Today, we are here talk about the centralized splitting and distributed splitting in ODN network structure.

Centralized splitting

Brief

Centralized splitting means that all the OLTs are placed in the Central office, and all the fibers are pulled out from the office room, or a small amount of backbone fibers are pulled to a central point, such as an optical distribution cabinet, and then the splitter is used to pull the distribution cables and drop cables to the user’s home.

In the ODN network structure of centralized splitting, the backbone cables are distributed by splitter, and one core fiber is allocated to one splitter, corresponding to a PON port on the OLT. Therefore, the amount of backbone cables used is relatively small. The distribution cables are allocated according to the number of users, usually with a ratio of 1:1 or 1:0.8 for residential areas, and the amount used is relatively large. Since centralized splitting achieves the centralized placement and positioning of the splitter.

Advantages

• It saves PON resources. Centralized splitting can maximize the efficient use of expensive OLT modules. Under this method, each user is directly connected to the splitter of the optical distribution cabinet through a fiber optic cable, and the splitter can flexibly cover the users, rather than one splitter can only correspond to one building or one unit. This achieves 100% efficiency and reduces the number of splitters, saving PON resources. This also enables a wider physical distribution of OLT ports, which is important in the initial stages of FTTH construction when the user activation rate is relatively low.

• It is convenient for network testing. The second advantage of centralized splitting is its ability to provide rapid testing and fault diagnosis. As centralized splitting achieves point-to-point fiber optic cable connection from the Central Office end to the user’s home, there are no other devices except for patch connecting such as adapters. Therefore, optical power meters, OTDR, and other tools can quickly test the attenuation and fault location of the optical path of each user, thereby improving the speed and quality of installation and maintenance.

• It can minimize splitter signal loss. Every time an optical signal encounters a network component connection such as a splitter, it will cause a certain degree of signal loss. Therefore, when multiple splitters are cascaded together, losses will occur at each device. The cumulative loss effect will reduce the distance that the signal can be transmitted, limiting the transmission distance of the fiber. Centralized splitting minimizes this signal loss by eliminating redundant connectors or adapters from the ODN network.

Of course, centralized splitting also has its drawbacks, such as: (1) It consumes more pipeline and fiber optic resources due to the large amount of distribution fibers used; (2) It requires a larger investment.

Distributed splitting 

Brief

Distributed splitting means that a building or a certain area is taken as a splitting point, and a main optical cable is led from the OLT to the splitting point, and then an optical cable is pulled from the splitting point to the user’s home.

In distributed splitting, the OLT device at the central office is connected to the corridor splitter through a backbone optical cable. One core backbone optical cable is connected to one splitter. There are several differences compared with centralized splitting: 1. The distribution cable is removed, and the backbone optical cable is extended to replace the distribution cable; 2. The splitter is changed from centralized to distributed and placed in the building corridor unit; 3. The splitter is placed in the corridor.

Advantages

1. Save optical cable and pipeline resources and reduce investment. This is the biggest advantage of this method. Since the splitter sinks to the corridor, the amount of distribution cable is eliminated. Although the amount of backbone optical cable increases appropriately compared with the centralized splitter, it can be ignored compared with the reduced distribution cable.

• Ensure user bandwidth. Since each unit or each building is equipped with a splitter, the number of users on each splitter is relatively small, so each user occupies a higher network bandwidth. Generally, the bandwidth of an EPON port is 1.25G. If there are 12 users in each unit of a 6-floor residential building, each user can get a bandwidth of 106M. The bandwidth of a GPON port is 2.5G, so the bandwidth allocated to each user is higher.

The disadvantage of distributed splitting is that because each building or each unit requires a splitter, one splitter corresponds to one PON port, so there is a high demand for PON ports. Generally, one PON port only covers one building or one unit. If there are few users in the building, the resources of the PON port cannot be fully utilized.

Applications of different optical splitting forms in various scenarios

From the above introduction, centralized and distributed
 optical splitting have their own advantages and disadvantages. In the construction of FTTH, we should choose the appropriate optical splitting method based on different factors such as community structure, user density, and building distribution. Here are several common scenarios for the application of centralized and distributed optical splitting, with the goal of minimizing investment and maximizing return while ensuring efficient construction.

Villa

Villa communities typically have scattered buildings with large distances between them, low user density, and high property management requirements that do not allow aerial cables. If distributed optical splitting is used, the backbone route will be longer, and each building will need a splitter, resulting in significant resource waste. Therefore, it is recommended to use centralized optical splitting in villa communities. A central point is set up within the community, and the backbone cable is led to the splitter at the central point, and then the distribution cable is led to the fiber distribution box in each building, and then the user’s indoor fiber cable is connected.

Multi-story residential communities

Multi-story residential communities are the most commonly encountered scenario in FTTH construction. The buildings are regularly distributed, and the distances between them are relatively short, with a high user density. There are many line scheme combinations available, and both centralized and distributed optical splitting can be used, depending on the distance between the community and the access network equipment room and whether there is an equipment room within the community. In FTTH construction planning, there are requirements for the setting of OLT equipment in the OLT equipment room. Generally, only one OLT equipment room is set up within a diameter of 5 km. Therefore, if the community does not have an OLT equipment room and is far away from the equipment room, it is recommended to use distributed optical splitting to save fiber optic resources. The main backbone cable is directly led from the OLT equipment room to the building corridor, and a splitter is installed in each building corridor, and then the fiber cable is led to the user’s indoor cable. If the community is close to the OLT equipment room or has the conditions to build an OLT equipment room, centralized optical splitting can be used, and the splitter is set up in the equipment room. The distribution cable is led from the equipment room to the fiber distribution box in the corridor, and then the user’s indoor fiber cable is connected. This simplifies the complexity of the community and internal building wiring, and reduces the construction and maintenance costs and difficulties.

High-rise residential communities

High-story residential communities have a high concentration of users and fewer buildings, and the buildings have a relatively high vertical height. If centralized optical splitting is used, the amount of distribution and indoor fiber cable required will be relatively large. Therefore, it is recommended to use distributed optical splitting. The main backbone cable is led from the OLT equipment room to the middle floors of the high-story buildings, and the number of splitters is determined based on the number of users, and then the fiber cable is led to the user’s indoor cable. This can greatly save the length of the indoor fiber cable and reduce the project investment. At the same time, reducing the amount of fiber cable can free up the valuable vertical corridor in high-story buildings and reduce the complexity of indoor fiber cable, which is beneficial to reducing construction and future maintenance difficulties.

Rural Areas

Rural areas have some similarities to villa communities in terms of building distribution, which is scattered and irregular, with a large distance between buildings and a low user density. Therefore, a centralized splitting form is suitable for use. Set up an optical convergence point in a relatively concentrated area, equip it with a splitter, and use it as the deployment point to cover the surrounding area.

One prominent issue with this transformation method is that the backbone optical cable needs to be laid from the OLT machine room in the urban area to the optical convergence point, which may result in a very long distance and high investment. The solution to this problem is to “make use of old resources”. Generally speaking, the construction of mobile base stations in rural areas by various operators during the first few years of mobile network construction was relatively complete, and optical cables have already been laid. The working of the base station generally only requires 2 or 4 cores, leaving many spare cores. Use these redundant fibers as the backbone optical cable for FTTH construction, and use the location of the mobile base station as the equipment room to cover the surrounding area, achieving the full utilization of resources.

Conclusion

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.