An intercom, also called an intercommunication device, intercommunicator, or interphone, is a stand-alone voice communications system for use within a building or small collection of buildings which functions 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 other intercom systems. Some intercom systems incorporate control of devices such as signal lights and door latches. Intercoms are used on a wide variety of properties; from houses that only require one connection between a resident and the property's entrance to multi-unit apartments that require intercom hardware to be installed in every individual apartment. Some are equipped with video and its wiring (electrical installation) can be connected to the outside with a few pairs (4-6 pairs) while controlling an electric strike. The latest generations are even compatible with computers and some models include TCP/IP compatibility. Permanent systems Traditional intercoms and public address systems are composed entirely of analogue electronics components but many new features and interfacing options can be accomplished with new intercom systems based on digital connections. Video signals can be carried as well as voice. Digital intercom stations can be connected using Cat 5 cable and can even use existing computer networks as a means of interfacing distant parties. Intercom cameras can be used in modern offices and hotels. Many schools and office buildings now use audio / video systems to identify visitors trying to gain access to a locked building and can be interfaced with the building's access control system. Besides fixed locations, intercom systems are used on many types of vehicles including trains, watercraft, aircraft and armoured fighting vehicles. Portable systems Portable intercoms are commonly used by special event production crews and professional sports teams. Performing arts venues such as theaters and concert halls often have a combination of permanently mounted and portable intercom elements. Motorsports race tracks often have both portable and permanent intercom stations mounted at critical points around the racecourse for use by race officials and emergency medical technicians. Basic intercom system terms Intercom installers, suppliers, and manufacturers often use the following terms to refer to intercom components. Master station or base station: These units control the whole system. Master stations initiate calls to substations and can be configured to make announcements that are transmitted over the entire intercom system. Substation: Units that initiate calls with a