An OLT optical power budget is the margin left after the OLT's transmitted light is attenuated through the splitter, fiber, and connectors yet still readable by the ONU. In a GPON system the ONU's normal receive window is roughly −8 to −27 dBm; weaker and it drops registration, stronger and it saturates. The "output power" you read is set by the OLT transmitter, the ONU receiver sensitivity, and the link loss together.
The OLT output power and ONU receiver sensitivity frame the window: −8 dBm is the "too bright" ceiling, −28 dBm the "too dark floor, with −8 to −27 dBm the normal run band.
Downstream budget is now ~37 dB with a +9 dBm high-power OLT module, but the upstream is the tighter side because the ONU only transmits around 0 dBm.
Always leave engineering margin: aim ONU receive around −15 to −23 dBm, away from both the overload and sensitivity limits.
The receive window is walled by two endpoints: transmit power sets how bright the start is, receiver sensitivity sets how dark the end can still be.
Rayin's GPON OLT uses a high-power PON module transmitting about +9 dBm. The ONU still transmits at Class B+ level, typically −1 to +4 dBm with a nominal ~0 dBm. That ~9 dB gap is why upstream is tighter than downstream — under the same loss the OLT's light simply starts much brighter.
Receiver sensitivity is the weakest light a receiver can still read. A Class B+ ONU sensitivity is generally ≤ −27 dBm, good modules reach −28 dBm; the OLT side is typically ≤ −28 dBm. Smaller (more negative) is more sensitive, tolerating more link loss. The other endpoint is overload power, typically ≥ −8 dBm — beyond that it saturates and bit errors climb. So −8 dBm is the "too bright" ceiling and −28 dBm the "too dark" floor; between them, −8 to −27 dBm is the common operating window.
Tip: Keep engineering margin. In real projects aim ONU receive at −15 to −23 dBm: that leaves 4–5 dB for aging and bends while staying clear of the −8 dBm overload risk.
Link budget totals every loss between OLT transmit and ONU receive, then compares against the safety margin. A standard GPON Class B+ link budget is about 28 dB, Class C+ about 32 dB. With Rayin's +9 dBm high-power OLT module the downstream usable budget is about 37 dB (9 − (−28)); upstream is still capped around 28 dB by the ONU transmitter, and XGSPON sits in the same ballpark.
| Loss item | Typical | Note |
|---|---|---|
| Splitter 1:64 | ~20.5 dB | 1:32 ~17.5 dB, 1:128 ~23.5–24 dB |
| Single-mode fiber (1490 nm down) | ~0.28 dB/km | 1310 nm up ~0.35 dB/km |
| Flange / connector | ~0.5 dB each | Fusion splice typically 0.1 dB each |
| Design margin | 1.5–3 dB | Buffer for aging, micro-bend, maintenance |
| Total (1:64 + 5 km + 2 flanges + margin) | ~26 dB | Verify per site item by item |
Warning: A common mistake is counting only the splitter and forgetting the flanges. Every loss on the OLT fiber path between equipment room and corridor can be 3–4 connectors at 0.5 dB each — 1.5–2 dB that is enough to push an otherwise "passing" link below −28 dBm.
The OLT output power and ONU parameters differ clearly. The central-office OLT transmits higher and receives a bit better; the ONU transmits weaker but needs a receive range close to the OLT's. When reading receive power, the two web pages show different directions: the ONU's RX is downstream OLT→ONU, the OLT's RX is upstream ONU→OLT.
| Parameter | OLT (central office) | ONU (user side) | Note |
|---|---|---|---|
| Transmit power | +9 dBm (high-power module) | −1 to +4 dBm (Class B+) | OLT far brighter |
| Receiver sensitivity | ≤ −28 dBm | ≤ −27 dBm | Weakest readable light |
| Overload power | ≥ −8 dBm | ≥ −8 dBm | Beyond → saturation |
| Optical power budget | Down ~37 dB | Up ~28 dB | Max allowed link loss (different directions) |
| Typical receive window | −8 to −27 dBm | −8 to −27 dBm | Normal operating band |
Note: The ONU side more often misbehaves: downstream from a +9 dBm OLT is bright, so on short links or small splits the ONU overloads more often than the OLT; upstream the ONU only sends ~0 dBm, so the OLT side more often alarms "light too weak." Read both ends — reading one side alone misleads.
