The Application Scenarios of Non-Uniform Optical Splitting in ODN

Due to the low insertion loss of the cascading ports of non-uniform optical splitters, they are suitable for chain-type networking in ODN links. This article will discuss the specific scenarios where non-uniform optical splitting is applicable, as well as the networking solutions for these specific scenarios.

1. Analysis of Scenarios for Non-Uniform Optical Splitting Adaptation

The introduction of traditional uniform optical splitting ODN typically adopts a tree-shaped networking structure for optical cable line routes. Each distribution box is analogous to a “leaf” and generally allocates 2 cores (one main and one backup), as shown in Figure 1. This structure can accommodate the access requirements of various scenarios.

If ODN adopts non-uniform optical splitting, a single-core optical cable can be used to cascade the distribution boxes along the optical cable route, forming a single-chain structure, as shown in Figure 2. However, when there are numerous branching routes in the optical cable routing or when the branch route lengths are long, the single-chain structure can lead to a doubling of the link length due to the winding routing of the optical fiber links. This, in turn, increases the project cost and reduces the safety of the link.

Figure 2: Non-Uniform Optical Splitting Scheme 1

To reduce the winding routing of optical fiber links, the ODN networking with non-uniform optical splitting can adopt dual-chain and multi-chain networking. The terminal sections of optical cable routing without branches use single-core optical cables, while the sections with branches use regular optical cables, as shown in Figure 3. The 1×2 for dual chains or 1×4 for multi-chains optical splitters are centrally placed in the distribution box/fiber distribution box.

Figure 3: Non-Uniform Optical Splitting Scheme 2

In the actual introduction of ODN optical cable routes, the branching of optical cable routing is very complex. Organizing the network in the manner shown in Figure 3 would make the setup and connection relationships of optical splitters complex. This complexity poses significant challenges for engineering design, construction, and ODN maintenance. Therefore, when there are many branching routes in the optical cable routing, non-uniform optical splitting is not suitable for ODN.

Non-uniform optical splitting is suitable for use in scenarios where the optical cable routing forms a chained structure. Because non-uniform optical splitting typically uses single-core optical cables, it is particularly suitable for scenarios where the optical splitting extends from the distribution box to the terminal, as shown in Figure 4. Traditional uniform optical splitting ODN distribution boxes are equipped with reserved fiber cores, making non-uniform optical splitting especially suitable for extending from the distribution box in the original route to the terminal.

Figure 4: Networking Utilizing Reserved Fiber Cores

2. Non-Uniform Optical Splitting ODN Networking

Based on the distribution of branches in the optical cable routing and the number of cascaded distribution boxes on each optical cable link, the ODN with non-uniform optical splitting is suitable for dual-chain or single-chain networking. When 4 to 6 distribution boxes are cascaded on the optical cable link, a single-chain network is suitable for ODN. When 2 to 3 distribution boxes are cascaded, a dual-chain network is suitable for ODN.

2.1 Single-Chain Non-Uniform Optical Splitting

The optical cable networking for single-chain non-uniform optical splitting is shown in Figure 5. If there are short-distance branch optical cables in the routing, the distribution boxes of the branch optical cables (distribution box ⑤ in Figure 5) can be cascaded into the chain.

Figure 5: Single-Chain Structure

When the number of distribution boxes on the chain exceeds 6, it is advisable to use non-uniform optical splitting for the terminal 4 to 6 distribution boxes, while the other distribution boxes should use uniform optical splitting.

2.2 Dual-Chain Non-Uniform Optical Splitting

When extending 2 to 3 distribution boxes from an existing optical cable route to the terminal, as shown in Figure 4, a 1×2 uniform optical splitter can be set in the distribution optical cross-connect to merge the two support branches under the same distribution optical cross-connect into one PON port. This way, the ODN networking becomes a dual-chain structure. The connection relationships in the dual-chain structure of the ODN, as shown in Figure 6, are illustrated in Figure 4.

