A red LOS light usually means the ONT isn't receiving the optical signal it expects, although the exact LED behavior depends on the model. Confirm the indicator, check the operator's service status, and inspect only the fiber path you are authorized to access. Measure receive power under known conditions before reaching for an OTDR to locate an event. This workflow is for an authorized fiber installer or maintenance technician dealing with a failed handover or service interruption. The ONT is the optical network terminal between the provider's fiber and the customer's equipment. LOS means loss of signal, and the ONT manual defines what its LED pattern means. If you're the customer, write down the ONT model, the complete LED pattern, when the outage began, and any visible cable damage. Check the provider's outage page and make sure the equipment has power. Leave connectors and provider boxes closed, never look into a fiber end, and pass those observations to the provider. Choose the branch supported by your observations The operator reports an upstream outage Received power is outside the specified window Power appears acceptable but LOS remains An isolated section needs fault location 1 Confirm the indicator and the test boundary Note whether LOS is steady or flashing, and record the PON and power indicators at the same time. Then check the documentation for that exact ONT. Optical reception, synchronization, and service registration are separate states; a generic internet LED chart can send the diagnosis down the wrong path. Ask the operator whether it sees a shared outage or upstream fault. When several subscribers are affected, investigate that common branch before disturbing a customer drop that may be perfectly healthy. Define the test boundary before disconnecting anything. Be clear about which patch cords, distribution sections, and connectors you may inspect, and arrange any service interruption in advance. Never look into a fiber end because live optical radiation may be invisible. 2 Inspect accessible connections first Start with what you can see: a pinched cord, a tight bend, a pulled connector, or damage near recent work. Compare any bend with the cable maker's minimum radius. Don't straighten a cable aggressively or unplug every connector just to see what happens. Where access is authorized, use an inspect-clean-inspect routine with the correct scope and cleaning materials. Match connector and polish types; mating APC and UPC connectors isn't a troubleshooting shortcut. Check contamination and poor connections before deciding that the buried cable is broken. Fluke Networks fiber troubleshooting guidance Field photo showing careful handling of test leads and accessible cable sections. The pictured instrument is not an NF-8607; use the NF-8607 and fiber-specific accessories for the optical checks described here. If you substitute a compatible known-good patch cord, keep every other condition the same and retest. An improvement points toward that cord or connection, but reconnecting may also have disturbed contamination. Inspect the removed parts before assigning the final cause. 3 Measure received power with defined conditions Get the operator's receive-power window for this ONT and service before measuring. Confirm the test point, expected wavelength, required instrument, and connection method, then keep those details with the reading. There is no useful universal dBm pass/fail number for every service. An optical power meter reports the power at its own input. dBm is an absolute level referenced to 1 mW; dB is a ratio or difference, such as relative loss. A reference-mode value in dB can't be substituted for an absolute receive-power reading in dBm. Where the approved method allows it, the NOYAFA NF-8607 can measure through its OPM interface. Connect at the authorized point with the matching adapter, select the calibration wavelength, and record the absolute reading with its units. If you also use a relative REF value, keep it clearly separate. Field measurement conditions should be recorded with the result. The pictured instrument is not an NF-8607 and does not represent an optical-power reading. Selecting a wavelength on a conventional broadband OPM changes its calibration; it doesn't separate every optical channel on a live PON. If the job requires an individual service wavelength, use the operator-approved wavelength-selective instrument and procedure. Read negative dBm values correctly Here's a simple reading example. Under comparable conditions, -20 dBm changing to -23 dBm is a 3 dB drop, which is roughly half the earlier received power. The more negative number means less power. These values are for interpretation only; they aren't NF-8607 measurements or ONT limits. Compare the actual reading with the correct equipment window: Actual result Check before assigning a cause Next action Below the specified window Point, wavelength, connector, adapter, and meter range Isolate the authorized section or report the low reading Above the specified window Reading validity and possible receiver overload Follow the operator's procedure rather than improvising attenuation Inside the window but LOS remains Relevant channel, exact LED meaning, and ONT/service pairing Ask the operator to check signal and ONT status When the power reading appears acceptable First confirm that the reading represents the expected service channel at the required point. Combined light on a non-selective meter may not answer that question. Even a valid receive-power reading says nothing by itself about synchronization, registration, or throughput. When escalating, include the ONT identity, LED pattern, reading, units, wavelength, and measurement point. That information is far more useful than saying only that the fiber has light. 