Fiber Distribution Box (FDB)

The last split before the subscriber — where PLC splitters turn one feeder fiber into dozens of drop connections.

A fiber distribution box (FDB) sits downstream of the fiber distribution cabinet (FDC) and upstream of the terminal box. Its job is straightforward: receive a small number of feeder fibers from the FDC, run them through PLC splitters, and deliver drop cables to individual subscribers. In most FTTH architectures, this is the enclosure that actually holds the splitter — the FDC handles splicing and cross-connect, but the FDB does the splitting.

Jergeo manufactures FDBs from 12 to 96 ports in SMC, ABS, and PC+ABS. All models are wall-mount or pole-mount — no ground-mount option, because these boxes serve a small coverage area (typically one building or one pole location) and need to be compact. Every outdoor model is IP65 rated and operates from -45\u00b0C to +65\u00b0C. Internal configurations support both splitter-based (LGX module slots) and pre-terminated (direct adapter termination) designs.

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JFDB-64A Fiber Distribution Box

JFDB-64A Fiber Distribution Box

64 ports FDB with SMC material and integrated splitter

64 Ports IP65 SMC
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JFDB-16C Fiber Distribution Box

JFDB-16C Fiber Distribution Box

Compact 16 ports FDB with SMC material

16 Ports IP65 SMC
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JFDB-96A Fiber Distribution Box

JFDB-96A Fiber Distribution Box

High-capacity 96 ports FDB with ABS material

96 Ports IP65 ABS
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JFDB-48B Fiber Distribution Box

JFDB-48B Fiber Distribution Box

48 ports FDB with SMC material

48 Ports IP65 SMC
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JFDB-32A Fiber Distribution Box

JFDB-32A Fiber Distribution Box

32 ports FDB with dual-layer design

32 Ports IP65 SMC
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JFDB-32B Fiber Distribution Box

JFDB-32B Fiber Distribution Box

Lightweight 32 ports FDB with PC+ABS material

32 Ports IP65 PC+ABS
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JFDB-24A Fiber Distribution Box

JFDB-24A Fiber Distribution Box

Compact 24 ports FDB with multiple splitter options

24 Ports IP65 PC+ABS
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JFDB-16A Fiber Distribution Box

JFDB-16A Fiber Distribution Box

16 ports FDB with SMC material and dual-layer design

16 Ports IP65 SMC
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JFDB-16B Fiber Distribution Box

JFDB-16B Fiber Distribution Box

Lightweight 16 ports FDB with PC+ABS

16 Ports IP65 PC+ABS
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JFDB-12A Fiber Distribution Box

JFDB-12A Fiber Distribution Box

Compact 12 ports FDB with PC+ABS material

12 Ports IP65 PC+ABS
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JFDB-12B Fiber Distribution Box

JFDB-12B Fiber Distribution Box

12 ports FDB with SMC material

12 Ports IP65 SMC
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The Split Happens Here — And That Changes Everything About How You Size It

Where the FDB Fits in the ODN Chain

The signal path in a typical FTTH network looks like this:

Central Office → Feeder Cable → FDC → Distribution Cable → FDB (this box) → Drop Cables → Terminal Box → ONT

The FDC receives the feeder cable, splices fibers, and routes them to distribution cables. The FDB receives those distribution cables and runs the fibers through PLC splitters. That is the critical difference: the FDC is a cross-connect point; the FDB is a split point.

This distinction matters for two reasons:

1. The splitter ratio determines your subscriber reach.

A 1×8 PLC splitter turns one input fiber into 8 output fibers. A 1×16 splitter turns one into 16. So a 32-port FDB with four 1×8 splitters serves up to 32 subscribers using only 4 feeder fibers from the FDC. Without splitters, you would need 32 feeder fibers — which means a bigger FDC, more cable, and more splicing labor. The splitter is what makes FTTH economical at scale.

2. The splitter insertion loss eats your optical budget.

Every split costs dB. A 1×8 PLC splitter has approximately 10.8 dB insertion loss (theoretical 9.03 dB per ITU-T G.671, plus connector and manufacturing tolerance). A 1×16 splitter costs approximately 13.5 dB. This is not a rounding error — it is the single largest loss contributor in the drop segment. When you choose between a 1×8 and a 1×16 configuration in the same FDB, you are not just deciding how many subscribers to serve. You are deciding how much optical budget is left for the drop cable run to the ONT.

This is why Jergeo FDBs offer both splitter-based and adapter-based (pre-terminated) internal configurations. In splitter mode, the box houses LGX PLC splitter modules with SC adapters on the output face. In pre-terminated mode, pigtails are fusion-spliced directly and terminated on adapter holders — no splitters, lower loss, but limited to the number of feeder fibers you bring in. Your network architecture decides which mode you need; the box should support both.

