How to Distinguish Optical Access Network OLT, ONU, ODN, ONT?

An optical access network is using light as a transmission medium to access every building. An optical access network generally consists of three parts: an optical line terminal OLT, an optical network unit ONU, and an optical distribution network ODN. OLT and ONU are the key to optical access network.
What is OLT?
The full name of OLT is Optical Line Terminal, also known as optical line termination, acts as the endpoint hardware device in a passive optical network.OLT, the central office equipment of telecommunications is used to connect optical fiber backbone. It works as a switch or router in a traditional communication network. In simple terms, It 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.
What is ONU?
ONU is the Optical Network Unit which converts optical signals transmitted via fiber to electrical signals. 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 optimizes and reorganizes the data stream so it would be delivered more efficiently. Different deployments of ONUs in FTTx networks have different access methods. For example, FTTC (Fiber To The Curb): ONU is placed in the central computer room of the community; FTTB (Fiber To The Building): ONU is placed in the fiber terminal box of the corridor; FTTH (Fiber To The Home): ONU is placed in home users
What is ONT? ONT is the Optical Network Terminal, the user side terminal unit of FTTH, commonly known as “optical modem”, similar to xDSL electric modem. ONT is an optical network terminal and is used for end users while ONU refers to an optical network unit. There may be other networks between it and the end user. ONT is an integral part of ONU.
What is the relationship between ONU and OLT?
The OLT is the management end, and the ONU is the terminal; the ONU service provisioning is delivered through the OLT, and the two are owner-member relationship. Multiple ONUs can be members via splitter under an OLT.
What is ODN? ODN is Optical Distribution Network, which is the physical channel of optical transmission between Optical Line Terminal and ONU. The main function is to complete the bidirectional transmission of optical signals. Usually these are connected by fiber optic cables, optical connectors, optical splitters, and optical distribution frame.