Optical splitter is the core optical device in passive optical network (PON) system, widely used in fiber-to-the-home (FTTH), which has two different distribution methods in FTTH network: centralized distribution and cascaded distribution.
Centralized distribution
Centralized distribution means that the optical splitters between optical line terminal (OLT) and optical network unit (ONU) are in parallel, basically in the form of “OLT → optical splitter → ONU”, where the splitting ratio of optical splitter is usually 1:32. In centralized distribution, the optical splitters are centrally distributed in fiber optic distribution boxes and In centralized distribution, optical splitters are centrally distributed in fiber optic distribution boxes and connected directly to the OLT at the central office through a single fiber, while the other end is connected to multiple ONTs at the subscriber end through multiple fibers. Its use is generally divided into four cases:
- placed in the bureau-end server room.
- placed in the cell room.
- placed in the cell optical handover box.
- directly placed in the building.
Centralized distribution has the advantages of high flexibility, low cost and easy maintenance, and is generally suitable for use in the city center or town with a high concentration of users.
Cascaded distribution
Cascaded distribution means that the optical splitters between optical line terminals (OLTs) and optical network units (ONUs) are cascaded, basically in the form of “OLT → optical splitter 1 → optical splitter 2 → ONU”, where the splitting ratio of optical splitter 1 is usually 1:4 and that of optical splitter 2 is usually 1:8. In cascaded In cascaded distribution, optical splitter 1 is usually installed near the central office, and optical splitter 2 is usually installed near the user side, such as in the building. Its use is generally divided into three cases.
- primary optical splitter is placed in the bureau-end server room, and secondary optical splitter is placed in the cell optical handover box.
- primary optical splitter placed in the cell room, secondary optical splitter placed in the cell optical handover box.
- The primary optical splitter is placed in the cell optical handover box, and the secondary optical splitter is placed in the building.
Cascade distribution has the advantages of low user access cost and flexible adjustment of splitting ratio, generally suitable for use in villages with more dispersed users.
Difference Centralized Splitting vs Cascading Splitting ?
Optical fiber splitter can be placed at different locations in a PON based FTTH network, which involves using either a centralized (single-stage) or cascaded (multi-stage) tap configuration in the distribution portion of the FTTH network. In fact, both approaches have their own advantages and disadvantages. So which one should you deploy? Below are comparison
Centralized Splitting
Centralized distribution typically uses a combined split ratio of 1:64, 1:2 splitters at the local end, and 1:32 in the cabinet. these single-stage fiber splitter can be placed in multiple locations in the network or installed in a central location. In most cases, however, centralized plc splitter are placed in the external plant (OSP) to reduce the total amount of fiber required. The active port of the OLT (optical line terminal) in the central office (CO) will connect/splice to the fiber leaving the CO. The fiber passes through a different enclosure to the input port of a fiber optic splitter that is normally placed in the cabinet. The output port of this fiber splitter enters the FTTH distribution network and reaches the potential customer’s home through different enclosures and indoor/outdoor terminal boxes called ONTs (Optical Network Terminals). Thus, in this centralized split topology, the PON connects one OLT port to 32 ONTs.
Cascading Splitting
The cascaded splitting method has no fiber splitter at the central office. The OLT ports are directly connected / spliced to the external plant fiber. The first stage splitter (1:4 or 1:8) is installed in a closed location not far from the central office. The input of the first stage fiber splitter is connected to the OLT fiber from the central office. The second stage fiber taps (1:16 or 1:8) are located in a termination box very close to the customer’s location, covering 8 to 16 homes per tap. The input to these PON taps is the fiber from the output of the first stage taps described above.
From above description, it is clear that in a centralized split network, all PON Splitters are located in a closed closure, which will maximize OLT utilization and provide a single point of access for troubleshooting. However, since the splitter must be terminated to the customer via separate connectors or splices, the cost of distribution cables will be very high.
In cascaded splitting, PON splitters are located in two or more different closures, which minimizes the number of fibers that need to be deployed to provide service. However, this can result in inefficient use of OLT PON ports and may increase testing and turn-up time for customers.
How To Choose Centralized Splitting Or Cascading Splitting ?
FTTH network architecture will depend on a variety of factors, except the deployment difference of centralized vs. cascading, your budget and expected future scalability will be considered too.
In addition, the density of your customer base is one of the most important deciding factors. In urban areas, distributed splitting will be the best option to quickly scale and connect many customers. On the other hand, a centralized split is better given the flexibility of rural or less populated areas.
