I am using BSNL 3G data card with usb device by the same. I always do recharge for 1GB data. However one can use only around 600MB in the total. The device stops connecting internet after this much data is used.
I have verifyied this 3 times by clearing the device database in my system everytime I make recharge.
When I contacted customer care, I NEVER get satisfactory answers.
BSNL is CHEATER. They do not provide usage up to the promised limit.
NEVER EVER GO FOR BSNL PREPAID.