Three Materials, Three Price Points, Three Use Cases

Pick Based on Where the Box Hangs

FDBs are smaller and lighter than FDCs, and they hang on walls or poles instead of standing on the ground. That means the material choice is driven by different factors. Here is the practical breakdown.

SMC (Glass-Fiber Reinforced Polyester)

  • Does not corrode. No rust, ever. UV-stable when properly formulated.
  • Heavier than ABS or PC+ABS at the same port count. A JFDB-32A (SMC) weighs more than a JFDB-32B (PC+ABS) at identical dimensions.
  • Best long-term thermal stability. SMC maintains mechanical properties across the full -45°C to +65°C range without becoming brittle in cold or softening in heat.
  • Operating range: -45°C to +65°C.

Best for:

Pole-mounted deployments where the box is exposed to full weather for 15+ years, or any project where the spec calls for SMC-equivalent material.

PC+ABS (Polycarbonate + ABS Blend)

  • Lighter than SMC at the same size. Easier to handle on a pole or wall bracket — one person can install it without a helper.
  • Good impact resistance. PC+ABS absorbs knocks and vibration better than SMC, which matters for pole-mounted boxes that get bumped by maintenance crews or sway in wind.
  • Lower material cost than SMC. Typically 20–30% less at the same dimensions.
  • Operating range: -45°C to +65°C. In sustained ambient temperatures above 55°C (direct sun on a dark surface), PC+ABS may begin to show surface discoloration before SMC does.

Best for:

Wall-mounted deployments on building facades where weight matters, mild to moderate climates, and projects where unit cost is a primary driver.

ABS (Acrylonitrile Butadiene Styrene)

  • The lightest option. Used in the JFDB-96A — the highest-port FDB — because the 550×415×150mm size would be prohibitively heavy in SMC for a wall/pole mount.
  • Good impact resistance, adequate for sheltered outdoor locations.
  • Lower UV resistance than SMC or PC+ABS unless a UV-stabilized grade is specified. In direct tropical sun without shade, ABS may yellow and become brittle faster.
  • Operating range: -45°C to +65°C.

Best for:

High-port-count applications (64+ ports) where weight is the binding constraint, or indoor/outdoor sheltered locations.

The Decision Matrix

Deployment Material Why
Pole, exposed, 15+ year spec SMC Longest service life, no UV degradation, no corrosion
Wall, building facade, moderate climate PC+ABS Lighter install, lower cost, adequate durability
High-port-count, weight-limited mount ABS Only way to get 96 ports into a wall-mount box
Coastal or industrial zone SMC Chemical and salt resistance exceeds PC+ABS and ABS

One Splitter Choice Costs You 3 dB — Here Is How to Calculate the Trade-Off

Splitter Configuration — The Math That Determines Your Network Design

The PLC splitter inside the FDB is not an afterthought. It is the component that determines how many subscribers one box can serve and how far those subscribers can be from the box. Get the splitter ratio right, and your optical budget works with margin. Get it wrong, and you are explaining to the customer why the ONT at the end of the street keeps dropping offline.

Splitter Insertion Loss (typical values per ITU-T G.671)

Splitter Ratio Theoretical Loss Typical Insertion Loss (with connectors)
1×4 6.02 dB 7.5 dB
1×8 9.03 dB 10.8 dB
1×16 12.04 dB 13.5 dB
1×32 15.05 dB 17.0 dB

* Actual loss varies by manufacturer grade (Grade A/B/C) and connector type. Values above are for Grade A PLC splitters with SC/APC connectors, which is what Jergeo supplies as standard.

Pick 1×16 when:

You have limited feeder fibers available. One input fiber serves 16 subscribers. The trade-off is 2.7 dB more insertion loss (13.5 dB vs 10.8 dB), which reduces the maximum drop cable length by roughly 8–10 km at 1310nm (assuming G.652.D fiber at ~0.35 dB/km). If your subscribers are all within a single building or a short drop run, the extra loss is acceptable.

Pick 1×8 when:

You have more feeder fibers available and want longer drop cable reach. Two input fibers serving 16 subscribers gives you 2.7 dB more optical budget per subscriber — that translates to roughly 8 km more reach on the drop side. Use this for suburban pole-mounted deployments where drop cables may run 1–3 km.

Quick Reference for Jergeo FDB Configurations

Model Splitter Setup Input Fibers Output Ports Total Splitter Loss
JFDB-64A 1×8 × 8 8 64 10.8 dB each
JFDB-32A/B 1×8 × 4 4 32 10.8 dB each
JFDB-24A (Type 1) 1×16 × 1 1 16 13.5 dB
JFDB-24A (Type 2) 1×8 × 2 2 16 10.8 dB each
JFDB-16A/B/C 1×8 × 2 2 16 10.8 dB each
JFDB-12A/B 1×8 × 1 1 8 10.8 dB

The JFDB-96A and JFDB-48B use adapter-based (pre-terminated) configurations without internal splitters — they terminate feeder fibers directly on adapter holders. Use these when your splitter is located elsewhere in the network (e.g., a splitter module inside the FDC), or when you are running a point-to-point architecture without passive splitting.

