These systems can be audio or video. A system usually consists of a door station, one or more handsets, a door release mechanism and a power supply. There can be one or more door stations and these are located outside and adjacent to the entrance door/s. Handsets can be of the telephone type or the 'hands-free' type and are strategically positioned for ease of use.
Also connected to the door station will be door release mechanisms which allow the door/s to be unlocked remotely from the handsets to allow visitors entry. These release mechanisms will be rated to suit the security level of the building. The power supply is connected into a local mains electrical supply and will provide power to the system.
Video Systems have a CCTV camera either built into, or associated with, the door stations to provide a colour or monochrome (black & white) image of the visitor. This aids identification of the visitor and provides for higher security.