@ Jithin
As said above I’ve not used BSNL through USB till date. On a wider scope you may want to go through these steps to get things working though.
1) Give the router the following values before you start off:
IP address: 192.168.1.2
Subnet Mask: 2555.255.255.0
Gateway:192.168.1.1
This can be done through System> Administration> Network
2)Check to see if your drivers are up to date.Probably if you could give me your modem details and h/w details I may be able to help you out.
3)Could you let me know what the following commands in the terminal give:
a)Sudo pppoeconf-> Configuring the pppoe
b)Sudo lsusb -> Believe this should help in seeing if the modem is being recognised by the system.