The monitors are installed within each internal apartment or unit on one interconnected network. The door station is installed outside the front entrance of the building and is connected to each internal monitor on the same network.
When a visitor approaches, they press the appropriate call button on the door station. The door station triggers the corresponding internal monitor and activates its camera. Inside, the monitor rings out loud to indicate to the resident that someone is requesting access. Most systems will display the video from the door station camera while the monitor is ringing, allowing the resident to see the visitor and decide whether to answer the call or not.
When the resident answers the call, audio communication is also activated, and the two parties can speak to each other. Some systems allow two-way video, while others only allow the resident to see the visitor and not the other way around.