Most of the Airtel Broadband plans available now are high speed plans( >2mbps). But none of them are truly unlimited. All the plans have high speed bandwidth transfer limit after which the speed is reduced to mediocre 256kbps.
Airtel has provided way to buy additional high speed Bandwidth for extra cost: Airtel SmartBytes. But they are very costly: 1GB costs Rs100/- 10GB for Rs 500/- up-to maximum of 150GB for Rs1500/-.
Though you will not be inclined to buy BW at very high cost from Airtel, it can be used to check your Data Usage very easily: that too without logging into the Airtel account.
1) Just go to the Smart-bytes Link
2) Select the option to check the option to see usage and buy BW( This works only from your Broadband connection not from external Internet connection)
You get to see the usage as below:
(This is for Airtel 1699 4 mbps 75GB plan)