how to know if your modem supports annex adsl modes? Are those needed?


Hi, BSNL allows user to choose their own modem after initial stages, where only data one modems were in vogue. I was having terracom tdsl300w2 modem from BSNL, which did not supply any firmware update, but working .
Now i have changed that to D-Link 2730 u indian model, and it is also working.
I connect only a modem at a time to get my connection.
Terracom default adsl mode is adsl+2 (auto sync) and annex mode is annex A.
I tried annex M modem available and once saved and rebooted , the dsl light does not stay at all.
I know that i have changed to annex M and so have the problem.
I agin switched back to annex a, and my dsl light stays to give me internet and phone connect.
My query to the experts
how to know if my modem supports annex M practically, or for that matter any annex mode.
Is there really any connection or speed difference if you select those modes or if it is enough, if you just tick the adsl2+ onlly in the dsl settings.
I also read in one of the site, that adsl+2 can give double download rate, ie 20 mbps within 1.5 km (or 5000 feet) and after it , it is educed due to length of wire ove copper decreases.
How one knows if his modem supports any annex model for eg annex a annex L annex M. There are readings lilke 992.3,992.5 butwill that be known from the router page on any tab?
when i changed to annex M, with adsl2+ my dsl is not stay or could not. It only blinked.
experts please
Adsl2+ annex a is sufficient.