Run adsl-setup. Most likely it is already installed on your system.
Otherwise you can also install it on yum enabled distributions like Fedora Core simply by:
yum install rp-pppoe
It asks questions like your login name and password (provided by BSNL),
your ethernet interface (choose the default provided - eth0; works for most cases) etc.
After the setup you should preferably reboot
(or restart the network - service network restart) and you will find
your DSL connection up and running.
You can also manually bring up DSL connection by:
ifup ppp0
You can bring it down by:
ifdown ppp0