What is a good ONU signal level?
For class B+ GPON optics, an ONU works between -8 dBm and -27 dBm. The closer it sits to -27 dBm, the less margin it has left.
By the Autoprator team. Updated .
GPON optics are specified in ITU-T G.984.2. For the common class B+ optics, the ONU receiver works from -8 dBm, where it overloads, down to -27 dBm, its sensitivity limit. Class C+ optics reach further, to about -30 dBm.
Source: ITU-T G.984.2: G-PON physical media dependent layer
How do I read the number?
The numbers are negative, so -20 dBm is stronger than -25 dBm. Too weak, and the link starts to drop. Too strong, near -8 dBm, and the receiver can overload, which mostly happens on very short runs.
Should I compare ONUs?
Yes. One ONU that is several dB weaker than its neighbours points to its own drop cable or connector. A whole port that gets weaker together points to the feeder fibre or a splitter.
Why watch the trend?
A signal that slowly falls gives you a warning before the customer loses service. Keep a history for each ONU and look at the change, not only today's reading.