Indicates whether network connectivity is possible. A network is unavailable when a persistent or semi-persistent condition prevents the possibility of connecting to that network. Examples include

• The device is out of the coverage area for any network of this type.
• The device is on a network other than the home network (i.e., roaming), and data roaming has been disabled.
• The device's radio is turned off, e.g., because airplane mode is enabled.

[Android Documentation]