An intercom/Door Phone (intercommunication device), talkback or doorphone is a stand-alone voice communications system for use within a building or small collection of buildings, functioning independently of the public telephone network. Intercoms are generally mounted permanently in buildings and vehicles. Intercoms can incorporate connections to public address loudspeaker systems, walkie talkies, telephones, and to other intercom systems. Some intercom systems incorporate control of devices such as signal lights and door latches. There are simple house intercoms and intercoms developed for collective apartments. Some are equipped with video, and its wiring, can be connected to the outside with few pairs (4-6 pairs) while controlling an electric strike. The last generations are even compatible with computers and some models include TCP/IP.