>> Isn't there a possibility that it is not possible to gain more speed because of the signal limitation with regards to distance of my house from the exchange?
According to the max speed calculator your expected limit based on your attenuation is around 18Mbps with a target SNR of 6dB, which you are near as damn it now.
If you have a DMT tweakable router then you should be able to override the Target SNR Margin up to a certain %. The maximum tweakable figure seems to be % based and it may depend upon the DSLAM, but you should be able to over-ride up to at least a further 3dB.
eg
if your target SNR is 9dB then you should be able to take it down to 6dB
if your target SNR is 6dB then you should be able to take it down to 3dB.
My own Target SNR is 3.5dB, and I can actually tweak down to 0.5dB - not that I'd want to because the line would become way too unstable and I'd clock up too many errors since its too near the 0dB mark which doesnt give any headroom - but it is possible for me to do so and still get sync.Your sync speed depends upon
bit allocation, which in turn depends upon the 'real'
SNR minus the '
Target SNR Margin'.
Theres a page which Ive done which isnt actually live yet (still needs proof reading and a couple of edits etc), but if you want to have a read through it - it should give you the general idea of how you sync speed is actually achieved.
Warning - this particular page is a bit more techy than some of the others, and needs and understanding of SNR and SNR Margin.
Anyhow the page which explains your sync speeds is and how adsl actually works is:-
ADSL Technology and DMT.
