Module 2

PON Architecture: OLT, ODN, ONT

One laser at the telephone exchange. Nothing but glass and mirrors in between. Light arriving at sixty-four homes at once. This is the single most important picture in all of fiber access — learn it well and the rest of the course clicks into place.

The tree of light

Picture a single thread of glass leaving the telephone exchange. A few kilometres out it reaches a small, sealed box — a splitter — with no power cable, no fan, no blinking lights. Inside, that one beam of light is divided and handed out to dozens of homes. Just glass, branching like a tree.

Copper and cable put powered boxes — DSLAMs, amplifiers, switches — out in street cabinets that need electricity and upkeep. A PON strips all of that out of the middle. Only the two ends are awake.

                                  ┌──► ONT  (home 1)
                                  ├──► ONT  (home 2)
  OLT ──── feeder fibre ────► SPLITTER ──► ONT  (home 3)
 (telco)    (one fibre)     (passive)   ├──► ONT  (home 4)
                                  └──► ... up to ~64 homes

  ▲ powered          ▲ just glass — no power ▲          ▲ powered
  one laser          the entire middle is passive       one box per home

The three boxes you must know

Almost everything in PON architecture is one of three things. Learn these three acronyms and where each lives, and you have the skeleton of the whole network.

Here they are with their full names and where you would physically find each one:

A footnote you'll meet constantly: ONT vs. ONU

You will see both ONT and ONU in the wild, often for what looks like the same box. There are two ways the terms differ, and it helps to keep them straight.

• Where the term comes from (most reliable distinction): ONT is the ITU-T term, used in the GPON / XGS-PON family. ONU (Optical Network Unit) is the IEEE term, used in the EPON family. Functionally they are the same kind of device.
• Where it physically sits (practical convention): an ONT sits at or inside the home and presents the final user ports (fibre-to-the-home). An ONU may sit short of the individual user — in a basement, a curb cabinet, or a distribution point — with a remaining copper or Ethernet segment finishing the run. The handy rule: all ONTs are ONUs; not all ONUs are ONTs.

What "passive" means — and why it matters

"Passive" is a precise claim about the outside plant between the OLT and the ONT: it contains only unpowered optical components — fibre and splitters. Nothing in the field needs mains power, cooling, battery backup, or a maintenance visit. The only powered elements in the entire access network are the two ends: the OLT in the Central Office and the ONT in the home.

That sounds like a small technical detail. It is actually the entire business case for fibre-to-the-home. Walk through what it removes from the field:

• No power. A splitter in a sealed box draws zero watts. There is no electricity bill for the middle of the network, and no power outage can take a street's fibre offline.
• No cooling, no batteries. Powered field gear needs fans, heat management, and backup batteries that age and must be replaced. Glass needs none of it.
• No maintenance trucks in the middle. Active electronics fail, get firmware updates, and need technicians. A passive splitter just sits there, often for decades.

How 64 homes share one fiber

Downstream is easy: one source, light fanning to everyone. Upstream is the puzzle — if 64 homes shine their lasers up the same fibre at once, the signals collide into noise at the splitter. The fix is to split the directions in two completely different ways.

Downstream (OLT → homes): broadcast to everyone

The OLT transmits continuously and broadcasts to every ONT — a form of TDM (Time-Division Multiplexing). The splitter copies that light down every branch, so all homes physically receive everything; each ONT keeps only the frames addressed to it.

That means your neighbour's ONT also receives your data — so downstream is encrypted (typically AES, Advanced Encryption Standard). The light reaches every door, but only the intended home holds the key.

Upstream (homes → OLT): take turns with time slots

Two ONTs firing at once would collide at the splitter, so upstream uses TDMA (Time-Division Multiple Access): the OLT grants each ONT an exclusive, non-overlapping time slot. Each transmits only in its window — in burst mode, laser on just for that burst — so nothing ever collides. The OLT also leaves a small guard time between one ONT's burst and the next, so that even with slight timing error the adjacent bursts never overlap.

Two more pieces make this work in practice:

• DBA (Dynamic Bandwidth Allocation) — the OLT shrinks and grows each home's time slots in real time based on demand. A home streaming video gets bigger slots; an idle home gets tiny ones. This lets the operator safely oversubscribe the shared capacity — selling more total capacity than the fibre physically has, betting that the homes are almost never all busy at once. (The general name for this bet is statistical multiplexing.)
• Ranging — homes sit at different distances from the OLT, so their bursts take different times to arrive. The OLT measures each ONT's round-trip delay and tells it exactly when to fire, so every burst lands neatly in its slot despite the different fibre lengths.

Both directions on one fiber: different colours of light

Down and up travel on the same fibre at the same time without interfering, because they ride different wavelengths — different "colours" of infrared light. This is WDM (Wavelength-Division Multiplexing): like two radio stations sharing the same air on different frequencies.

Put it together and the shared tree carries private, two-way internet for every home at once: TDM broadcast down, TDMA turns up, WDM keeping the directions apart. The widget below lets you watch it move.

PON vs. Active Ethernet (point-to-point)

The main alternative to PON is an AON (Active Optical Network), usually built as Active Ethernet: the opposite philosophy. Instead of a passive shared tree, run a dedicated fibre to each subscriber through powered switches in the field. Both deliver fibre to the home — they just trade cost, fibre, and bandwidth differently.

The splitter — and the lack of power in the middle — is the tell. The full side-by-side:

In one line: PON trades dedicated bandwidth for radically cheaper, simpler outside plant — which is why it dominates residential FTTH. Active Ethernet wins where a customer needs a guaranteed pipe or very long reach (enterprise, backhaul) and will pay for the fibre and field gear.

(PON's shorter ~20 km reach isn't a hard limit of the glass — it's that the splitter eats most of the optical loss budget, leaving less for fibre length. Use a smaller split and a PON can reach farther; Active Ethernet has no splitter to pay for, so it goes ~70–100 km.)

When the tree breaks: protection & redundancy

A PON is a tree, and a tree has a single trunk. Cut the feeder fibre, or lose the OLT port that feeds it, and every home downstream goes dark at once. The same passive simplicity that makes PON cheap also concentrates risk at the root — so resilience is a real design choice, not an afterthought.

The 2:N protection splitter

An ordinary splitter has one input (1:N). A 2:N splitter has two inputs feeding the same set of outputs. That second port lets a second feeder fibre — often from a second OLT port — reach the same tree. If the primary feeder is cut, traffic switches to the standby feed and the homes downstream stay lit. This is what the splitters module meant by "2:N adds redundancy": it is the hook on which PON protection hangs.

ITU-T G.984.1 protection types

The ITU-T standard G.984.1 defines four levels of PON protection. They differ by how much of the path you duplicate — and you pay accordingly. More duplication protects more of the network, but costs more.

The sweet spot for most operators is Type B: it duplicates the two things that hurt the most homes when they fail — the OLT port and the feeder — without paying to duplicate the distribution fibre, every drop, and every ONT.

Route diversity, separacy & dual-homing

Spare gear only helps if it sits on a different physical path. Three terms pin this down:

• Route diversity — running two or more independent physical paths between the same points, so one cut can't take out both. But "diverse" routes may still share a stretch of duct.
• Separacy — the stricter form: the paths share no duct and no cabinet anywhere. True separacy is what survives a single backhoe.
• Dual-homing — connecting a node to two geographically separate upstream hubs, so losing one whole hub doesn't isolate it.