GPON Splitter Choice for a 20-ONT Rural Link
A BSNL JTO is planning a GPON FTTH link from an OLT to a gram panchayat 12 km away. The OLT PON port launches +3 dBm; ONT receiver sensitivity is -27 dBm and overload -8 dBm. Feeder fiber loss is 0.35 dB/km including splices. There are 0.5 dB connectors at the OLT, at the splitter input, at each splitter output used, and at the ONT. The drop fiber to the farthest ONT is 1.5 km at 0.35 dB/km. The village needs 20 ONTs now. The vendor offers either a 1:32 splitter with 17 dB insertion loss or a 1:64 splitter with 21 dB insertion loss and argues that 1:64 is better because it allows future growth. The JTO must choose a splitter and keep at least 3 dB engineering margin above receiver sensitivity for aging, repairs, and temperature. During commissioning, one ONT reports -29 dBm and fails to range, while others are normal.
1. Calculate the worst-case received optical power at the farthest ONT for both splitter options. Recommend one option and justify it against the 20-ONT requirement, receiver overload, and the 3 dB engineering margin.
Read the worked answer
Use the farthest ONT because it has the greatest loss.
Total connector loss = OLT connector 0.5 + splitter input 0.5 + splitter output 0.5 + ONT connector 0.5 = 2.0 dB. Feeder loss = 12 km x 0.35 dB/km = 4.2 dB. Drop loss = 1.5 km x 0.35 dB/km = 0.525 dB.
For the 1:32 splitter: P_rx = 3 - 4.2 - 2.0 - 17 - 0.525 = -20.725 dBm. Margin to sensitivity = -20.725 - (-27) = 6.275 dB. This is above the required 3 dB. The level is also well below the -8 dBm overload point.
For the 1:64 splitter: P_rx = 3 - 4.2 - 2.0 - 21 - 0.525 = -24.725 dBm. Margin to sensitivity = -24.725 - (-27) = 2.275 dB. This fails the 3 dB engineering margin. Overload is not a problem, but the design is too tight for splices, ageing, and repairs.
Recommendation: use the 1:32 splitter. It serves the required 20 ONTs with 12 spare ports and gives 6.275 dB margin. The 1:64 option leaves only 2.275 dB, so future growth alone does not justify accepting a marginal optical link. If more than 32 ONTs are needed later, use another PON port or a higher-power OLT/optics rather than compromise the current margin.
2. The failing ONT measures -29 dBm and does not range. List the first three troubleshooting steps you would take and state the most likely cause, distinguishing it from a splitter-port failure.
Read the worked answer
The measured -29 dBm is below the -27 dBm sensitivity, so the ONT cannot synchronise and range.
First three steps:
- Inspect and clean the ONT connector and the splitter output connector, then re-measure with an optical power meter. A dirty connector can easily add several dB.
- Check the drop cable for tight bends, pinches, or a damaged patch cord, and measure at the splitter output with a known-good patch cord. If the splitter output is normal but the ONT end is low, the drop cable is suspect.
- Swap the ONT with a known-good unit on the same port and re-test. If the level remains low, the fault is in the fiber/connector path; if the level becomes normal, the ONT is faulty.
Most likely cause: contaminated or damaged connectors, or a tight bend in the drop fiber. A splitter-port failure is less likely because the other ONTs are normal; to confirm, measure another known-good ONT on the same splitter port. An OTDR trace from the splitter to the ONT is useful if the visual inspection does not reveal the fault.