Take Rayin's GPON OLT with the +9 dBm high-power module: downstream OLT transmit +9 dBm, split 1:64, fiber 5 km, two flanges, design margin 3 dB. Loss totals ~26 dB, ONU receive = +9 − 26 = −17 dBm — squarely in the healthy band, with wider margin than a standard module.
Upstream, ONU transmits 0 dBm, same 26 dB loss, OLT receive = 0 − 26 = −26 dBm. Still above the −28 dBm sensitivity, but only 2 dB of margin. Add one more transition flange or 1 dB of bend and the OLT side throws a low-light alarm; the ONU shows registration jitter or drops.
Warning: That is why upstream is the tighter side: downstream budget is now ~37 dB, but the ONU transmits nearly 9 dB weaker than the OLT, so the upstream margin is inherently much smaller. On long-reach, large-split builds, do the upstream budget math first.
Link too short, split too small — ONU drops anyway. Equipment room to floor closet may be only a few hundred meters, with a 1:8 split (~9.7 dB). Downstream from +9 dBm can deliver −5 dBm or brighter to the ONU, slamming into the −8 dBm overload ceiling. The high-power module makes short-link overload more frequent than a standard +3 dBm module. Symptom: power "looks strong" but bit errors and latency jump. Add a fixed attenuator or move to a larger split.
Reading only the OLT RX, not the ONU RX. The OLT page shows the ONU's upstream light; the ONU page shows the OLT's downstream light. The two directions can be asymmetric — especially since upstream uses 1310 nm and downstream 1490 nm, and fiber attenuates the two wavelengths differently.
Forcing 1:128 split at 20 km. A 1:128 splitter alone loses ~23.5 dB; add 20 km fiber (1490 nm ~5.6 dB), flanges, and 3 dB margin for a total ~33.3 dB. Downstream with +9 dBm still holds (9 − 34 ≈ −25 dBm, inside −27), but upstream the ONU sends only 0 dBm, 0 − 34 = −34 dBm, long past −28 dBm sensitivity. So 1:128 at 20 km is actually capped upstream; clean links usually stay within 10–15 km, and beyond that step back to 1:64.
Treating receiver sensitivity as the target. −28 dBm is the "still receivable" limit, not the "healthy" target. Running long-term at −26 to −27 dBm, module aging, winter joint tightening, or micro-bends can push it below −28.
What receive power is normal for a GPON ONU?
−8 to −27 dBm is the normal window. Engineering prefers −15 to −23 dBm — clear of both the −8 dBm overload and the −28 dBm sensitivity floor.
Why do OLT and ONU receive readings differ?
Different directions. The OLT's RX is the ONU's upstream light; the ONU's RX is the OLT's downstream light. The OLT transmits ~9 dB brighter than the ONU, so downstream has wide margin while upstream is tight — which is why the OLT side more often alarms low light.
What happens if receive power is too strong?
ONU receive above −8 dBm can trigger receiver saturation: rising bit-error rate, failed ranging, PING jitter, even dropped registration. Common causes are too-short links or too-small splits — add an optical attenuator.
When the OLT optical power budget is short, what to check first?
First confirm split ratio and distance match, then count flanges/connectors, then check fiber bends and splices. Upstream breaks budget before downstream — if needed, lower the split ratio or use a higher-transmit OLT module (e.g., +9 dBm) to add downstream margin, but upstream is still limited by the ONU transmitter.
An OLT optical power budget is not a complex formula — it is aligning the OLT output power, the ONU receiver, and the link loss. GPON's −8 to −27 dBm is the normal window; engineering-wise keep the ONU side around −15 to −23 dBm. Rayin's OLT uses a +9 dBm high-power module to push downstream margin to ~37 dB, yet upstream remains the bottleneck — guard against overload on short links and sensitivity shortage on long, large splits. Do the budget math and you pull fewer fibers in troubleshooting. For GPON OLT and 1:128 splitter measured parameters, see Rayin's GPON OLT lineup.
Customer Manager at Shenzhen Rayin Technology Co., Ltd.
