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How to Design Your FTTH Network Splitting Level and Ratio?

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Posted on June 1, 2016
September 27, 2020
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In FTTH network design, optical splitters play an important role in the passive optical network (PON) by allowing a single PON interface to be shared among many subscribers. Optical splitters are installed in each optical network between the PON OLT (Optical Line Terminal ) and the ONTs (Optical Network Terminals) that the OLT serves. To design and deploy an FTTH network successfully, the FTTH splitting level and ratio are a problem that should be solved.

FTTH network design

Choose FTTH Splitter: PLC Splitter or FBT Splitter?

Choosing the right FTTH splitter is the first step to start the splitting level and ratio design. There are two types of optical splitters in our current FTTH network design—PLC splitter and FBT splitter. The differences between these two optical splitter types determine which one is more suitable for FTTH networks. Here is a simple comparison of them:

Parameters PLC Splitter FBT Splitter
Wavelength Range 1260-1650 nm Single/dual/triple window
Splitting Ratio Equal division (1x2/4/8/16,etc.) Equal or unequal division
Dimensions Small Large size for multi-channel
Wavelength Sensitivity Low High
Cost Low splitting channel, high price Price is lower for small channel splitter

With the rapid growth of FTTH worldwide, the requirement for larger split configurations (1x32, 1x64, etc) in these networks has also grown in order to serve mass subscribers, since PLC splitters offer very accurate and even splits with minimal loss in an efficient package, they offer a better solution for today’s FTTH networks than FBT splitters.

FTTH Network Splitting Level Design

The PON is the optical fiber infrastructure of an FTTH network. It's a point-to-multipoint fiber optical network with no active elements in the signal’s path. It uses a shared optical fiber to connect the central office with a passive optical splitter that is able to accommodate multiple optical connections with customers. The PON splitting may be achieved by centralized splitting (one-level) or by cascaded splittings (two-level or more).

A centralized approach typically uses a 1x32 splitter located in a fiber distribution hub (FDH). The splitter is directly connected via a single fiber to an OLT in the central office. On the other side of the OLT splitter, 32 fibers are routed to 32 customers’ homes, where it is connected to an ONT. Thus, the PON network connects one OLT port to 32 ONTs.

ftth network design, centralized splitting

A cascaded approach may use a 1x4 splitter residing in an outside plant enclosure. This is directly connected to an OLT port in the central office. Each of the four fibers leaving this lever 1 splitter is routed to an access terminal that houses a 1x8 level 2 splitter. In this scenario, there would be an also total of 32 fibers (4x8) reaching 32 homes. It is possible to have more than two splitting levels which is called multi-level splitting in a cascaded system, and the overall split ratio may vary (1x16 = 4x4, 1x32 = 4x8, 1x64 = 4x4x4).

ftth network design, centralized splitting

A centralized architecture typically offers greater flexibility, lower operational costs, and easier access for technicians. A cascaded approach may yield a faster return-on-investment with lower first-in and fiber costs. Usually, the centralized splitting solution is used in the crowded city center or town areas, in order to reduce cost and easy to maintain the optical distributed network (ODN) nodes. On the other hand, the two-level and the multi-level cascaded splitting solutions are used in curb or village places, to cover widely ODN nodes, conserve resources, and save the money.

FTTH Network Splitting Ratio Design

The most common FTTH splitters deployed in a PON system is a uniform power splitter with a 1:N or 2:N splitting ratio (N=2~64), where N is the number of output ports. The optical input power is distributed uniformly across all output ports.

Different ratio splitters may perform differently in your network, which affects the splitting ratio design in FTTH networks. According to the FTTH splitting ratio design mentioned above, a 1x32 or 1x64 OLT splitter may be used in the centralized splitting solution. But a 1x4 and 1x8 OLT splitter may be used more often in the cascaded splitting solution. Usually, there are two main splitting ratio solutions in FTTH-PON networks. when the splitting ratio is 1:32, your current network can receive a qualified fiber optic signal in 20 km. If your distance between OLT and ONU is small, like in 5 km, you can also consider about 1:64 splitting ratio.

Conclusion

When to design your FTTH network splitting level, in fact, centralized splitting and cascaded splitting both have its advantages and disadvantages. We had to evaluate these factors and select an appropriate splitting level for our network. As for splitting ratio design, to ensure a reliable signal transmission, the longer the transmission distance, the lower the splitting ratio should be used.