All ADSL/telephone line based broadband connections require username/password provided by ISP.Just see the username/password from your airtel modem settings page by logging into the modem at its IP(usually 192.168.0.1 or 192.168.1.1,default username password for modem usually is admin/admin).Also I don't know how good W8968 auto setup is but it has to correctly select the VPI/VCI values of Airtel ADSL line for link led to become stable.Username password only comes into the picture once the ADSL link led is stable.