Conflicts are detected by data assistants, who report them to the actual controllers; the data assistants suggest how to solve the conflict, but the controller is not obligated to follow the suggested instructions. A conflict occurs when the distance between aircraft in flight violates a defining criterion, usually considered as 5 nautical miles (9 km) of horizontal and/or 1,000 ft (300 m) of vertical separation. These distances define an aircraft's protected zone, a volume of airspace surrounding the aircraft which should not be infringed upon by any other aircraft.[4]