PON problems: ONU registration, LOS & levels

Passive optical networks add their own failure modes on top of ordinary optics: one transmitter talks to dozens of receivers through a splitter, the two directions use different wavelengths, and the OLT decides who may join. This page covers the faults specific to PON — especially with SFP ONU "sticks" and OLT modules, where CodingBox users spend most of their PON time.

ONU does not register

The ONU sees light but the OLT never activates it. Work down the list:

Check	Why it blocks registration	How to fix
PON standard	a GPON ONU on an XGS-PON OLT (or vice versa) uses different wavelengths and never hears the OLT	match the module to the OLT: GPON 1490/1310, XGS-PON 1577/1270 (PON wavelengths)
Serial number / whitelist	most OLTs activate only known ONU serials (GPON SN, vendor ID + 8 hex)	register the ONU's serial on the OLT — or set the stick to present the serial the OLT expects
LOID / PLOAM password	some operators authenticate by logical ID or password instead of / in addition to the serial	configure LOID/password in the stick's firmware
Vendor ID	operator OLTs may accept only approved ONU vendor codes	the stick must present an accepted vendor ID (the 4-letter prefix of the serial)
ONU Rx power out of class	below ~−27/−28 dBm (class B+) the ONU cannot decode downstream; above ~−8 dBm it saturates	fix the plant, or add an attenuator on a too-short drop
Ranging / distance	ONU outside the OLT's differential reach window (typically 20 km, sometimes 60)	check fibre length and OLT settings
OLT port	port disabled, ONU limit reached, wrong profile	OLT-side configuration
Rogue ONU	another ONU transmitting continuously blinds the OLT receiver for everyone	isolate by disconnecting drops one at a time; OLTs report rogue detection

Two identities live in an ONU stick: the MSA module identity (SFF-8472 — what the host router reads) and the PON identity (serial, vendor ID, LOID/password — what the OLT sees). CodingBox codes the former; the latter is set through the stick's own management interface. See PON transceivers: OLT & ONU.

ONU shows LOS

No downstream light at all:

Dirty or damaged SC/APC — PON connectors are APC; one dirty end-face on a drop with 28 dB of budget already spent is enough.
UPC in an APC plant — a blue-boot patch in a green-boot network costs several dB and reflects into the OLT (Physical mismatches).
Splitter port unused / damaged; fibre break between splitter and ONU (OTDR from the ONU side sees it; from the OLT side the splitter hides individual drops).
Wrong wavelength module (see the table above) — reads as LOS, not as "wrong standard".

Levels

Reading	Normal	Too low	Too high
ONU Rx (GPON B+)	−8 … −27 dBm	budget miss: dirt, splitter ratio, distance	drop too short → attenuator
OLT Rx per ONU	−8 … −28 dBm	one drop's problem; compare ONUs on the same port	burst overload
ONU Tx	+0.5 … +5 dBm (GPON)	dying laser	—

The OLT reports per-ONU upstream power (through ranging), while the OLT SFP's own DDM Rx is a burst average — use the OLT system's per-ONU figure for diagnosis (DDM levels).

Intermittent service

Marginal ONU Rx (−26…−28 dBm) — works until a warm day or a bent patch cord.
Overheating stick — SFP ONUs run hot inside consumer routers with no airflow; symptoms are drops in the afternoon and a temperature alarm in DDM (Power & thermal).
Host compatibility — the router's SFP port must support the stick's rate (GPON sticks present 1G or 2.5G; XGS-PON sticks need a 10G SFP+ port) and mode (SGMII / 1000BASE-X) (Speed & rate).
OMCI / provisioning — service configured but no traffic: an OLT profile issue, not optics.

OLT-side optics

OLT modules are class B+/C+/C++ (see PON transceivers); a C+ plant fed by a B+ module loses ~4 dB of budget.
OLT SFPs are often vendor-locked to the OLT chassis exactly like switch optics (Vendor lock).

Diagnostic sequence

Confirm standard and wavelength of the module against the OLT.
Read ONU Rx — from the stick's DDM on the bench or via the router.
Inspect and clean the SC/APC drop; check polish colour.
Verify serial / vendor ID / LOID against what the OLT expects.
Check the OLT: port state, ONU list, rogue alarms, per-ONU Rx.
Swap in a known-good ONU on the same drop.

In CodingBox

CodingBox reads a PON stick or OLT module like any SFP: wavelength and rate codes on the Check transceiver screen (is it GPON or XGS-PON? OLT or ONU side?), live Rx/Tx and temperature on DDM, and the MSA identity a host router expects in the EEPROM editor.