Decoding OLT, ONU, ONT, and ODN in PON Network
PON (passive optical network) is a fiber-optic network that employs a point-to-multipoint topology and fiber optic splitters to transmit data from a single source to multiple user endpoints. Unlike an Active Optical Network (AON), where multiple customers are linked to a single transceiver through a branching tree of fibers and passive splitter/combiner units, a PON operates solely in the optical domain without the need for power.
PON Network Components
In the PON network, there is an OLT at the service provider's central office and a number of ONU devices or optical ONT devices near end users, as well as the optical splitter (SPL). In addition, the ODN is also used during the transmission between OLT and ONU/ONT.
The following illustration shows the network architecture of PON:
SNI: Service Node Interface
IFpon: PON Interface
UNI: User to Network Interface
CPE: Customer Premises Equipment
Optical Line Terminal (OLT)
OLT is short for optical line terminal, which is a device used to connect olt fiber and transfer signals. PON OLT is the starting point, which is connected to an aggregation switch through Ethernet cables. It makes a significant difference in PON.
OLT Device Components
The OLT equipment typically includes a rack, CSM (Control and Switch Module), ELM (EPON Link Module, PON card), redundancy protection with power supply modules, and fans. Notably, the PON card and power supply support hot-swapping. The OLT operates in two directions: upstream (distributing data and voice traffic from users) and downstream (receiving data, voice, and video traffic from the network and sending it to all ONT modules on the Optical Distribution Network, ODN).
The OLT device is a crucial component in PON network, serving as a multi-service providing platform that simultaneously supports IP services and traditional TDM services. Positioned at the edge of the Metropolitan Area Network (MAN) or the exit of the community access network, it consolidates access services and transmits them to the IP network.
In addition to its role in service aggregation, the OLT also functions as a centralized network management platform. It enables device-based network element management, service-based security management, and configuration management. The OLT can monitor and manage devices and ports, facilitate service provisioning and user status monitoring, and allocate bandwidth according to the Quality of Service (QoS) and Service Level Agreement (SLA) requirements of different users.
Its functionalities include:
1. Sending Ethernet data to ONU in broadcast mode.
2. Initiating and controlling the ranging process, and recording ranging information.
3. Allocating bandwidth for ONU, controlling the start time of ONU data transmission, and regulating the size of the sending window.
The OLT device collaborates with various types of ONU to establish diverse access networks such as FTTC, FTTH, FTTO, FTTM, etc. Signals carrying a variety of services are aggregated at the central office and transmitted to end-users in the access network according to a specific signal format. Simultaneously, signals from end-users are routed to various service networks based on different service types.
In the figure below, we illustrate an FTTH application of the OLT3610-08GP4S. The OLT device is connected to the management switch and ONU, with a splitter positioned between the OLT and ONU. The entire PON infrastructure can deliver multiple services such as IPTV, VOIP, IP Camera, etc., catering to several families.
Optical Network Unit (ONU) / Optical Network Terminal (ONT)
ONU converts optical signals transmitted via fibers into electrical signals, which are then forwarded to individual subscribers. Typically, there exists a distance or another access network between the ONU and the end-user's premises. Additionally, the ONU can send, aggregate, and groom various types of data originating from the customer and transmit it upstream to the OLT. Grooming involves optimizing and reorganizing the data stream to enhance delivery efficiency.
The OLT supports bandwidth allocation, facilitating the smooth delivery of data upstream to the OLT, often arriving in bursts from the customer. ONUs can be connected using various methods and cable types, including twisted-pair copper wire, coaxial cable, optical fiber, or through Wi-Fi. End-user devices may also be referred to ONT. In essence, ONT device and ONU device are the same. ONT is an ITU-T term, whereas ONU is an IEEE term. Belonging to different standard bodies, they both denote user-side equipment in the EPON system. However, in practice, there is a slight difference between ONT and ONU based on their location.
Optical Distribution Network (ODN)
ODN, an integral part of the PON system, provides the optical transmission medium for the physical connection of the ONUs to the OLTs with 20 km or farther reach. Within the ODN, fiber optic cables, fiber optic connectors, passive optical splitters, and auxiliary components collaborate with each other. The ODN specifically has five segments which are feeder fiber, optical distribution point, distribution fiber, optical access point, and drop fiber. The feeder fiber starts from the optical distribution frame (ODF) in the central office telecommunications room and ends at the optical distribution point for long-distance coverage. The distribution fiber from the optical distribution point to the optical access point distributes optical fibers for areas alongside it. The drop fiber connects the optical access point to ONTs, achieving optical fiber drop into user homes. In addition, ODN is the very path essential to PON data transmission and its quality directly affects the performance, reliability, and scalability of the PON system.
OLT, ONU or ONT, and ODN are the main components in a PON network, which has been widely used in FTTH applications thus far. The reduced cabling infrastructure (with no active elements) and flexible media transmission make passive optical networks more ideal for home Internet, voice, and video applications. Additionally, passive optical networks can also be applied in college campuses and business environments, providing cost-effective solutions. As PON technology has continued to improve, the potential applications have expanded as well.