Civilian Air Traffic Controllers' licensing is standardized by international agreement through ICAO. Many countries have Air Traffic Control schools, academies or colleges, often operated by the incumbent provider of air traffic services in that country, but sometimes privately. These train student controllers from walking in off the street to the standards required to hold an Air Traffic Control license, which will contain one or more Ratings. These are sub-qualifications denoting the air traffic control discipline or disciplines in which the person has been trained. ICAO defines five such ratings: Area (procedural), Area Radar, Approach (procedural), Approach Radar and Aerodrome. In the United States, controllers may train in several similar specialties: Tower, Ground-Controlled Approach (GCA), Terminal Radar Control, or En route Control (both radar and non-radar). This phase of training takes between 6 months and several years.