An FAU can be put inside a reconfigurable optical add-drop multiplexer (ROADM) and function as an optical transmission for the wavelength selective switch (WSS) to switch traffic remotely from a wavelength division multiplexing (WDM) system at the wavelength layer.

There are other functions within long-haul and metro networks that require FAUs, and they are amplifier/CP module, coherent mixer, multiport wavelength switch, multicast switch, and optical channel monitor.

(That's a mouthful)