Architecture drives costs in FTTH networks, and there are different types to choose from: point-to-point, centralized split versus distributed split, star versus daisy-chain, and all-spliced versus pre-terminated connectivity. A typical PON covers an area of 20 kilometers in length.
Let’s look at the benefits of different approaches.
In a point-to-point network, nodes are directly connected with a single connection line. No additional functioning nodes are required and there is no redundancy, but this is a cost effective and reliable solution. This approach is typically utilized for business-grade services or backhaul for converged networks.
Bandwidth isn’t shared, so each port on a premise offers uninterrupted high speeds. Installation, maintenance and repairs are relatively easy. However, there is no option for branching out and adding more connections. Rollout can take longer and be more costly—and the approach is less than ideal for rural regions.
The following network types of FTTH access networks are all point-to-multipoint. The optical splitter used in PON-based point-to-multipoint networks can be placed at different locations in the network.
- Centralized splitting architecture
- Distributed split (cascaded) architecture
- Daisy-chain architecture
- Star architecture
- Optical fiber tapping
- Fiber indexing
Centralized splitting architecture
The centralized approach uses single-stage splitters in a central hub in a star or daisy-chain topology. This provides optimal flexibility in management of subscriber connections and utility of connected equipment—and the advantage of having an easily accessed testing point.
However, it requires a “fiber rich” network from the splitter location to the premises. Centralized splitting architecture has been used extensively to reach subscribers in initial FTTH deployments. This approach typically uses multiple 1×32 splitters located in a fiber distribution hub (FDH), which may be located anywhere in the network.
The 1×32 splitter is directly connected via a single fiber to a GPON optical line terminal (OLT) in the central office. On the other side of the splitter, 32 fibers are routed through distribution panels, splice ports, and/or access point connectors to 32 customers’ homes.
Here, they are connected to an optical network terminal. In this way, the centralized PON architecture connects one OLT port up to 32 ONTs. Cross-connection capability in the FDH makes it possible to connect any outlet port from the splitter to a port in the patch panel, which can bring savings on labor and material costs.
Centralized splitting also introduces a physical location at the center of the optical distribution network, which can conveniently be used for testing. However, in areas with lower uptake rates, building on a per-home basis becomes more costly.
To reduce costs and speed up deployments, alternatives need to be considered. Pre-terminated connectivity is one way of reducing deployment time. The other key solution is using distributed splitting.
Distributed split (cascaded) architecture
A cascaded architecture utilizes multiple splitters in series to achieve the overall desired split ratio. For example: a 1×4 splitter residing in an outside plant enclosure is directly connected to an OLT port in the central office.
Each of the four fibers leaving this Stage 1 splitter is routed to an access terminal that houses a 1×8, Stage 2 splitter. In this scenario, a total of 32 fibers (4×8) would reach 32 homes. It is possible to have more than two splitting stages in a cascaded system, and the overall split ratio may vary (1×16 =4×4; 1×32 = 4×8 or 8×4; 1×64 = 4x4x4).
This approach reduces the amount of fiber in the distribution area by moving part of the splitting process to the access point where the subscriber drops are connected. There is, however, a trade-off: a cascaded PON network typically has poorer OLT port utilization than a centralized split architecture.
Cascaded architectures are also highly dependent on the “take up rate” and the number of customers being fed from the PON. Research shows that FDH capacity can be reduced by 75 percent, allowing smaller cabinets, easier placement, and the prospect of moving from a cabinet to a splice closure.
The distribution fibers required have been reduced by 75 percent as well, lowering CapEx for cables, splice closures and splicing labor. The access point now includes a splitter, so a modest change here permits significant savings in the entire approach.