FTTH Access
FTTH for Multi-Dwelling Units: Deployment Models, Per-Segment Cable, and Splitting
An MDU (multi-dwelling unit) is any building with multiple homes behind one street connection — apartments, condos, and mixed-use blocks. Bringing fiber to an MDU is different from a single house because the fiber has to be distributed vertically and horizontally inside the building: down a shared feeder, up the risers, and along each floor to individual units. That shared internal network — the deployment model, the splitter placement, and the cable in each segment — is what this guide covers, and it is where MDU fiber projects succeed or stall.
Key takeaways
- MDU ≠ single-home FTTH: the distinctive work is the in-building vertical (riser) and horizontal (in-unit) network, not the drop to the door.
- Three deployment models — home-run, centralized split, distributed split — trade fiber count against splitter placement and flexibility.
- The cable changes by segment: feeder, vertical riser, and horizontal drop each want a different construction and fire rating.
- Fire rating is a code decision inside the building — risers and plenums have specific requirements (OFNR/OFNP/LSZH) that a single-home drop never faces.
- Greenfield vs brownfield changes everything: a new build can hide fiber in the structure; a retrofit must route it through existing, occupied space.
What makes an MDU different
In a single house, fiber deployment is one drop to one ONT. An MDU multiplies that by every unit and adds a shared spine. The signal arrives at the building once — at a basement or ground-floor entry — and must then be split and distributed to dozens or hundreds of units through risers (the vertical shafts between floors) and horizontal runs (along each corridor to unit doors). Three things follow from that structure:
- Splitters live inside the building, not just in a street cabinet — and where they sit (basement vs per-floor) defines the deployment model.
- Cable runs through shared, fire-regulated space, so riser and plenum fire ratings apply.
- Retrofit (brownfield) is the hard case — the fiber must be routed through occupied buildings with limited pathway, which is why compact, bendable, self-supporting cables matter.
The three MDU deployment models
How the split is distributed through the building is the core architectural choice:
Model | How it works | Splitter placement | Best for |
|---|---|---|---|
Home-run (point-to-point) | A dedicated fiber from a central point to every unit | None in-building (or central only) | Premium builds, business tenants, max flexibility |
Centralized split | One splitter serves the whole building from a single location | One splitter in the basement/entry (e.g. 1×32) | Mid/high-rise, predictable full occupancy |
Distributed (cascaded) split | A small feeder split, then per-floor splitters | 1×4 at entry → 1×8 per floor | Dispersed units, phased take-up, retrofit |
The trade is fiber count against flexibility. Home-run uses the most fiber (one per unit all the way back) but gives every unit a dedicated path — best where tenants may want point-to-point services. Centralized split uses one splitter for the building, simple to manage but commits the whole building's split ratio up front. Distributed split pushes the final split onto each floor, which cuts riser fiber count and lets you light floors as they fill — the model that most often fits a retrofit. As with any PON, the power budget sees the total split, so the choice is about fiber and flexibility, not loss — see the PLC splitter guide for the budget math.
Matching the model to the building
Building type | Typical model | Note |
|---|---|---|
Garden / low-rise (2–4 floors) | Centralized or distributed | Short risers; often a single 1×16/1×32 |
Mid-rise (5–12 floors) | Distributed split | Per-floor splitters cut riser count |
High-rise (12+ floors) | Distributed split | Riser fiber count and access dominate the design |
Any, greenfield (new build) | Any — designer's choice | Fiber hidden in structure; plan risers now |
Any, brownfield (retrofit) | Distributed split | Minimize riser fiber; use compact/self-support cable |
The two axes that drive the choice are height (taller → more benefit from per-floor splitting to keep riser counts manageable) and greenfield vs brownfield (a new build can run generous fiber inside walls; a retrofit must thread limited existing pathways, favouring the distributed model and smaller cables).
