So here is my update. The modem's light on when it is plugged into usb port.I tried running brctl to establish bridge and i am successful in establishing it.To be sure that the connection is established i used dmesg.The output of dmesg shows adsl line is up and displays up and down speed.But still i am not able to connect to internet in ubuntu. I am posting my dmesg output on ubuntu pastebin. Here is the lin .
WHAT THIS PRODUCT DOESThe D-Link ADSL2/2+ Ethernet/USB Modem (DSL-2320B) enables you to access a high-speed DSL Internet connection. Homes with a single computer can connect directly to the DSL-2320B and access the Internet. To share your high-speed Internet connection with multiple computers, simply add a multi-port router2. In addition, the DSL-2320B automatically scans and detects your DSL settings, helping minimize set up time and hassle.
About the USRobotics USB ADSL Modem The USRobotics USB ADSL modem is compatible with any PC equipped with a USB port. As a Plug and Play device, Windows 98 First Edition (FE), 98 Second Edition (SE), 2000, and Millennium (Me) will automatically recognize the U.S. Robotics USB ADSL modem and initiate the installation process.