In the dual-chain structure shown in Figure 4, the two chains only need to belong to the same distribution optical cross-connect, and they do not necessarily have to terminate in the same introduced optical cable under the distribution optical cross-connect.

Figure 6: Dual-Chain ODN Example 1

In the optical cable networking structure shown in Figure 5, if the branch optical cable of distribution box ⑤ is relatively long, it is not advisable to cascade distribution box ⑤ into the chain. Instead, distribution boxes ⑤ and ④ can be independently formed into a support branch, as shown in Figure 7.

Figure 7: Dual-Chain Optical Cable Networking Example 2

For the ODN in Figure 7, it is advisable to form a dual-chain structure as shown in Figure 8. In the figure, distribution box ④ is equipped with a 1×2 uniform optical splitter and a 1×9 optical splitter.

Figure 8: Dual-Chain ODN Example 2

3. Comparison of Investment with Uniform Optical Splitting

If the scenario shown in Figure 5 adopts uniform optical splitting, the optical cable networking structure is as shown in Figure 9.

Figure 9: Optical Cable Networking with Uniform Optical Splitting

Based on Figures 5 and 9, the investment in ODN under different optical splitting methods can be calculated (with reference to the cost level in a certain province in the central region in 2023), as shown in Table 1. The lengths of various optical cable sections in Figure 5 are referenced from Figure 9.

Table 1: Investment Comparison

From Table 1, it can be observed that when utilizing the spare fibers of existing optical cables, the investment with non-uniform optical splitting can be significantly reduced, and the longer the length of the spare fibers in the existing optical cables, the greater the reduction in investment. Since non-uniform optical splitting uses far fewer fiber cores than uniform optical splitting, even if the lengths of the constructed optical cables are the same, the cost of ODN with non-uniform optical splitting is only about 89% of that with uniform optical splitting.

4. Other Characteristics of Uniform Optical Splitting

Apart from the differences in networking structure and investment, ODN with non-uniform optical splitting has the following distinctions from ODN with uniform optical splitting:

• Low Safety: Due to its chained structure, any failure in the ODN link will affect users below the point of failure.
• Easy Management: As each cascade of the splitter introduces approximately 2.0 dB of insertion loss, the connection relationship between distribution boxes and users in the ODN link can be inferred from the received optical power of the user’s ONU. In addition, the fiber cores terminated at the distribution optical cross-connect in non-uniform optical splitting are often only 10% to 20% of those in uniform optical splitting, significantly reducing the difficulty of fiber core management.
• Low Construction Difficulty: The construction often involves only single-core optical cables, and terminations are only required at the distribution box. In some cases, there may not even be optical cable joints in the line.
• Poor Expansion Capability: Each distribution box in uniform optical splitting usually reserves one fiber core for expansion. However, the expansion capacity of non-uniform optical splitting is limited. Expansion is possible only when there is a certain redundancy in the optical power of the link, by either replacing a 1×5 splitter with a 1×9 or adding splitters to the link.
• Poor Business Integration Capability: Uniform optical splitting ODN has a higher number of fiber cores, making it capable of integrating various services such as wireless fronthaul and dedicated data lines. On the other hand, non-uniform optical splitting ODN often uses single-core optical cables, limiting its ability to connect only to PON services.

5. Recommendations for the Application of Non-Uniform Optical Splitting

In summary, non-uniform optical splitting is not suitable for scenarios requiring the integration of various services such as wireless, dedicated data lines, and home broadband, or scenarios with many branching routes in the optical cable routing.

Non-uniform optical splitting is suitable for scenarios where the optical cable adopts a chained networking structure, the users’ services are connected via PON, and the security requirements for access services are not high. It is particularly well-suited for situations where existing distribution boxes have spare fiber cores extending to the terminal.

Currently, non-uniform optical splitting primarily uses single-core optical cables, leading to lower link security, difficulties in expansion, and challenges in adapting to local winding in the fiber optic link (such as the access link of distribution box ⑤ in Figure 5). Using dual-core optical cables might be a better choice to address these challenges.