Optical Network Unit (ONU)
ONU converts optical signals transmitted via fiber to electrical signals. These electrical signals are then sent to individual subscribers. In general, there is a distance or other access network between ONU and end user’s premises. Furthermore, ONU can send, aggregate and groom different types of data coming from the customer and send it upstream to the OLT. Grooming is the process that optimises and reorganises the data stream so it would be delivered more efficiently. OLT supports bandwidth allocation that allows making smooth delivery of data float to the OLT, that usually arrives in bursts from the customer. ONU could be connected by various methods and cable types, like twisted-pair copper wire, coaxial cable, optical fiber or Wi-Fi.
Optical Network Terminal (ONT)
Actually, ONT is the same as ONU in essence. ONT is an ITU-T term, whereas ONU is an IEEE term. They both refer to the user side equipment in GEPON system. But in practice, there is a little difference between ONT and ONU according to their location. ONT is generally on customer premises.
Optical Line Terminal (OLT)
OLT a equipment integrating L2/L3 switch function in GEPON system. In general, OLT equipment contains rack, CSM (Control and Switch Module), ELM (EPON Link Module, PON card), redundancy protection -48V DC power supply modules or one 110/220V AC power supply module, and fans. In these parts, PON card and power supply support hot swap while other module is built inside. The main function of OLT is to control the information float across the ODN, going both directions, while being located in a central office. Maximum distance supported for transmitting across the ODN is 20 km. OLT has two float directions: upstream (getting an distributing different type of data and voice traffic from users) and downstream (getting data, voice and video traffic from metro network or from a long-haul network and send it to all ONT modules on the ODN.
