GPON Drop Fault After Splitter
A GPON FTTH network uses a 1:8 field splitter and a 1:4 building splitter. The OLT launches +3 dBm at 1490 nm. The feeder is 8 km at 0.35 dB/km, the field splitter has 10.5 dB insertion loss, the distribution fibre is 2 km at 0.35 dB/km, the building splitter has 7.2 dB loss, and the drop fibre is 80 m at 0.35 dB/km. Six connectors are in the path, each assumed 0.3 dB. One ONT on port 3 of the building splitter reports LOS; the other three ONTs on that splitter work. A test at the building splitter port with a power meter reads -19.5 dBm, but there is no reading at the customer ONT end. The technician has an optical power meter, a visual fault locator, a known-good ONT, and a spare drop cable. The customer needs service restored within two hours.
1. Calculate expected optical power at the ONT and compare with -19.5 dBm at the building splitter port. Does the result point upstream or downstream?
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Losses: feeder 8 x 0.35 = 2.8 dB; field splitter 10.5 dB; distribution 2 x 0.35 = 0.7 dB; building splitter 7.2 dB; drop 0.08 x 0.35 = 0.028 dB; six connectors x 0.3 = 1.8 dB. Total = 2.8 + 10.5 + 0.7 + 7.2 + 0.028 + 1.8 = 23.028 dB. Expected ONT power = +3 - 23.028 = -20.03 dBm, about -20.0 dBm. The measured -19.5 dBm at the building splitter port is close to the calculated power at that point, before the drop and its connectors, so the feeder, field splitter, and distribution path are healthy. The problem is downstream of that port: drop fibre, connector, splitter port, or ONT.
2. How should the technician localise the fault with the given tools? Include a port-swap test.
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Because the other three ONTs on the same building splitter work, the common feed and splitter input are good. First connect a known-good ONT to port 3: if it also reports LOS, port 3 or the drop is faulty; if it works, the original ONT is faulty. Next connect the power meter to port 3; no light or very low power means the port or drop path is faulty. Swap the customer drop to a spare port: if service returns, port 3 is bad; if not, the drop is bad. Use the visual fault locator on the drop to find a break or sharp bend; inspect and clean the connector, then remeasure. Do not look into the fibre; use the meter and VFL only.
3. What repair and verification would you authorise within the two-hour window?
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Authorise the least invasive fix that matches the isolation: clean or replace the drop connector, replace the drop cable if the VFL shows a break, or move the drop to a spare splitter port if port 3 is dead. Replace the ONT only if a known-good ONT works on the same port and the original still fails. After repair, verify optical power at the ONT is within the receiver range (about -8 to -27 dBm for many GPON ONTs, but use the manufacturer's limit), confirm the ONT registers, and pass a throughput and stability test. Record the measured power and the replaced item. If power is still near -20 dBm but the ONT does not register, escalate as an ONT or OLT provisioning issue rather than replacing more fibre.