The cable in each segment — the part most guides skip
An MDU network is not "one fiber cable." Each segment has a different job, and the construction and fire rating change with it. This is the mapping to get right:
Segment | Job | Construction | Fire rating |
|---|---|---|---|
Feeder | Street/entry → building distribution point | Single-mode (OS2) outdoor or entry cable | Per entry pathway |
Vertical riser | Up the shaft, floor to floor | Self-supporting / tight-buffered riser cable | Riser-rated (OFNR) |
Horizontal / in-unit drop | Corridor → unit → ONT | Flat drop (compact, bendable) | Riser/plenum per pathway |
How the constructions map to these segments: the horizontal in-unit drop is a flat drop cable (2–3 mm) — small enough to route along corridors and into units, and bendable for the tight turns of finished interiors. The vertical riser wants a self-supporting construction that can bear its own weight up a multi-storey shaft without an external messenger every floor — the same self-support principle used in aerial drops like GJYXFCH (tensile 600 N), but specified for the indoor riser with a riser-rated jacket (see the rating note below). The feeder is standard single-mode outdoor cable into the building's distribution point.
The rating column is the part single-home guides never touch: inside a shared building, cable in a vertical shaft must be riser-rated (OFNR), and cable in an air-handling plenum must be plenum-rated (OFNP) — with LSZH required in many jurisdictions. Getting this wrong is a code violation, not a preference. See the OFNP vs OFNR vs LSZH fire-rating guide for the decision framework.
GPON or XGS-PON for an MDU?
The PON generation sets how much bandwidth the building's split shares:
GPON | XGS-PON | |
|---|---|---|
Downstream / upstream | 2.5 / 1.25 Gbps (shared) | 10 / 10 Gbps (shared, symmetric) |
Shared across the split | Fine for typical residential MDUs | Headroom for dense or high-tier buildings |
Choose when | Standard residential, cost-led | High density, business tenants, futureproofing |
Both run over the same ODN and the same PLC splitters, so the cabling does not change with the PON generation — the choice is an electronics and bandwidth-planning decision. A common approach is to build the passive plant (fiber + splitters) once and choose or upgrade the OLT optics later, which is why getting the cable segments and split model right matters more than the PON generation on day one.
Hardware checklist for an MDU build
- OLT — in the central office/head-end, serving many buildings.
- Feeder into the building — single-mode outdoor cable to the entry/distribution point.
- PLC splitters — one centralized, or feeder + per-floor for distributed. Ratio set by unit count and power budget (see the splitter guide).
- Riser cable — self-supporting, riser-rated, up the shafts.
- Floor distribution boxes — where the riser splits to the corridor.
- Horizontal drops — flat drop cable to each unit's ONT.
- ONT — in each unit.
For the broader access-network context, see the FTTH deployment guides; product detail is on the DYS FTTH range.
Frequently asked questions
What is an MDU in FTTH?
MDU stands for multi-dwelling unit — a building with multiple residences behind one connection, such as an apartment block or condo. In FTTH, an MDU needs an internal fiber network that splits and distributes the incoming signal vertically up risers and horizontally to each unit, which is what distinguishes it from single-home (SDU) deployment.
What is the difference between centralized and distributed splitting in an MDU?
Centralized splitting places one splitter (e.g. 1×32) at the building entry, serving the whole building from a single point — simple to manage. Distributed (cascaded) splitting uses a small split at the entry then per-floor splitters (e.g. 1×4 then 1×8), which reduces riser fiber count and suits phased take-up and retrofits. The power budget sees the same total split either way.
What cable is used for MDU risers?
Vertical riser runs use a self-supporting or tight-buffered cable that can bear its own weight up the shaft, and it must be riser-rated (OFNR) — or plenum-rated (OFNP) in air-handling spaces — per building fire code. Horizontal runs to units use compact flat drop cable.
Do MDU cables need a special fire rating?
Yes. Because MDU cable runs through shared vertical shafts and sometimes air-handling plenums, it must carry the corresponding fire rating — OFNR for risers, OFNP for plenums, with LSZH required in many regions. This is a code requirement inside occupied buildings, unlike a single-home drop.
Should an MDU use GPON or XGS-PON?
GPON (2.5/1.25 Gbps shared) suits standard residential MDUs; XGS-PON (10/10 Gbps symmetric) gives headroom for dense buildings, business tenants, or futureproofing. Both use the same fiber and splitters, so you can build the passive plant once and choose or upgrade the OLT optics later.
What is the difference between greenfield and brownfield MDU deployment?
Greenfield is a new building where fiber pathways and risers can be designed into the structure. Brownfield is a retrofit of an existing, occupied building, where fiber must be threaded through limited existing pathways — which favours a distributed split model and compact, bendable, self-supporting cables to minimize disruption.