GPON optical power budget: is the ONU low because of distance or because of a fault?

An ONU receiving −25 dBm could be perfectly fine or about to drop. It depends on where it is. At 18 km behind a 1:64 splitter, −25 dBm is expected; at 800 meters behind a 1:8, it's a dirty connector or a bent fiber. The number alone isn't enough: you have to compare it with what that ONU should be receiving.

Where the signal loses power along the way

Between the optical module on the PON port and the ONU, the light loses power in three places:

• The fiber. On G.652.D, in the 1490 nm downstream window, about 0.25 dB per kilometer.
• The splitter. This is the biggest loss. A 1:8 PLC loses around 10.5 dB; a 1:16, 13.8 dB; a 1:32, 17.1 dB; a 1:64, 20.5 dB.
• Connectors and splices. On a typical drop, about 2 dB combined.

The expected power at the ONU is what the port's module transmits minus that sum.

An example

An ONU at 3.2 km, behind a 1:32 splitter, with a module transmitting +3 dBm:

1. Fiber: 3.2 km × 0.25 dB/km = 0.8 dB.
2. 1:32 splitter: 17.1 dB.
3. Connectors and splices: 2 dB.

Expected loss: 19.9 dB. The ONU should receive around −16.9 dBm.

If the ONU reports −24.5 dBm, the actual loss is 27.5 dB: there are 7.6 dB that distance and the splitter don't explain. It's still within what the optical class tolerates, so the subscriber can browse, but something on that drop is wrong and it's going to get worse.

When the gap is a problem

No plant is exact: splices vary, and so do splitters. That's why it makes sense to allow some deviation before raising the alarm. A rule of thumb:

• Less than 3 dB off from expected: the ONU is where it should be.
• 3 dB or more of extra loss: something on that drop needs checking (connector, splice, bend).
• Total loss exceeds the optical class budget: the ONU is beyond what the link can sustain, even if it's still online.

The optical class budget

GPON modules are sold by class, and each class defines how much total loss the link can absorb (ITU-T G.984.2):

• B+: 28 dB.
• C+: 32 dB.
• C++: 35 dB.

With a B+ module, a network with 20 dB of expected loss has 8 dB of margin for aging, repairs and future splices. With a C+, 12 dB. That margin is what a degrading plant eats away, little by little.

Cascaded splitters

If the network uses two levels (for example, a 1:8 in the cabinet and another 1:8 in the NAP box), the subscriber ends up behind an effective 1:64, but with the loss of both splitters added together: 10.5 + 10.5 = 21 dB, a bit more than a single 1:64. That's how it should go into the calculation.

Doing it with real data

The calculation is useful when it's done with each ONU's own data: its distance, the splitter in its NAP box and the power transmitted by its port's module. In eCloud OLT Controller, each ONU's detail page calculates it automatically from that data, and if any of it isn't real, instead of giving a verdict it tells you what still needs to be entered.

To try it with your own numbers, there's the site's optical power budget calculator: the same model, no login required.