Video-door systems use two means of communication: sound and image. IP technology allows for digital video and audio transmission and also protects against eavesdropping. Full duplex systems are encountered (transfer both audio and video simultaneously) but the most popular is an audio transfer in both sides and caller preview. In addition to video and audio transmission, the video door systems allow you to control the actuators (locks, gates) as well as recording any combination connection or connection attempts. With IP technology, videophones have high scalability and expandability.