Below is a rough guide to indicate what maximum speeds you are likely to achieve on the higher rate adaptive speed products.
Your modem/router should report your existing downstream attenuation. Details how to get your line stats.
Please note the figures are only a guide, and no-one can say for sure how your line will perform when on dslmax or adsl2+, until its actually connected on that service.
Attenuation is the reduction in signal strength on your phone line. In adsl this may be reported as "loop loss" and is the natural deterioration of the adsl signal over distance from the exchange. Whilst attenuation is normally directly linked to the length of your line, there are a few other important factors to bear in mind as to why the above calculator can only be used as a guide. Copper is traditionally used in the local loop and the higher gauge of copper will give the best signal, however some lines may have some aluminium or aluminium joints on the line which will increase resistance... as will oxidisation of joints.
It is also worth pointing out that attenuation is calculated differently for adsl and adsl2+, and its not unusual to see an increase of 3-4 db when moving from adsl1 to adsl2+.
The figures used are based on attenuation measured at 300Khz for adsl1. If you are already on adsl2+, then you will likely be using additional higher frequencies with an increase in attenuation which should be bourne in mind.
The calculator is based on a default Target SNR of 6dB. Each 3dB of SNR is worth anywhere between 400 to 1200kbps of speed depending upon your bit loading. Therefore if you have a higher/lower target SNR set on the DSLAM then this will affect your maximum speed.
Explanation and credits.
Thanks to Internode from whose graph these guidelines were obtained.
Many thanks go to Richard Moulynox on the whirlpool forums for his linear calculations which I used and adapted for this calculation with his permission.
Finally thanks to the many users on UK forums who posted their linestats where I random checked the stats to ensure a fair and representative average figure.