ONU Rx Power (dBm): Good Range, Bad Range and How to Fix It
ONU Rx power is the light level, in dBm, that the customer’s ONU or ONT receives from the OLT. On a GPON network a healthy reading is roughly -8 to -27 dBm. Weaker than about -28 dBm, or stronger than about -8 dBm, means a fault or a design problem to fix.
By Madan Kc, ISP/GPON engineer at DG Link Network · Updated 8 November 2026
display ont optical-info on a Huawei OLT. Between about -8 and -27 dBm is the working range, and roughly -15 to -25 dBm is comfortable. Below -27 dBm the light is too weak: clean connectors, check bends and splices, and re-check the splitter budget. Above -8 dBm it is too strong: fit an optical attenuator.How to read the Rx power
On a Huawei OLT, enter the GPON interface and ask for the optical information of one ONU. The two numbers are the GPON port and the ONU ID.
enable
config
interface gpon 0/5
display ont optical-info 0 0Find the line for the power the ONU receives (usually labelled Rx optical power). The OLT also reports the upstream power it receives from that ONU. Field names vary a little between firmware versions, so check yours. The other ONU commands are in the Huawei ONU information commands guide. If the ONU is offline there is no live reading, so start with the ONU offline or LOS guide.
What the numbers mean
| Reading | Meaning | Action |
|---|---|---|
| Stronger than -8 | Too strong (overload) | Add an attenuator. See the fix below. |
| -8 to -15 | Valid but hot | Fine on most hardware. Check the ONU datasheet if it errors. |
| -15 to -25 | Comfortable | Nothing to do. |
| -25 to -27 | Near the edge | Works, but has no margin. Clean and inspect before it gets worse. |
| Weaker than -27 | Too weak | Expect drops. Find the loss. See the fix below. |
Where do the limits come from? A Class B+ GPON optic has an OLT transmit power of +1.5 to +5 dBm, an ONU receive sensitivity of about -28 dBm and an overload point of about -8 dBm. Class C+ is stronger: +3 to +7 dBm out, -32 dBm sensitivity and -12 dBm overload. These figures are the typical published ones, so check the datasheet of your own optic and ONU. The “comfortable” band of -15 to -25 dBm is a field rule of thumb that keeps some margin for ageing and dirt. It is not a standard.
Estimate it before you blame the fiber
You can predict the Rx power by subtracting every loss from the OLT transmit power. Use planning values, then compare with what you measure. A big gap points to a fault.
| Item | Typical loss |
|---|---|
| Fiber | about 0.3 dB per km |
| Splitter 1:2 / 1:4 / 1:8 | about 3.5 / 7.2 / 10.5 dB |
| Splitter 1:16 / 1:32 | about 13.7 / 17.1 dB |
| Connector pair | 0.3 to 0.5 dB |
| Fusion splice | about 0.1 dB |
Worked example
OLT transmit +3 dBm. A 1:4 splitter at the cabinet (7.2 dB), then a 1:8 splitter near the street (10.5 dB), 5 km of fiber (1.5 dB), four connector pairs (2.0 dB) and six splices (0.6 dB).
Expected Rx = 3 – 7.2 – 10.5 – 1.5 – 2.0 – 0.6 = -18.8 dBm. If one ONU reads -29 dBm on this link, about 10 dB is being lost somewhere it should not be. Look at that customer’s drop and connectors first.
To try your own numbers, use the fiber link loss calculator, and see the fiber splitter calculator for split ratios.
Fix: Rx power too weak
First compare with the other ONUs on the same PON port. This tells you where to look.
| What you see | Likely place |
|---|---|
| One ONU weak, the rest normal | That customer’s drop cable, connector or ONU port |
| All ONUs behind one splitter weak | The splitter, its input connector or the feeder after the first split |
| Every ONU on the port weak | The feeder fiber, the OLT optic or the PON port |
- Clean and inspect connectors. Dirt is the most common cause of a few extra dB. Use a click-type cleaner or lint-free wipe with isopropyl alcohol, and look at the end face with a fiber scope if you have one.
- Check connector types. GPON uses green APC connectors. A blue UPC plug forced onto an APC port wastes light and can damage the ferrule.
- Look for bends and pinched cable. Staples, tight corners and cable ties can add loss. As a common guideline, avoid bending standard fiber tighter than about 30 mm radius.
- Re-test splices and joints. A poor fusion splice or a mechanical splice that has moved can add several dB.
- Re-check the design. A longer run or an extra splitter than planned may simply not fit the budget. Recalculate as above.
Fix: Rx power too strong
A reading stronger than about -8 dBm usually means a short link with a low split, such as an ONU close to the OLT or a 1:2 or 1:4 split with little fiber. Too much light can overload the receiver, causing errors or drops, and can shorten its life.
- Fit an inline optical attenuator at the ONU end to bring the reading into range, for example 5 to 10 dB for a hot link. Choose one that matches the connector type (SC/APC for GPON).
- Re-read the power after fitting it. Aim for the comfortable band, not just the edge.
- Check whether the port is a C+ optic. If so the overload point is lower and a smaller reading may already be too hot.
Fix: reading that jumps around
A level that changes by several dB between reads is a sign of a loose or damaged connection, not a design problem. Reseat and clean the connectors, check that the drop cable is not moving or under strain, and look for water or damage on outdoor joints. Do this before you replace the ONU. For a flapping ONU that also shows a red LOS light, see the ONT red LOS light guide.
Frequently asked questions
What is a good ONU Rx power?
Between about -8 and -27 dBm is the working range on GPON. A reading of -15 to -25 dBm is comfortable and leaves some margin.
Is -28 dBm bad?
It is at or past the receive limit of many ONUs. It may still work for a while, but expect drops and errors. Find and fix the loss.
Is -5 dBm too strong?
Yes. Stronger than about -8 dBm is above the overload point of a typical Class B+ link. Fit an optical attenuator.
What is the difference between Rx power and Tx power?
Rx is the light the device receives. Tx is the light it sends. On an ONU, Rx comes from the OLT, and the ONU’s own Tx is what the OLT receives upstream. Check both when a customer drops.
Can a splitter cause low Rx power?
Yes. Every split costs light. A 1:8 splitter costs about 10.5 dB and a 1:32 about 17 dB, so a high split ratio on a long link can push the reading below the limit.
Related guides
Sources and testing: Optic power classes are from published Class B+ and C+ specifications, as summarised by Netceed; see also ITU-T G.984.2. Splitter, fiber and connector losses are typical planning values. The worked example is a calculation, not a field measurement. I did not run these commands on hardware for this update, so confirm output fields on your own OLT and firmware.