Testing PONs
This domain tests passive optical network testing, where splitters, multiple wavelengths, and live traffic change the rules. The exam expects you to know PON wavelength assignments, splitter loss, PON power meter use, filtered OTDR testing, and safe measurement at the ONT.
PON wavelengths and power
GPON and similar PONs use about 1490 nm downstream, 1310 nm upstream, and often 1550 nm for video overlay. A PON power meter filters these bands and displays each wavelength separately; a generic power meter may sum them and give a misleading number. Downstream power is measured at the ONT; upstream is bursty and is normally measured with the PON meter’s upstream function or at the OLT, not with a continuous source.
Splitters and OTDR
A 1×32 splitter has an ideal splitting loss of about 15 dB, plus excess loss. A standard OTDR cannot see through a live splitter at 1310/1550 nm in the normal way; use a filtered PON OTDR at an out-of-band maintenance wavelength such as 1625 nm and follow the PON test plan. A launch cable is still needed, and the splitter’s high loss limits dynamic range.
Worked example
A 1×32 PON segment has 20 km of single-mode fiber, four connector pairs, two splices, and a splitter with 15.5 dB including excess loss. Fiber: 20 × 0.35 = 7.0 dB. Connectors: 4 × 0.3 = 1.2 dB. Splices: 2 × 0.1 = 0.2 dB. Total = 7.0 + 1.2 + 0.2 + 15.5 = 23.9 dB. Against a 28 dB Class B+ budget, the margin is 4.1 dB before measurement uncertainty and aging.
Traps
- Measuring the downstream 1490 nm signal with a meter set to 1310 nm.
- Using a generic power meter that sums all PON wavelengths.
- Trying to see through a splitter with a standard OTDR and calling the trace a fiber fault.
- Measuring upstream with a continuous source instead of the burst-mode PON meter.
- Forgetting splitter loss and excess loss in the budget.