LuxHow an FDH cabinet and its splitters work
Cabinets
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Where the FDH sits, and the feeder that comes in
A fiber distribution hub, or FDH, is the cabinet where a few fibers from the central office meet the many fibers that run to homes. Inside, splitters share each feeder fiber among as many as 32 homes, and a technician connects each new customer with one patch cord. This guide walks through the cabinet from the feeder in to the ports out, then sizes one for a neighborhood.
In a PON network, the fiber between the operator and the home has two parts. The feeder runs from the central office or hut to the FDH and carries relatively few fibers. The distribution side runs from the FDH out along the streets, with a fiber for each home or terminal. The FDH is where the two meet: usually a pad-mounted cabinet at the edge of a neighborhood, or a smaller unit on a pole or a wall.
The feeder cable enters through the base. Its fibers are spliced to factory-made fibers with connectors on the end, either in the cabinet or in a closure nearby, so each feeder fiber ends on a connector ready for a splitter. An FDH holds no electronics and needs no power.
Splitter modules: 1:32, or 1:4 then 1:8
A splitter module is a small cassette with one input and a set of output cords. A common choice in an FDH is 1:32: one feeder fiber in, thirty-two outputs, each able to serve one home. Splitting costs light, about 3 dB each time the signal is halved. A 1:32 split halves it five times, so with the module's own losses it costs about 17 dB.
Some networks split in two stages instead, called a cascaded or distributed split. A 1:4 splitter in the cabinet sends each feeder fiber to four fibers, and each of those meets a 1:8 splitter in a closure or terminal on the street. Four times eight is 32, so each feeder fiber still reaches 32 homes with about the same total loss, but far fewer fibers have to leave the cabinet.
The choice is a trade. Centralized 1:32 splitting keeps every splitter in one place, so splitters fill evenly and a new splitter, with its OLT port, is added only when enough homes sign up. Cascaded splitting saves distribution fiber and cabinet space, but spare outputs end up scattered along the street.
Distribution ports, parking and patching
Distribution ports
The distribution panel is a field of connectors, one for each distribution fiber, often hundreds of them. Each port is wired to a fiber that runs out to a terminal, and each terminal serves specific addresses. The port map, which port serves which address, is the record the whole cabinet depends on.
Parking
Splitter outputs that serve no one yet are not left hanging. Each one is plugged into a parking position, a holder that keeps the connector clean and the cord tidy until a customer needs it.
Patching
When a home signs up with the ISP, a technician takes a splitter output from parking and plugs it into that home's distribution port. That one patch connects the home, through the splitter and the feeder, to a port on the OLT. Disconnecting the home reverses it. Good patching keeps cords short, routed through the guides and labeled, because a tangled cabinet is slow to work in and easy to damage.
Worked example, key terms and questions
Worked example: a neighborhood of 288 homes
A neighborhood has 288 homes. Here are the two ways to split it.
With 1:32 splitters in the FDH. 288 divided by 32 is 9 splitter modules, fed by 9 feeder fibers. Every home gets its own distribution fiber, so the cabinet needs 288 distribution ports. A 288-port cabinet is full on day one; a larger one leaves room to grow.
With a cascaded 1:4 and 1:8 split. The same 9 feeder fibers feed nine 1:4 splitters in the FDH, for 36 outputs. Those 36 fibers run to 36 closures or terminals, each with a 1:8 splitter for 8 homes. 36 times 8 is 288.
Either way. The feeder also carries spare fibers for growth and repairs, and each of the 9 feeder fibers needs its own PON port on the OLT once its homes are connected.
Key terms
FDH: Fiber distribution hub: the cabinet where feeder fibers meet splitters and distribution fibers.
Feeder: The cable from the central office or hut to the FDH.
Distribution: The cables from the FDH out to the terminals that serve homes.
Splitter module: A cassette that divides one feeder fiber among many outputs, such as 32.
Cascaded split: Splitting in two stages, such as 1:4 in the cabinet and 1:8 in the street.
Parking: Holders where unused splitter outputs wait, clean and protected, until a customer needs one.
Patch cord: A short fiber cord with connectors that links a splitter output to a distribution port.
Port map: The record of which distribution port serves which address.
Common questions
What is the difference between an FDH and a splice closure?
A splice closure is a sealed box for permanent splices, opened rarely. An FDH is a cabinet built to be opened often: its splitters and ports use connectors, so a technician can connect and move customers with a patch cord.
Should we use 1:32 or 1:64 splitters?
A 1:64 split serves more homes per OLT port but costs about 3 dB more light, which shortens reach or uses up margin. 1:32 is a common choice; the right one depends on your optical budget, the PON standard and how far away the homes are.
How many homes does one FDH serve?
It depends on its size. Cabinets are commonly built in sizes such as 144, 288 or 432 ports, and the design picks one that fits the homes in the area plus room to grow.
How Lux does this work
Lux drafts a design from an area you select: closures and cabinets placed, cable routed to every home and every quantity counted, checked against your rules. Try the design demo.
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