ONU registration is the process that lights up the link between the user-end gpon onu and the central-office OLT: discover, authenticate, pull up service. Under GPON it runs four steps — the ONU finds the OLT and reports its serial number, the OLT assigns an ONU-ID and equalizes delay, OMCI pushes the configuration, and traffic goes live. In the field, a failed registration is often not a dead box; it is a dirty connector, a wrong ID, optical loss past the line, or a VLAN that never aligned. When engineers weigh onu vs ont for a rollout, both are user-end terminals and the fiber onu (or ont fiber) simply lands at the subscriber's premises — the registration steps below apply to either. Below, the causes are listed from most to least frequent.
Most "won't register" cases are optical or config, not hardware: dirty fiber, wrong LOID/SN, loss over budget, or VLAN mismatch.
A gpon onu normally receives -8 to -28 dBm; below -28 dBm trips LOS, above -8 dBm risks optical saturation. Measure Rx at the ONU first.
1:128 splitting alone costs about 21 dB; together with connectors and fiber it eats the Class B+ 28 dB budget, leaving only a few dB of margin.
Walk the link in order — optical power, then SN/LOID, then VLAN — and you usually locate the fault within ten minutes.
Many installers suspect the OLT or the ONU first, but the first thing to check is the end face. APC/UPC connectors with dust, fingerprint oil, or even splice debris on a single end can eat 0.5–3 dB of insertion loss. A marginal link that already has only a few dB of margin drops straight below the LOS threshold once a layer of dust is added.
An optical power meter tells the story. On a GPON link, the ONU side normally receives between -8 and -28 dBm. Below -28 dBm it triggers LOS (loss of signal); above -8 dBm it may saturate the receiver. The OLT PON port usually transmits at 7–9 dBm. Measure at the ONU end with a power meter; if the reading drifts below -30 dBm or shows "no light," wipe both APC end faces with anhydrous ethanol and lint-free paper, then measure again. Do not use a shirt sleeve or tissue — it leaves fibers that make things dirtier.
Bends and crushing count as "dirt" too. A drop cable pinched in a door gap into a sharp bend, or a patchcord squeezed at a splitter port, spikes insertion loss suddenly. A red-light pen shows the leak at the bend.
Note: A reading of "no light" almost always means dirty or broken fiber, a severe bend, or a severed link — not a registration parameter. Wipe the end faces and trace with a VFL before touching any config.
Light is present, but the ONU sits at "authenticating / logging" — that is usually an identity mismatch. GPON uses SN (serial number, 12 hex characters) or password authentication; some GPON also uses LOID (logical ID, usually 12 characters) for access authorization. If the SN/LOID in the OLT authorization list does not match the label on the ONU body, the OLT rejects it outright.
Two checks: first, print the Auth state from display ont info by-port on the OLT. If it says "auth fail," verify that the configured SN/LOID matches the ONU bottom label character for character — case, leading zeros, everything. Second, confirm the ONU was actually added to the authorization table; a missed add also keeps it offline. After the fix, power-cycle the ONU once to re-run authentication.
Even with clean faces and correct IDs, loss past the line still registers unstably. At a 1:128 split ratio the splitter's even split already costs about 21 dB; add 4–6 active connectors at 0.3–0.5 dB each, plus fiber at 0.3–0.4 dB/km, and total insertion loss easily approaches the Class B+ 28 dB budget. Once the budget is spent, only a few dB remain for surprises.
The cruelest case is "reads light but never stable." A reading of -27 or -28 dBm, hugging the LOS threshold, drops registration intermittently the moment the daytime temperature rises or a connector loosens. Measure with a power meter several times — once in the morning, once in the afternoon — it is far more reliable than a single reading. GPON acceptance commonly requires the ONU to receive within -25 dBm (Class B+ / C+ thresholds differ slightly by vendor; C+ can relax to -32 dBm sensitivity).
Warning: A reading near -27 dBm is marginal. Class B+ sensitivity is typically -27 / -28 dBm, so it drops offline intermittently. Keep the ONU receive power within -25 dBm to leave real margin.
Light and authentication pass, the ONU shows "normal" but no traffic — the problem moves to configuration. The OLT binds a line-profile and a service-profile to the ONU, and the VLAN must match both the upper layer and the ONU UNI port. If the three layers are not aligned, service packets cannot leave.
Three common mistakes: the VLAN is not permitted on the PON port (ONT side tagged, OLT side not transparent); the service-port is bound to the wrong ONU or port; the ONU UNI port VLAN mode disagrees with the upper layer (access/trunk reversed). Pull both sides' VLAN with display service-port and display ont port vlan and compare — a single mismatched tag breaks everything. This is tightly tied to PON port planning; decide the VLAN scheme at selection time. Rayin's GPON OLT gives enough room on the PON ports and service templates to lock the VLAN topology down during planning.
| Symptom | Likely cause | Steps / tool |
|---|---|---|
| OLT shows offline, no Rx / LOS alarm | Dirty fiber, end-face contamination, broken fiber, severe bend | Power meter at ONU Rx (normal -8 to -25 dBm); ethanol + lint-free wipe on APC faces; VFL for breaks |
| Light present, ONU stuck "authenticating / logging" | Wrong LOID/SN, not authorized | Match OLT config to ONU label; display ont info Auth state; reboot ONU after fix |
| Light and auth OK, no service | Service/line-profile or VLAN mismatch | Check template binding; display service-port, display ont port vlan for VLAN permit |
| Some ONUs drop intermittently, ping loss | Loss at margin (-26 to -27 dBm), ranging >20km, >1:128 per PON | Measure at several times; display ont ranging for EqD; shorten split or add optical amplifier |
1. An ONU stays offline and the power meter shows no light — what do I check first? Wipe the APC end faces at both the OLT and ONU ends, then trace for breaks with a red-light pen. No light almost always means dirty, broken, or severely bent fiber, not a registration parameter.
2. Why won't it register even with normal optical power? Look at the OLT Auth state. If light is present but it sits at "authenticating," the SN/LOID is wrong or it was never authorized; if the state is normal but service fails, the VLAN or template is not aligned.
3. Is -27 dBm receive power normal? Marginal. GPON Class B+ sensitivity is typically -27 / -28 dBm, so hugging the threshold drops offline intermittently. Keep the ONU receive within -25 dBm for margin.
4. What is onu versus the OLT, and who owns a registration failure? The OLT is the central office managing all ONUs; the ONU is the user end. Locate the failure in the optical layer or the auth/config layer: no light is the optical link (including both transceivers), "authenticating" is OLT authorization or ONU ID, no service is the OLT-side template/VLAN.
Customer Manager at Shenzhen Rayin Technology Co., Ltd.
