An ONU (Optical Network Unit) is a key device in Fiber-to-the-Home (FTTH) and other FTTx networks, operating within a Passive Optical Network (PON) architecture. A passive optical network (PON) is a fiber-optic telecommunications network that uses only unpowered devices to carry signals, as opposed to electronic equipment. Cisco introduces GPON with the Catalyst GPON platform. In simple terms, it's a device that receives the optical signal from your Internet Service Provider (ISP) via a fiber optic cable and converts it into electrical signals that your router, computer, phone, and other devices can understand and use. To truly understand how an optical access network functions, you must know what each acronym stands for and what role it.