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A Guide to PON Network Components: OLT, ONU, ONT, and ODN

Author Moore

Date 07/10/2024

This article will introduce passive optical networks (PON), in which we will introduce everything about OLTs, ONTs, ONUs, and ODNs, including their operation principles and functions.

PON (Passive Optical Network) refers to a fiber optic network built using a point-to-multipoint topology and fiber optic splitters. This network is distinguished by its capability to make the data transmission from a single source to multiple user terminals. In contrast to an active optical network (AON), which connects various users to a single transceiver through a fiber optic branching tree and passive splitter/combiner unit, a PON is different in that the network operates only in the optical domain and does not require power.


PON Network Components Introduction


In a PON network, an OLT is deployed at the service provider's central office, and many ONU devices or optical ONT devices and optical splitters (SPLs) are near the end users. In addition, ODNs are used for data transmission between OLTs and ONUs/ONTs.

PON Networks




SNI: refer to Service Node Interface


IFpon: refer to PON Interface


UNI: refer to User to Network Interface


CPE: refer to Customer Premises Equipment


What is Optical Line Terminal (OLT)?


OLT Definition


OLT stands for Optical Line Terminal, a device that connects optical fibers and converts signals. This component plays a vital role in PON, as the PON OLT is the starting point of the entire passive optical network, which is connected to the aggregation layer switches using Ethernet cables.


OLT Device Components


The OLT equipment consists of a rack, a control and switching module (CSM), an EPON link module (ELM), a PON card, a power module, and a fan. It has redundancy protection, and the PON card and power module usually support hot-swapping. The OLT runs in two directions: the uplink direction, which is used to distribute data and voice traffic from subscribers, and the downlink direction, which is used to receive data, voice, and video traffic from the network and deliver the traffic to all ONT modules across the Optical Distribution Network (ODN).


OLT Function


OLT devices support multi-service delivery platforms for IP and traditional TDM services and play an important role in PON networks. They are located at the edge of the MAN (metropolitan area network) or at the egress of the community access network, where they aggregate the entire access service and transport it to the IP network.


Besides network aggregation, the OLT is also responsible for centralizing the network management platform, it supports device-based network element management, service-oriented security management, and configuration management. The OLT also monitors and manages device ports, facilitates service provisioning, monitors subscriber status, and allocates bandwidth based on the quality of service and service level agreement (SLA) requirements of different subscribers.


1. Sends Ethernet data to the ONU in broadcast mode.


2. Start and control the ranging process and record the ranging information.


3. Allocates bandwidth to the ONU, controls the start time of ONU data transmission, and adjusts the size of the sending window.


OLT Application


OTL equipment access network is assisted by several types of ONUs, for example, FTTC\FTTH\FTTO\FTTM, etc., and the various signals it carries are finally aggregated in the central office and then delivered to the end-users through a specific signaling format in the access network. Meanwhile, the signals from the end-users will be routed to their corresponding service networks according to their different types of services.


The figure below shows a simple FTTH application in which OLT devices are connected to the management switch and ONU, and a splitter is deployed between them. This PON network system can provide various services to meet different network requirements, including IPTV, VOIP, IP cameras, and many others.


OLT device in FTTH application


What is ONU (Optical Network Unit) / ONT (Optical Network Terminal)?


The ONU makes the optical signals to electrical signals conversion from fiber optic transmissions and then forwards this data to individual subscribers, generally over a distance from the ONU and the end-user's access port or to another access network. In addition, the ONU can send, aggregate, and reorganize various types of data from the user and transmit it to the upstream OLT. It can also organize, optimize, and reorganize the data flow to improve network transmission efficiency.




The OLT also supports bandwidth allocation to facilitate the smooth transmission of upstream data to the OLT, which typically arrives in bursts from the customer. ONU connections can be made using various methods and cable connections, including twisted-pair copper, coaxial cable, fiber optic, or wireless access points. End-user equipment can also be called an ONT, and ONT and ONU equipment are essentially the same. In terms of expression, ONT comes from the ITU-T terminology, while ONU comes from the IEEE terminology, and although defined by different standards bodies, they both refer to the user terminal equipment in an EPON system. ONT and ONU are slightly different in the network due to their other locations.

Optical Distribution Network (ODN): Connectivity Between ONU & OLT


ODN is also an indispensable part of the PON system, which serves as the physical transmission medium between the ONU and the OLT and the maximum transmission distance reach up to 20 km or more through optical fiber for connection. In ODN, fiber optic cables, connectors, passive optical splitters, and auxiliary components work together. ODN is mainly divided into five parts(in bold below): Feeder fiber begins at the optical distribution frame (ODF) in the central office telecom room and ends at the optical distribution point used for long-haul coverage. Distribution fiber extends from the optical distribution point to the optical access point, providing fiber optic connectivity to the immediate area. The in-home fiber then connects the optical access point to the ONT to achieve fiber to the home. In addition, ODN is the critical channel for PON data transmission, with its quality directly impacting the PON system's performance, reliability, and scalability.



OLTs, ONUs, or ONTs and ODNs are now widely used in FTTH as critical components of PON networks, and the unique benefits of passive optical networks include reduced cabling resources (no active components) and flexible media delivery, making them more suitable for home Internet, voice and video solutions. In terms of applications, several university campuses and commercial environments can also use passive optical networking solutions to build more cost-effective and efficient networks. In the future, as PON technology continues to evolve, there will be many more potential application areas that will be expanded.


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