In a passive optical local area network (POL), where each end-user or workstation is equipped with its own optical network terminal (ONT) connected via a fibre optic cable, any period of downtime can result in decreased productivity and potential revenue loss. Whether you are an IT manager, a network maintenance technician, or a fibre contractor, your primary objective is to restore network access to users as expeditiously as possible. If the issue affects only a single user or a small group within the same area, it is likely associated with the fibre optic connection. By employing the appropriate troubleshooting tools and adhering to best practices, you can effectively address the problem and minimise the mean time to repair (MTTR).

Understanding the Importance of Swift Resolution

In a POL environment, any network downtime can lead to:

  • Loss of productivity
  • Potential revenue loss
  • Frustrated end-users

When users are unable to access the network, it is imperative to identify and resolve issues promptly. If the problem affects a single user or a small group within the same zone, the fibre optic link is often the root cause.

EXFO Passive Testers and Inspection Tools

Passive Optical LAN Testing

As the trend towards fibre-to-the-home (FTTH) continues to expand bandwidth offerings, a novel technology is emerging to enhance bandwidth, provide additional services, and ensure future-ready networks for enterprises: passive optical LAN (POL). Also known as OLAN or fibre-to-the-desktop, this GPON-based solution establishes a highly cost-effective local area network (LAN) with virtually limitless capabilities.

The increasing demand for services, experiences, and data has propelled the rapid deployment of fibre optic networks to the forefront. The advent of 5G and the transformative potential of the Internet of Things (IoT) and Industrial Internet of Things (IIoT) have underscored the significance of fibre optic networks. Consequently, the testing of fibre optic networks has become a critical component of this infrastructure.

What Needs to Be Tested?

The fibre architecture of POL networks comprises in-building cabling that utilises single-mode fibres powered by GPON technology and optical splitters. EXFO recommends a four-step approach for testing passive optical LANs.

 

Step 1: Connector Inspection with a Fibre Inspection Probe

This serves as the foundation of any fibre manipulation, as faulty connectors are the primary cause of network failures. Inspection ensures that the connector endface is clean and provides a software analysis indicating whether a specific patch cord can be connected. Clean connectors optimise performance and maintain a healthy network, enabling high-speed bandwidth provision.

 

Step 2: Full Fibre Characterisation with OTDR-Based Technology

EXFO offers an OTDR-based solution called iOLM. This technology characterises the fibre and examines every component within the link, including all splitters and connectors, to provide a comprehensive end-to-end loss analysis and identify potential issues, such as fibre breaks. GPON architecture typically requires a splitter (usually 1:16 or 1:32), which are high-loss components that pose a testing challenge. Therefore, it is crucial to utilise an industry-standard “PON-optimised OTDR.”

 

 

Step 3: Testing Service Activation with a PON Power Meter (PPM)

The PPM is a specialised power meter that validates the upstream and downstream power levels prior to activation, ensuring the proper functioning and readiness of the active equipment (both the OLT and ONT) for user access.

 

Step 4: Reporting

Conducting multiple tests with various instruments necessitates the concatenation and aggregation of all test results to furnish lucid reports to network owners. These reports should encompass validations, evidence, and job closures.

 

Best Practices for Troubleshooting

1. Inspect Optical Ports and Connectors

The initial step in troubleshooting entails examining the optical ports of the affected ONT and the optical connectors of the fibre jumper connecting the ONT to the wallplate (or other connecting interface).

Tools required:
  • Automated fibre inspection probe
Process:
  1. Utilise the inspection probe to examine connector endfaces.
  2. Identify a clear pass/fail outcome.
  3. Should the inspection fail, proceed to cleaning.

2. Clean Faulty Connections

Numerous network issues originate from dirty or damaged connectors. Appropriate cleaning can frequently resolve these problems.

Tools required:
  • Fibre optic cleaning tools
Process:
  1. Employ suitable cleaning tools to remove particles from the connector.
  2. Re-inspect using the automated probe.
  3. Repeat cleaning if necessary until the inspection test passes.

3. Verify Fibre Link Integrity

Once all ports have been inspected and cleaned, it is imperative to assess the overall integrity of the fibre link.

Tools required:
  • Optical Fibre Multimeter (OFM)
Process:
  1. Connect the OFM to the jumper cable connecting to the ONT.
  2. Monitor incoming optical power levels.
  3. Should low or no light be detected, utilise the OFM’s troubleshooting mode.

4. Identify Faults

If the link is damaged, pinpointing the precise fault location is crucial for prompt repairs.

Tools Required:
  • Optical Fault Locator (OFM) with fault location capabilities
Process:
  1. Activate the OFM’s troubleshooting mode.
  2. The OFM will display the precise distance to the fault location.
  3. Utilise this information to pinpoint the exact location for inspection and resolution of the issue.

Technical Considerations

  1. Downstream Signal: Pay meticulous attention to the optical power levels of the downstream signal originating from the Optical Line Terminal (OLT).
  2. Fibre Quality: Assess the quality of the fibre optic cable from the first optical splitter (zoned or centralised).
  3. Power Levels: Normal power levels can fluctuate depending on the specific POL system, typically ranging from -15 dBm to -28 dBm at the ONT.
  4. Attenuation: Be cognisant of the inherent attenuation levels in your fibre links. Unexpectedly high attenuation could indicate a bend, break, or other form of damage.
  5. Splitter Loss: Account for the loss introduced by optical splitters within your network. A typical 1:32 splitter may introduce approximately 17 dB of loss.

Conclusion

Efficient troubleshooting of POL fibre optic links necessitates a combination of best practices and appropriate tools. By adhering to this technical guide and employing equipment such as automated inspection probes and optical fibre multimeters, technicians can swiftly identify and resolve network issues. This approach not only minimises network downtime but also maximises the return on investment for POL infrastructure.

Remember that in the realm of fibre optics, meticulous cleanliness is paramount. Always commence with inspection and cleaning, progressing to more advanced troubleshooting techniques. Equipped with these skills and tools, you will be adequately prepared to maintain a robust and dependable POL network.