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Français ODN classes & the PON link budget
A PON's optical distribution network (ODN) has a loss budget defined by class: the window of attenuation between the OLT and any ONU within which every compliant pair of modules must work. Splitters eat most of it, fibre and connectors the rest, and what is left is the margin that decides whether a customer at the far end of a 1:64 split stays online on a hot day. This page gives the classes, the loss elements and a worked design.
Classes
| System | Class | ODN loss window, dB | Notes |
|---|---|---|---|
| GPON (G.984.2) | A | 5–20 | rare |
| GPON | B | 10–25 | legacy |
| GPON | B+ | 13–28 | the residential workhorse |
| GPON | C | 15–30 | — |
| GPON | C+ | 17–32 | higher split or distance; APD receivers at both ends |
| GPON | C++ (vendor) | ~20–35 | extended |
| XG(S)-PON (G.987.2 / G.9807.1) | N1 | 14–29 | nominal 1 |
| XG(S)-PON | N2 | 16–31 | nominal 2 |
| XG(S)-PON | E1 | 18–33 | extended 1 |
| XG(S)-PON | E2 | 20–35 | extended 2 |
| 10G-EPON (802.3av) | PR10 / PR20 / PR30 | 20 / 24 / 29 max | IEEE naming |
The minimum loss matters too: an ONU too close to the OLT with too little splitting can overload its receiver (GPON B+ ONU overload −8 dBm) — a short drop may need an attenuator (Rx power & link budget).
Module classes behind the ODN classes
| Class | OLT Tx, dBm | OLT Rx sensitivity, dBm | ONU Tx, dBm | ONU Rx sensitivity, dBm |
|---|---|---|---|---|
| GPON B+ | +1.5 … +5 | −28 | +0.5 … +5 | −27 |
| GPON C+ | +3 … +7 | −32 | +0.5 … +5 | −30 |
| XGS-PON N1 | +2 … +6 | −28 (1270 nm) | +4 … +9 | −28 |
| XGS-PON N2 | +4 … +8 | −30 | +4 … +9 | −28 |
| XGS-PON E1/E2 | +6 … +10 / +8 … +12 | −30 / −32 | +4 … +9 | −28 |
The OLT and ONU modules must be of matching (or better) class; a B+ OLT on a C+-designed ODN loses 4 dB of budget (PON transceivers, Typical values).
Loss elements
| Element | Typical loss | Notes |
|---|---|---|
| Splitter 1:2 | 3.5 dB | ideal 3.0 + excess |
| 1:4 | 7.2 dB | — |
| 1:8 | 10.5 dB | — |
| 1:16 | 13.7 dB | — |
| 1:32 | 17.2 dB | typical residential GPON |
| 1:64 | 20.5 dB | XGS-PON or C+ GPON |
| 1:128 | 23.5–24 dB | rarely — needs E-class |
| Feeder + distribution fibre | 0.35 dB/km at 1310 nm (upstream), 0.25 dB/km at 1490/1577 nm | budget on the worst wavelength: 1310 nm upstream |
| SC/APC connector pair | 0.3–0.5 dB | 3–6 pairs in a typical ODN |
| Fusion splice | 0.05–0.1 dB | 5–15 per path |
| Coexistence element (CEx) | 1–1.5 dB | when GPON and XGS-PON share the ODN |
| Video overlay filter / WDM | 0.5–1 dB | if RF video is present |
| Ageing and repair margin | 1–3 dB | design reserve |
Element details: Connectors & fibre.
Worked design: GPON B+, 1:32, 12 km
| Item | Loss, dB |
|---|---|
| Feeder 10 km + distribution 2 km at 1310 nm | 12 × 0.35 = 4.2 |
| Splitter 1:32 | 17.2 |
| 5 connector pairs | 2.0 |
| 10 splices | 0.8 |
| Total ODN loss | 24.2 |
| Class B+ window | 13–28 → fits with 3.8 dB spare |
| Add CEx for XGS-PON coexistence | +1.5 → 25.7, spare 2.3 dB |
| Same ODN at 1:64 | 24.2 − 17.2 + 20.5 = 27.5 → spare 0.5 dB: needs C+ / N1 optics |
Expected ONU Rx (downstream 1490 nm, OLT +3 dBm): 3 − (12 × 0.25 + 17.2 + 2.0 + 0.8) = −20 dBm → 7 dB above B+ ONU sensitivity. Expected OLT Rx per ONU (ONU +2 dBm upstream): 2 − 24.2 = −22.2 dBm → 5.8 dB above −28 dBm.
Differential distance and split rules
- Differential fibre distance (nearest to farthest ONU) — 20 km standard, 40 km with extended ranging; it bounds the equalisation delay, not the loss.
- Maximum logical reach — 60 km in GPON with adequate class; physical reach is set by the loss budget in practice.
- Split ratio vs reach — every doubling of the split costs ~3.3 dB ≈ 9–13 km of fibre; 1:32 at 20 km or 1:64 at 10 km are the classic B+/C+ trade-offs.
- Cascaded splitters (e.g. 1:4 then 1:8) lose ~0.5–1 dB more than a single 1:32 but save feeder fibre.
Measuring an installed ODN
| Tool | Where | What it gives |
|---|---|---|
| Optical power meter with PON filters (1310/1490/1550) | at the ONU drop | downstream power at the customer — compare with expected Rx |
| Light source + meter | before OLT is live | end-to-end loss per path |
| PON-aware OTDR (1310/1490/1550 or 1625/1650 nm out-of-band) | from the ONU side, or from the OLT through the splitter with limits | breaks and high-loss events; from the OLT side the splitter hides individual drops |
| OLT per-ONU Rx | OLT CLI/EMS | upstream power per ONU from ranging — the field number that matters |
| ONU DDM | stick or ONU web UI, CodingBox on the bench | downstream Rx at the ONU |
Field faults and their signatures: PON problems.
In CodingBox
Before installation, reading OLT and ONU modules on the bench confirms their class (launch power and declared sensitivity), wavelength plan and identity — the inputs of the budget above. In the field, an ONU stick's DDM Rx compared with the design value tells whether the ODN is within its class window (DDM).