Active Optical Networks (AON) and Passive Optical Networks (PON) make FTTH broadband connections possible. To date, most FTTH deployments in planning and deployment have used PON to save on fiber costs. PON has attracted much attention in recent years due to its low cost and high performance.

PON standards: GPON (Gigabit Passive Optical Network) and EPON (Ethernet Passive Optical Network). But no matter what type of PON, they all have the same basic topology. A GEPON system usually consists of an OLT (Optical Line Terminal) at the service provider’s central office and multiple ONU (Optical Network Units) or ONT (Optical Network Terminals) close to the end user as optical splitters. In addition, the transmission between OLT and ONU/ONT adopts an optical distribution network (ODN).

Introduction of Optical Line Terminal (OLT)

The heart of any PON system is the optical line terminal (OLT). Think of the OLT as the brain of the network; it is the concentration point for upstream and downstream traffic. The OLT is typically located in a central office or data center and is responsible for managing and controlling the entire PON infrastructure.

The OLT communicates with the optical network unit (ONU) or optical network terminal (ONT) at the user end, coordinating the distribution of data and ensuring that each connected user receives the appropriate information.

1. Equipment Components

Generally speaking, OLT equipment includes a rack, CSM (control and switching module), ELM (EPON link module, PON card), redundant protection-48V DC power module, or a 110/220V AC power module, fans, etc. Among these parts, the PON card and power supply support hot swapping, and another module is built-in. OLT has two float directions: upstream (distributes different types of data and voice traffic from users) and downstream (gets data, voice, and video traffic from the metropolitan area network or long-distance network and sends it to all ONT modules) in ODN superior. )

2. Functions and Applications

OLT equipment is an important central office equipment in EPON. It is a multi-service providing platform that supports both IP services and traditional TDM services. Placed at the edge of the metropolitan area network or the exit of the community access network, the access services are aggregated and transmitted to the IP network respectively.

In addition to providing service aggregation functions, OLT also serves as a centralized network management platform. Send Ethernet data to ONU in broadcast mode; initiate and control the ranging process, record ranging information; allocate bandwidth to ONU; that is, control the start time of ONU sending data and the size of the sending window.

OLT equipment cooperates with different types of ONUs to implement various access networks such as FTTP, FTTH, FTTB, and FTTC.

• Uplink the upper network to complete the uplink access to the PON (Passive Optical Network) network.
• User equipment ONU is connected through the ODN network (composed of optical fiber and a passive optical splitter). Realize the control, management, and ranging functions of user equipment ONU.

Optical Network Unit (ONU)/Optical Network Terminal (ONT)

ONU converts optical signals transmitted through optical fibers into electrical signals. These electrical signals are then sent to individual subscribers. Generally, there is a distance or other access network between the ONU and the end user premises. In addition, ONU can send, aggregate, and collate different types of data from customers and send it upstream to the OLT. Grooming is the process of optimizing and reorganizing the flow of data so that it can be delivered more efficiently. The OLT supports bandwidth allocation to smoothly deliver customers’ typically bursty data to the OLT. The ONUs can be connected via a variety of methods and cable types, such as twisted pair copper wire, coaxial cable, fiber optic, or Wi-Fi.

ONT and ONU are essentially the same. ONT is an ITU-T term and ONU is an IEEE term. They all refer to user-side equipment in the GEPON system. While both devices act as endpoints in a PON, optical network terminals (ONTs) generally refer to the user-side devices in a PON system that serves residential or enterprise customers. ONTs are typically used in scenarios where the PON extends to the customer premises. It is the final gateway for fiber optic connections to user equipment, providing necessary interfaces such as Ethernet ports, voice ports, and video ports for seamless connectivity.

Optical Distribution Network (ODN)

Optical Distribution Network (ODN) is the PON physical layer that connects OLTs to ONUs or ONTs. Provides optical transmission media for physical connection from ONU to OLT over distances of 20 kilometers or more. In ODN, fiber optic cables, fiber optic connectors, passive splitters, and auxiliary components work together.

ODN specifically has five parts: feeder fiber, optical distribution point, distribution fiber, optical access point, and introduction fiber. The feeder optical fiber starts from the optical fiber distribution frame (ODF) in the central office room and ends at the optical distribution point for long-distance coverage. A distribution fiber from an optical distribution point to an optical access point distributes fiber to the area next to it. Introduce optical fiber to connect the optical access point to the terminal (ONT) to realize the introduction of optical fiber into the user’s home. In addition, ODN is the critical path for PON data transmission, and its quality directly affects the performance, reliability, and scalability of the PON system.

1. Spectrum

The most important component is the beam splitter. An optical distribution network (ODN) mainly has primary splitting and secondary splitting, or centralized splitting and cascade splitting. The structure of primary light splitting is an OLT-optical splitter-ONU, and the optical splitters from OLT to ONU are all connected in parallel. When using primary splitting, the splitter is generally set at the intersection of the wiring.

The structure of secondary light splitting is OLT-optical splitter 1-optical splitter 2-ONU. Optical splitter cascades from OLT to ONU. When using a two-stage splitter, the first-stage splitter is usually set at the intersection of the optical paths of the wiring, and the second-stage splitter is usually set at the fiber distribution box (FDB).

2. Split Differences

There is only one beam splitter in a split. Therefore, a beam splitter with a large number of splitting ratios, such as 1:32 or 1:64, is a good choice. On the contrary, the secondary optical splitting is often 1:8 or 1:16 in the first-level splitter, and 1:4 or 1:8 in the second-level optical fiber distribution box.

3. Application Differences

Typically, primary splitting solutions are used in densely populated urban centers or urban areas to reduce costs and make it easier to maintain optical distribution network (ODN) nodes. On the other hand, the secondary optical splitting solution is used on roadsides or in rural areas and can cover a wide range of ODN nodes, saving resources and money.

Conclusion

OLT, ONU, ONT, and ODN are the main components of the GEPON system and have been widely used in FTTH applications. As the demand for high-speed, reliable broadband continues to grow, PON technology is a powerful and scalable solution with an expanding range of potential applications.