What is the PON link budget formula?
The link budget is the difference between the OLT transmit power and the ONU receiver sensitivity. Every passive element between them subtracts from that budget: the splitter, the fibre, the connectors, the splices and any WDM coupler. The result must be greater than zero, with an ageing margin.
Budget = (OLT TX power) − (ONU RX sensitivity). Available margin = Budget − splitter loss − fibre loss − connector loss − splice loss − WDM loss. Require available margin ≥ 3 dB.
| Element | Typical loss | Notes |
|---|---|---|
| OLT TX power | +4.5 to +9 dBm (EPON example) | From the OLT datasheet |
| ONU RX sensitivity | −27 dBm (XPON example) | From the ONU datasheet |
| 1:32 PLC splitter | 16.5 dB | 1:64 is about 20.5 dB |
| Fibre 1310 nm | 0.35 dB/km | 0.2 dB/km at 1550 nm |
| Connector pair | 0.3 – 0.5 dB | Per mated pair; count both ends |
| Splice | 0.1 dB | Per splice |
| Ageing margin | 3 dB | Do not design to zero |
What does a 20 km GPON budget look like?
Take a Class B+ GPON OLT with +3 dBm transmit power and an ONU sensitivity of −27 dBm. Budget = 3 − (−27) = 30 dB. A 1:32 splitter takes 16.5 dB, 20 km of fibre at 0.35 dB/km takes 7 dB, six connector pairs at 0.4 dB take 2.4 dB, and four splices at 0.1 dB take 0.4 dB. Total = 26.3 dB. Margin = 30 − 26.3 = 3.7 dB, which passes with the 3 dB ageing reserve.
Change the splitter to 1:64 (20.5 dB) and the total becomes 30.3 dB, leaving −0.3 dB. The link fails. The fix is a 1:32 splitter, a shorter span, fewer connectors, or a higher-power OLT optic. This is why the split ratio is the first thing to check, not the last.
| Item | Calculation | Budget remaining |
|---|---|---|
| Class B+ budget | 3 − (−27) | 30.0 dB |
| 1:32 splitter | −16.5 dB | 13.5 dB |
| 20 km fibre at 0.35 dB/km | −7.0 dB | 6.5 dB |
| 6 connector pairs at 0.4 dB | −2.4 dB | 4.1 dB |
| 4 splices at 0.1 dB | −0.4 dB | 3.7 dB |
| Ageing reserve | −3.0 dB | 0.7 dB pass |
A 1:64 splitter adds 20.5 dB instead of 16.5 dB and leaves −0.3 dB: the link fails.
What should you measure in the field?
Measure the OLT output power with a calibrated power meter, then measure at the ONU input. The difference is the actual path loss. Compare it with the calculated value; a 2–3 dB discrepancy usually means a dirty connector, a bad splice, a wrong splitter or a macrobend.
Record the values per port on the as-built. A baseline lets the next technician see whether a link has degraded. Clean and inspect every connector before measuring; a dirty SC connector can add 1–3 dB and look like a splitter problem.
Why keep a 3 dB ageing margin?
Laser output drops with age and temperature, connectors degrade with mating cycles, and splices can drift. A design that works at 0 dB margin on day one will fail in year three. The 3 dB margin is a practical allowance for ageing, temperature and repair splices.
If the site has a high temperature swing or many field connectors, use 4–5 dB. If the margin is below 3 dB, reduce the split ratio or move the splitter; do not rely on a higher-power ONU alone.
How does a CATV overlay change the budget?
A 1550 nm CATV signal shares the ODN with the PON. The WDM coupler adds loss (typically 0.5–1.5 dB), and the CATV receiver has its own input range (+2 to −18 dBm). The PON budget and the CATV budget must both pass; a link that works for data may be too lossy for the RF channel.
Plan the two budgets separately, then combine them at the headend. If the CATV receiver is at the edge of its range, add an EDFA or reduce the split; do not boost the PON laser to compensate.
Use this on a real site
Send the endpoint count, the longest run, the mains condition and the load. We will apply this calculation to your project and return the optical/power budget with the bill of materials.