4 Choose a locator for the question you need to answer On a short, accessible, isolated section, a visual fault locator can help identify the fiber or reveal a gross light leak. It doesn't measure receive power, and a fault hidden inside the jacket may show no visible glow. Keep your eyes away from the beam and every fiber end. An OTDR analyzes returned light to estimate event positions and characterize loss and reflection. Use it when the question is where an optical event lies on an authorized test section. It doesn't register an ONT or prove internet speed. VIAVI's French-language OTDR explanation For the NF-8607 workflow in this article, test a dark, isolated fiber. General 1310/1550 nm operation is not evidence of filtered live-PON capability. Live-service work needs the right instrument, wavelength, and operator-approved procedure. Figure 1. Concept comparison of OPM, VFL and OTDR questions: optical power, visible fault clues and optical event location. These checks do not establish synchronization, registration or working service. Device drawings and colored light are illustrative. 5 Set the OTDR for the actual section The reviewed NF-8607 V2 manual includes Auto-OT, Expert-OT, and an event-map view. Open Setup to confirm the wavelength, units, and refractive index. Expert mode also exposes range, pulse width, and duration. Choose values for the actual fiber and path instead of copying the manual's example screen. A structured field check depends on a known test boundary and repeatable settings. The pictured locator is not an NF-8607; the photo illustrates workflow rather than fiber-test capability. Use a suitable launch fiber when you need to characterize the first connection, and a receive fiber when the far-end connection matters. Record their lengths and how you account for them. Otherwise, the displayed distance can include the launch lead and an end connection may sit inside a dead zone. Figure 2. OTDR setup screen excerpt from the NF-8607 V2 manual, printed page 06, with added explanations. Illustrated settings are not recommended defaults for every link. A shorter pulse can separate events that are close together, but it trades away range and signal. A longer pulse reaches farther yet may hide nearby events in the response around a reflection. Match the setting to the section; maximum range is rarely the best starting point for a short link. Fluke Networks OTDR parameter guide Use Start to acquire the trace, then read the curve together with the event list. Save the result with Storage before leaving the function. The reviewed manual warns that unsaved test data is cleared on exit, so reopen the saved file and confirm that the record is there. 6 Read a curve as evidence rather than a fault label Distance runs along the horizontal axis. The vertical trace represents returned light, not the ONT's receive-power measurement. Establish the fiber end and launch-lead allowance first, then work through the changes along the curve. Figure 3. Original conceptual OTDR curve for reading practice. Distances and trace shape are illustrative; it is not a measured NF-8607 result. Read the numbered features with the installation context: 1 Launch reflection: the response near the connection can hide nearby events. The shaded region is illustrative, not an NF-8607 dead-zone specification. 2 Reflective event: a connector or another reflective discontinuity may produce a peak. Check its position against the route and event list. 3 Non-reflective loss step: a bend, splice, or another loss mechanism may produce a step without a strong peak. The shape alone does not identify the physical cause. 4 End event: a reflection followed by the noise floor can indicate an open end. If the apparent end is earlier than the documented path, investigate that area and the setup. Events that sit close together may merge. A launch lead, appropriate pulse settings, and a comparison from the opposite end can help when the approved method allows it. Save the original trace with its direction and test conditions; a simplified drawing isn't a substitute for that evidence. 7 Verify the service after the authorized repair After the authorized repair, repeat the power measurement at the same point with comparable settings. Confirm the expected ONT indicators, obtain operator acceptance where required, and test the actual service. A cleared LED is encouraging, but it isn't the entire handover. Keep the conditions with the reading Record item Enter the actual information Service and symptom ONT model, fiber identity, LED pattern, failure time Access boundary Section authorized for inspection, disconnection, or testing Receive-power method Measurement point, channel or wavelength, meter mode and units Acceptance basis Equipment or operator window and document reference Before and after Actual readings under comparable conditions OTDR conditions when used Direction, wavelength, IOR, range, pulse width, duration, thresholds Distance allowance Launch and receive fibers and compensation method Completion Saved file identity, repair, optical checks and service result