNR. After sync, your SNR Margin is then able to fluctuate up and down according to the local condition.
Therefore if your SNR is improved, then this will directly have an impact on you being able to sync at a higher speed, but your SNR Margin will still appear the same
Whilst SNR Margin and Target SNR are related, theyre not the same thing. -
SNR and SNR MarginSo whats likely happened is removal of the bell wire has increased your Signal to Noise Ratio... but because there now more "spare" this has allowed your line to sync at a higher speed whilst still maintaining a xdB SNR Margin. Hope that makes sense
As regards the "spike" and negative SNR, roseways already explained most of that, but one of the reasons why in the past we recommended the Netgear is because of its ability to hang on to lines even if the SNRM went into negative figures. - Granted though you are likely to be racking up quite a few errors... but it does seem to "hang on in there".