Frequently Asked Questions

What is the difference between a fiber distribution box (FDB) and a fiber distribution cabinet (FDC)?
An FDC is the primary distribution node in the ODN — it receives the feeder cable from the central office, splices the fibers, and cross-connects them to distribution cables. FDCs are large (72 to 1152 ports), ground-mounted or wall-mounted, and house splice-and-distribution modules. An FDB is a smaller enclosure (12 to 96 ports) installed downstream of the FDC, closer to the subscriber. The FDB's primary function is to house PLC splitters and deliver drop cables to individual buildings. Think of the FDC as the neighborhood hub and the FDB as the building entry point.
Can I use an FDB without splitters (pre-terminated mode)?
Yes. The JFDB-96A and JFDB-48B are configured in adapter-based mode — they terminate feeder fibers directly on adapter holders without internal PLC splitters. Any FDB can also be ordered without splitters if your network design places the splitter elsewhere (e.g., inside the FDC). In pre-terminated mode, the box simply passes fibers through from the cable entry to the adapter ports, with splicing at the pigtail point. This gives you the lowest possible insertion loss per port, but limits you to one output port per input fiber.
Why do some FDBs have the same port count but different materials (e.g., JFDB-32A vs JFDB-32B)?
The JFDB-32A uses SMC and the JFDB-32B uses PC+ABS. They have the same dimensions (420×320×125mm) and the same port count. The difference is weight, cost, and long-term durability. SMC is heavier and more expensive but offers better UV resistance and a longer service life in harsh outdoor conditions. PC+ABS is lighter and easier to install on a wall bracket, and costs less. If the box is going on a pole in full weather exposure for 15+ years, choose the SMC version (JFDB-32A). If it is going on a building facade in a moderate climate, the PC+ABS version (JFDB-32B) is sufficient.
What is the maximum distance from FDB to ONT with a 1×8 splitter?
It depends on your transmitter output power, the total link loss budget, and the fiber type. As a practical reference: with a typical GPON OLT output of +3 dBm at 1490nm, a 1×8 PLC splitter loss of 10.8 dB, and G.652.D fiber at 0.25 dB/km (1550nm) to 0.35 dB/km (1310nm), a conservative maximum drop cable length is approximately 15–20 km before the ONT receive sensitivity is exceeded. In practice, most FDB-to-ONT runs are under 2 km in urban FTTH deployments, so optical budget is rarely the binding constraint with 1×8 splitters. With 1×16 splitters, the additional ~2.7 dB loss reduces this by roughly 8–10 km.
Can I change the splitter ratio after the FDB is installed?
Yes, if the FDB uses LGX-style splitter modules. The splitter modules are held in internal slots and can be swapped in the field. For example, you can replace a 1×8 LGX module with a 1×16 module in the same slot to increase subscriber capacity at that location. The fiber routing inside the box accommodates both sizes. You will need to re-splice the input fiber to the new module and re-terminate or re-route the output pigtails. This is a field task that takes approximately 30–60 minutes per module with a fusion splicer.
Are wall-mount and pole-mount hardware included?
The FDB body includes mounting ears for wall installation (screw holes are pre-formed). Pole mounting uses stainless steel banding straps that pass through slots in the back of the box — these straps are not included in the standard configuration because pole diameters vary by site. You can specify banding straps at order time, or source them locally. The box design is compatible with standard 20mm and 30mm stainless steel banding.
Why is the operating temperature limited to +65°C when FDCs go to +80°C?
FDBs use ABS, PC+ABS, or SMC shell materials in thinner wall sections than FDCs (to keep the box light enough for wall and pole mounting). ABS and PC+ABS have lower heat deflection temperatures than the thick-gauge SMC used in FDCs. The +65°C rating accounts for the material's behavior at sustained high ambient temperatures plus solar heat gain on the box surface. In practice, the internal temperature of a pole-mounted FDB in direct sun can exceed ambient by 15–20°C. The +65°C rating ensures the shell material remains within its structural limits even in these conditions. If your deployment location routinely exceeds 55°C ambient, consider specifying a shade or heat shield for the FDB.
What certifications do Jergeo FDBs carry?
Jergeo FDBs comply with CE and RoHS requirements. The IP65 protection rating is verified per IEC 60529. SMC material is tested per relevant IEC standards for UV resistance and flame retardancy. PLC splitters supplied with FDBs are compliant with ITU-T G.671 and Telcordia GR-1209. For project-specific certification requirements (e.g., local telecom authority approvals), contact us with your specification and we will confirm compliance or arrange testing.