PON Fault Scenarios
Scenario 1: Simple PON (only one customer is affected)
There are three potential faults when only one subscriber cannot receive service—fault in the distribution fiber between the customer and the closest splitter, or fault in the ONT equipment, or fault in the customer’s home wiring.
Scenario 2: Cascaded PON (all affected customers are connected to the same splitter)
When all customers connected to the same splitter cannot receive service, but others connected to the same OLT can, the cause may be one of the two—fault at the last splitter, or fault in the fiber link between the cascaded splitters.
Scenario 3: All customers are affected (at the OLT level)
Whether or not the PON is cascaded, all customers dependent on the same OLT may be affected. If all customers are affected, the cause may be from of the three—fault in the splitter closest to the OLT, or fault in the feeder fiber cable of the network, or fault in the OLT equipment.
Troubleshooting a PON first involves locating and identifying the source of an optical problem. Depending on how many customers are affected, and the best location to shoot an OTDR.
Generally, most PON problems can be located using PON power meter and PON-optimized OTDR. The power meter is connected as a pass-through device, allowing both downstream and upstream traffic to travel unimpeded. It measures the power at each wavelength simultaneously and can be used for troubleshooting at any point in the network. A monitoring OTDR provides a graphical trace that enables to locate and characterize every element in a link, including connectors, splices, splitters, couplers and faults. OTDRs designed specifically for in-service PON troubleshooting exist. These OTDRs feature a dedicated port for testing at 1625 or 1650 nm and incorporate a filter that rejects all unwanted signals (1310, 1490 and 1550 nm) that could contaminate the OTDR measurement. Only the OTDR signal at 1625 or 1650 nm is allowed to pass through the filter, generating a precise OTDR measurement. In-service OTDR troubleshooting of optical fiber should be done in a way that does not interfere with the normal operation and expected performance of the information channels. Testing with the 1625 or 1650nm wavelength does just that.
PON troubleshooting should first find the fault locations of the network.
