Thats why I had to take the 2250 plan with 175 GB fup limit.A session is started when you connect to the internet, and it ends once you logout/disconnect. If you aren't the one disconnecting every 30 minutes, then its possible that BSNL chops up sessions to 30 minute blocks. It seems like you have currently used up ~7 GB of data in 2 days, and at this rate, you would reach ~110 GB in a month.