The caste system in India divides Hindus into four main categories - Brahmins, Kshatriyas, Vaishyas and the Shudras. The main castes were further divided into about 3,000 castes and 25,000 sub-castes, each based on their specific occupation.
This is a layer of atmosphere about 60 to 70 km above the earth and extends to about 400km. It contains ionised particles which can reflect radio waves making radio communication round the world possible.