Guide for ISPs

PON signal levels in dBm: what is a good ONU signal?

A customer says the internet is slow. Is it their WiFi, their package, or the fibre itself? The fastest way to answer is the optical signal at their ONU. If you know what the number means, half of your support calls can be sorted before anyone leaves the office.

13 October 2026 · 5 min read · ISPOpera

What the number is

The PON signal, or receive power, is how much light from the OLT reaches the customer’s ONU. It is measured in dBm and is a negative number: -18 dBm is a stronger signal than -26 dBm. Every splitter, connector, splice and kilometre of fibre between the OLT and the ONU takes some of the light away.

Good, weak and bad

These are the ranges ISPOpera uses to colour a customer’s signal:

Good: from -8 dBm down to -24 dBm. The line is healthy.
Weak: from -24 dBm to -27 dBm. It works, but there is no margin left. A dirty connector or a hot day can push it over.
Bad: below -27 dBm. Expect slow speed and disconnections.
No signal: the ONU is switched off, or its fibre is down.

A signal can be too strong

Stronger is not always better. An ONU very close to the OLT, or behind too little splitting, can receive more light than its receiver is built for. ISPOpera shows anything stronger than -8 dBm as Weak for this reason. The usual fix is an optical attenuator on that line.

What makes a signal weak

A dirty or scratched connector. This is the most common cause, and the cheapest to fix.
A fibre bent too tightly, often behind furniture or at the entry point of the house.
A bad splice in a joint box.
Too much splitting on one PON port.
A long run of fibre added to a splitter that was already near the limit.

One customer or many?

Before sending anyone, look at the neighbours. If one customer under a splitter is weak and the others are good, the problem is on that customer’s own drop cable, connector or ONU. If every customer under the same splitter is weak, the problem is above them: the splitter, the cable feeding it or the OLT port.

Also check the ONU’s transmit power

The ONU sends light back to the OLT too. If the receive signal looks fine but the customer still drops, compare the ONU’s transmit power with other ONUs of the same model. One that is far off the rest may be a failing ONU.

How to see every customer’s signal without a visit

Reading the signal on the OLT one ONU at a time works, but nobody does it until a customer complains. It is far more useful when the signal is already next to the customer’s name.

With your OLT connected to ISPOpera, the signal at each customer’s ONU is read every 10 minutes and shown on the customer’s page and in the customer list as Good, Weak or Bad, with the ONU’s transmit power and the fibre distance. Customers are matched to their ONU automatically. From the same page you can change the customer’s WiFi name and password, set the PPPoE login and VLAN on their device, and see their live speed.