Testing, Fault Diagnosis and Maintenance
This domain tests systematic fault finding, instrument choice and maintenance records. The exam favours a logical isolation sequence over guessing.
Core rules
- Work layer by layer: check power, link LED, cable, port, configuration, then the upstream network.
- Instruments: multimeter for voltage and continuity, cable tester for pinout, TDR for copper length and fault location, OTDR for fibre events and distance, optical power meter for live level, VFL for breaks, tone generator for tracing.
- BER and loopback tests prove end-to-end integrity; ping and traceroute test IP reachability.
- For fibre, clean connectors, measure a reference power, then compare with the expected loss budget. OTDR dead zones hide events near the connector.
- Maintenance includes scheduled cleaning, UPS and battery checks, firmware updates, spare inventory and a fault log.
Worked example
A GPON ONT shows no service. The expected receive level is −20 dBm but the power meter reads −30 dBm, an extra 10 dB loss. The technician cleans the connector and re-measures −20.5 dBm. If cleaning had not helped, the next step is to test at the splitter with a VFL and OTDR, not to replace the ONT blindly. Record the before and after values and close the ticket with the cause.
Traps
Replacing the ONT first; measuring optical power with a VFL; using an OTDR on a live link without approval; confusing an OTDR dead zone with a break; accepting a link LED as proof of throughput; failing to document the root cause.