How to Design Your FTTH Network Splitting Level and Ratio?

Introduction

Within the realm of fiber-optic communication, designing an effective Fiber-to-the-Home (FTTH) network is a task of paramount importance. The FTTH network serves as the infrastructure enabling data transmission in the form of light signals over optical fiber from the operator’s switching equipment directly to a home or business. Key components such as the Optical Line Terminal (OLT), Optical Network Terminals (ONTs), and particularly optical splitters contribute significantly to the performance of the network. To deploy a successful FTTH network, one must consider factors such as the choice of splitter, splitting level, and splitting ratio. This guide delves into these pivotal aspects, offering a comprehensive understanding of FTTH network design.

Choosing the Right FTTH Splitter: PLC vs. FBT

Optical splitters play an instrumental role in the Passive Optical Network (PON), enabling a single PON interface to be shared amongst multiple subscribers. The two primary types of optical splitters employed in the current FTTH network design are Planar Lightwave Circuit (PLC) splitters and Fused Biconical Taper (FBT) splitters. Here's a tabulated comparison of these two types:

With the progressive growth of FTTH worldwide, the requirement for larger split configurations, such as 1x32, 1x64, and so forth, has increased. These configurations allow service to a greater number of subscribers. PLC splitters, offering precise and even splits with minimal loss in a compact package, are typically a more suitable solution for today’s FTTH networks compared to FBT splitters.

Designing FTTH Network Splitting Levels

Understanding the design of the Passive Optical Network (PON) splitting level is crucial for implementing a successful FTTH network. The PON comprises the optical fiber infrastructure of the FTTH network, constituting a point-to-multipoint fiber optical network with no active elements in the signal's path. It utilizes a shared optical fiber to connect the central office with a passive optical splitter capable of accommodating multiple optical connections with customers. This connection scheme allows for splitting designs that can be centralized (one-level) or cascaded (two-level or more).

Centralized Splitting

A centralized splitting approach often uses a 1x32 splitter situated in a Fiber Distribution Hub (FDH). This splitter is directly linked via a single fiber to an OLT in the central office. On the opposite side of the OLT splitter, 32 fibers are routed to 32 customers' homes and are connected to an ONT. In this manner, the PON network connects one OLT port to 32 ONTs. Centralized splitting is often used in densely populated areas like city centers or towns due to its higher flexibility, lower operational costs, and easier access for technicians.

Cascaded Splitting

A cascaded splitting design might involve a 1x4 splitter located in an outside plant enclosure, directly connected to an OLT port in the central office. Each of the four fibers exiting this level 1 splitter is routed to an access terminal housing a 1x8 level 2 splitter. In this case, there would be a total of 32 fibers (4x8) connecting 32 homes. While the two-level approach is common, more than two splitting levels, referred to as multi-level splitting, are possible in a cascaded system, with a variable overall split ratio such as 1x16 = 4x4, 1x32 = 4x8, or 1x64 = 4x4x4. The cascaded splitting solution is typically employed in curb or village locations to cover a wide range of Optical Distribution Network (ODN) nodes, conserve resources, and save costs.

While both centralized and cascaded splitting have their respective benefits and drawbacks, the choice between the two will depend on a careful evaluation of the specific requirements and constraints of the network deployment area.

FTTH Network Splitting Ratio Design

The design of the splitting ratio in an FTTH network is a pivotal aspect that directly impacts the efficiency and effectiveness of the network. Splitters employed in PON systems are typically uniform power splitters with a 1:N or 2:N splitting ratio, where 'N' is the number of output ports. In this context, the optical input power is uniformly distributed across all output ports.

Generally, splitters with a 1:N ratio are deployed in star networks, while those with a 2:N splitting ratio are deployed in ring networks to provide physical network redundancy. The specific design of your network will influence the choice of splitter ratio.

Star and Ring Networks

Star and ring networks represent two distinct network topologies, each with its unique advantages. In a star network, all nodes are individually connected to a central point, typically by optical fibers. This layout allows for easy addition and removal of nodes without disrupting the entire network. On the other hand, ring networks connect all nodes in a closed loop configuration, where each node is connected to its adjacent nodes. This setup provides robustness and redundancy since the network can continue to function even if one link breaks down.

For FTTH networks and other PON networks, the star configuration with a 1:N splitting ratio architecture is most commonly applied, providing a balance between network robustness, ease of maintenance, and cost-effectiveness.

Commonly Used FTTH Splitters

When it comes to FTTH splitter design, 1x32 and 1x64 OLT splitters are the most commonly used in centralized splitting solutions, due to their ability to serve a large number of customers while maintaining an efficient network operation. On the other hand, 1x4 and 1x8 OLT splitters are often used in cascaded splitting solutions due to their suitability for widely dispersed ODN nodes.

As a rule of thumb, the longer the transmission distance, the lower the splitting ratio should be used. For instance, when the splitting ratio is 1:32, your network can receive a satisfactory fiber optic signal with a transmission distance of 20 km. If the distance between the OLT and the ONT of your network is relatively short, say 5 km, a 1:64 splitting ratio can be considered.

Conclusion: FTTH Network Design Considerations

Designing an FTTH network involves numerous considerations, particularly concerning the selection of the appropriate splitter, and the design of the splitting level and ratio. Each network's unique requirements will inform the decisions made in these areas.

Both PLC and FBT splitters offer unique advantages, with the former generally preferred due to their broad wavelength range, smaller dimensions, and equal splitting ratio. The latter, although less preferred, offers cost advantages for small channel splitters.

The choice between centralized and cascaded splitting architectures also depends on the specific context and objectives of the network. Centralized splitting offers greater flexibility, lower operational costs, and easier access for technicians, making it ideal for dense urban areas. Cascaded splitting, on the other hand, may yield a faster return-on-investment with lower first-in and fiber costs, making it suitable for less densely populated areas.

In terms of splitting ratio design, the rule of thumb is to use a lower splitting ratio for longer transmission distances to ensure reliable signal transmission. The 1:N splitting ratio is most commonly applied in star configurations, which are commonly used in FTTH and other PON networks.

FTTH network design is a complex process that requires careful consideration of various factors. It's crucial to balance the demands of serving a large number of customers, maintaining network efficiency, and managing costs. With proper planning and design, it's possible to build an FTTH network that meets these challenges and serves your customers effectively.