Passive optical network (PON) based FTTH access network is a point-to-multipoint, fiber to the premises network architecture in which passive optical splitters are used to enable a single optical fiber to serve multiple premises. The optical fiber splitter can be placed in different locations of the PON based FTTH network, which involves using centralized (single-stage) or distributed (multi-stage) splitting configurations in the distribution portion of the FTTH network. In fact, both methods have their own advantages and disadvantages. Then which one should you deploy? A comparison between centralized splitting and distributed splitting will be explored in this article.
A centralized splitting approach generally uses a combined split ratio of 1:64 with a 1:2 splitter in the central office, and a 1:32 in a cabinet. These single-stage fiber splitters can be placed at several locations in the network or housed at a central location. But in most cases, the centralized fiber splitters are placed in the outside plant (OSP) to reduce the amount of overall fiber required. The OLT (optical line terminal) active port in the central office (CO) will be connected/spliced to a fiber leaving the CO. This fiber passes through different closures to reach the input port of the fiber splitter, normally placed in a cabinet. The output port of this fiber splitter goes to the FTTH distribution network, reaching the homes of potential customers through different closures and indoor/outdoor terminal boxes known as ONT (Optical Network Terminal). Thus, the PON connects one OLT port to 32 ONTs in this centralized splitting topology.
Unlike centralized splitting topology, a distributed splitting approach has no fiber splitters in the central office. The OLT port is connected/spliced directly to an outside plant fiber. The first level of splitting (1:4 or 1:8) is installed in a closure, not far from the central office. The input of this first level fiber splitter is connected with the OLT fiber coming from the central office. The second level of fiber splitters (1:16 or 1:8) resides in terminal boxes, very close to the customer premises with each splitter covering 8 to 16 homes. The inputs of these PON splitters are the fibers coming from the outputs of the first level splitters described above.
From the introduction of centralized splitting and distributed splitting networks above, it’s known that in centralized splitting networks, all PON splitters are located in one closure, which will maximize OLT utilization and provide a single point of access for troubleshooting. But since optical splitters must be terminated to the customer either through individual splices or connectors, the cost of distribution cables will be very high. In terms of distributed splitting methods, the PON splitters are located in two or more different closures, which will minimize the amount of optical fibers that needs to be deployed to provide service. But it may create inefficient use of OLT PON ports and may increase the testing and turn-up time of customers. The advantages and disadvantages of centralized splitting and distributed splitting networks are summarized in the table below:
|FTTH Splitting Types||Advantages||Disadvantages|
|Centralized Splitting||OLT utilization (pay as you grow)||More distribution fiber|
|Future proof & easy to change technology||Larger network elements in the OSP|
|Monitoring & maintenance||Possibly additional infrastructure|
|Distributed Splitting||Lower capital expense for customer connection||More actives and more splitters|
|Reduces splitter cabinet requirements||Less flexible network|
|Flexibility in split ratios in serving area||Fewer monitoring & maintenance capabilities|
Before deciding which splitting methods to use in a PON-based FTTH network, please always consider every unique aspect of your network case. Since both centralized splitting and distributed splitting networks have their pros and cons, the best architecture is the one that meets the requirements and expectations of the provider by reducing capital expenses, optimizing long-term operational expenses, and making a future-proof network that can cope with new technologies without dramatic changes.