An intercom is an electronic communications system within a building or group of buildings. Intercoms are generally composed of fixed microphone/speaker units, which connect to a central control panel. A small home intercom might connect a few rooms in a house. Larger systems might connect all of the rooms in a school or hospital to a central office. Intercoms in larger buildings often function as public address systems, capable of broadcasting announcements. It contains master station, sub-stations, door stations and power supply. Master station initiates a call with any of the stations. Sub-stations and door stations are capable of only initiating a call with a master station. Power supply is used to feed power to all units.