Back and forth
Sound transmission is often expressed in terms of simplex, half-duplex and full-duplex transmission.
Using a camera with built-in sound, you can easily set up gatekeeper functionality. Visitors can present themselves to the camera and state their business. Video and sound are monitored by an operator who can talk to the visitor through a loudspeaker connected to the camera. Once the visitor has been cleared for entry, the operator can open the door remotely, using e.g. the camera’s output port.