Application of Optical Splitter in PON Network: Comparison of Centralized Distribution and Cascaded Distribution

Optical splitter is the core optical device in passive optical network (PON) system, which is widely used in fiber to the home (FTTH). It has two different distribution modes in FTTH network: centralized distribution and cascading distribution.

Centralized Distribution

Centralized distribution means that the optical splitters between the optical line terminal (OLT) and the optical network unit (ONU) are parallel, and the basic form is "OLT→ optical splitter →ONU", where the optical splitter ratio is usually 1:32. In the centralized distribution, the optical splitter is centrally distributed in the fiber distribution box, and is directly connected to the OLT at the central office through a single fiber, and the other end is connected to multiple ONTs at the client end through multiple fibers. Its use is generally divided into four situations:

     1. Place it in the equipment room at the office end.

     2. Place it in the room of the community;

     3. Placed in the community light handover box;

     4. Place directly in the hallway.

The centralized distribution has the advantages of high flexibility, low cost and easy maintenance, and is generally suitable for the city center or town where users are concentrated.

Cascading Distribution

Cascade distribution means that the optical splitter between the optical line terminal (OLT) and the optical network unit (ONU) is cascaded, and the basic form is "OLT→ optical splitter 1→ optical splitter 2→ONU", where the optical splitter 1 is usually 1:4, and the optical splitter 2 is usually 1:8. In a cascading distribution, optical splitter 1 is usually installed near the central office end, and optical splitter 2 is usually installed near the client end, such as inside the corridor. Its use is generally divided into three situations:

1. The first-level optical splitter is placed in the office equipment room, and the second-level optical splitter is placed in the residential optical handover box;

2. The first-level optical splitter is placed in the community room, and the second-level optical splitter is placed in the community optical handover box;

3. The first-level optical splitter is placed in the community optical handover box, and the second-level optical splitter is placed in the corridor.

The cascading distribution has the advantages of low user access cost and flexible adjustment of optical splitting ratio, and is generally suitable for villages with scattered users.

The fiber splitter can be placed in different locations on the FTTH network based on the PON network, which involves using either a centralized (single stage) or cascading (multistage) shunt configuration on the distributed portion of the FTTH network. In fact, both approaches have their own advantages and disadvantages. So which one should you deploy? The comparison between centralized and cascading distributions will be explored below.

Centralized Distribution Network

In centralized distribution, the combined shunt ratio of 1:64 is generally adopted, with the splitter at the office end being 1:2 and the cabinet being 1:32. These single-stage fiber splitters can be placed in multiple locations in the network or installed in a central location. But in most cases, the centralized fiber splitter is placed in an external device (OSP) to reduce the total amount of fiber required. 

The OLT (optical line terminal) active port in the central room (CO) connects/splices to the optical fibers leaving the central room (CO). The optical fiber passes through different sealing devices to the input port of the optical fiber splitter usually placed in the cabinet. The output port of the fiber splitter enters the FTTH distribution network and reaches the potential customer's home through different closures and indoor/outdoor terminal boxes called ONT (Optical Network Terminals). Thus, in this centralized split topology, PON connects an OLT port to 32 OnTs.

Cascading Distribution Network

Unlike the centralized splitter topology, the cascading splitter method has no fiber splitter in the central office. OLT ports are directly connected/spliced to external plant fibers. The first level splitter (1:4 or 1:8) is installed in an enclosure not far from the central office. The input of the first-stage fiber splitter is connected to the OLT fiber from the central office. A second level fiber optic splitter (1:16 or 1:8) is located in the terminal box very close to the customer location, with each splitter covering 8 to 16 homes. The input to these PON splitters is the fiber from the output of the first stage splitter above.

Centralized Distribution And Cascading Distribution In FTTH-PON Network

From the above introduction to centralized and cascading distributed networks, we know that in a centralized distributed network, all PON splitters are located in a single closure, which maximizes OLT utilization and provides a single point of access for troubleshooting. But because the splitter must be connected to the customer via a separate connector or connector end, The cost of Distribution cable will be very high.

In a cascading distributed network, the PON splitter is located in two or more different closure devices, which minimizes the amount of fiber that needs to be deployed to provide service. However, this may result in inefficient use of the OLT's PON port and may increase testing and commissioning time for the customer.

How to choose between a centralized distribution network and a cascading distribution network?

In addition to the information on the differences between the two deployment approaches of centralized and cascading distributed networks, here are some tips on how to decide which one to choose. The FTTH network architecture will depend on a variety of factors, including your budget and expected future scalability.

In addition, the density of the customer base is one of the most important determinants. In urban areas, cascading distribution networks will be the best option for quickly scaling and connecting many users. On the other hand, given the flexibility of rural or less populated areas, centralized distribution networks are better.

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