Modern Video Door Phone
Installation of a doorbell in an apartment : Modern Video Door Phone
Quote from Article entitled Installation of a doorbell in an apartment:
Depending on the connection method, doorbells are divided into two types:wire;wireless.
Wire doorbells operate off the electricity, and off an independent power supply. The button and the sound panel of these devices are needed to be connected to each other by wires. Wireless bells do not require a physical connection of elements.
The full version of the article can be found here.