For using data in Airtel Prepaid, a base plan is definitely needed (for validity). All base plans with Airtel are now only plans with Unlimited Calls. Data Only plans could be activated as addons, only if there is an active base plan.
155 (Minimum Base Plan)
24 days validity (or 28 for some customers), Unlimited Calls, 1GB data (or 2GB for some customers) for the whole validity period.
1799 (Minimum Base Plan for 1 Year)
365 days validity, Unlimited Calls, 24 GB data for the whole validity period.
Once there is a base plan, data addons can be activated, which are typically valid until the end of the base plan's validity period.