Optical Network Unit (ONU)

Posted on Sep 19, 2009 in Knowledge Base

An optical network unit (ONU) is a device that transforms incoming optical signals into electronics at a customer’s premises in order to provide telecommunications services over an Optical Fiber network.


An ONU is a generic term denoting a device that terminates any one of the endpoints of a Fiber to the Premises network, implements a Passive Optical Network (PON) protocol, and adapts PON PDUs to subscriber service interfaces. In some contexts, an ONU implies a multiple subscriber device. An optical network terminal (ONT) is a special case of an ONU that serves a single subscriber.


ONU provides the subscribers with broadband Internet access.

An ONU is used in combination with an optical line terminal (OLT).

Optical network unit closure

An ONU closure is a mechanical compartment that houses the ONU equipment. The outer closure faces the outside environment and provides physical, mechanical, and environmental protection for cable (fiber and copper) components or equipment housed within it.

An ONU system consists of a closure that is a metallic or non-metallic enclosure that provides physical and environmental protection for the active electronic, optoelectronics, and passive optical components it houses. It terminates Optical Fibers from the ODN and processes the signals to and from the Customer Premises Equipment (CPE). It is the NE that provides the tariffed telecommunications as well as video service interfaces for multiple residential and small business customers.

Services on the customer side of the ONU are communicated over metallic twisted pairs and coaxial cable drops (in the future, possibly fiber cable or wireless) to a Network Interface (NI) where they are handed off to the customer’s network (usually, inside wiring). Depending on the deployment strategy, the ONU closure may provide one or more of the following additional features:

  1. Access to the fiber distribution cable
  2. Management of slack fiber and fiber splices
  3. Access to the Telephone Support Cable (TSC) for the purpose of powering the ONU
  4. Prevention of unauthorized entry.

Primary power for ONUs is derived from either an external DC or an external AC power source. Back-up power for ONUs can either be derived from an external power source or be internal to the ONU closure and be provided by the FITL system supplier. Primary power and external back-up power can be delivered to ONUs over either copper twisted pairs or coaxial cable facilities. These cable facilities are commonly referred to as the TSC.

Deployment of an ONU system requires access to the fiber distribution cable, TSC, and metallic customer drop wires. When access to these cables is provided internal to the ONU closure (i.e., by looping each cable through the closure), it is necessary that the ONU closure also provide splicing and storage facilities for each of these cables.

Telcordia GR-950, Generic Requirements for Optical Network Unit (ONU) Closures and ONU Systems, contains complete proposed specifications for the ONU closures and systems.