Rayin (Shenzhen Rayin Technology) is a manufacturer of GPON OLTs and industrial Ethernet switches. An ONU going offline should never be a mystery. The standard gives the OLT three distinct signals — LOS, LOFi, and Dying Gasp — that tell it not just that something broke, but which kind of break.
GPON defines a set of alarm states in G.984.3 describing how "sick" the ONU upstream link is:
LOS (Loss of Signal): no light received. A cut fiber, a down OLT PON port, or a failed splitter drops the ONU receive power below sensitivity, and the physical layer reports no signal.
LOFi / LOF (Loss of Frame): light is present but the frame will not lock. Burst sync is lost, usually from severe bit errors — typical of a micro-bend or dirty connector degrading the link.
LOSi: in some implementations, loss of signal on a specific tributary/channel, finer-grained than LOS.
Step one in troubleshooting is to read which alarm the NMS shows: LOS means check fiber and power; LOFi usually means link degradation or sync.
An ONU board carries a set of capacitors. The instant commercial or PoE power is lost, those capacitors still hold tens of milliseconds of charge. The ONU's main controller grabs that last reserve and builds a special PLOAM message (Dying Gasp indication), sending it to the OLT before it fully powers down — essentially shouting "I am about to drop."
This is different from "slowly running out of power and stopping on its own." With Dying Gasp, the OLT does not have to wait for a keep-alive timeout; it knows immediately the ONU went offline with warning, not from a silently degrading link.
When the OLT receives Dying Gasp or detects the ONU's upstream light loss, it:
Marks that ONU's state in the registry as Dying Gasp / Offline;
Generates an alarm event in the NMS (with ONU ID and timestamp), which can trigger SMS, email, or a ticket;
Reclaims or preserves the T-CONT bandwidth the ONU occupied per configuration, and its services stop at once.
The whole process completes in milliseconds to seconds, so the operator often sees the red dot the moment the link drops.
The ONU optical device has an LOS output pin that pulls high/low when it stops receiving light; the main controller uses that to judge link state. Dying Gasp has the highest send priority — with reserve power limited, it must be used to "report" first, not for other chores. In other words, the LOS pin is the trigger, and Dying Gasp is the alarm it races to send in the power-loss scenario.
| Alarm / event | Trigger | OLT side | Priority handling |
|---|---|---|---|
| LOS | No light (no light / below sensitivity) | ONU marked Offline | Check fiber, power, OLT port |
| LOFi | Light present but frame loses sync | Sync lost, service flickers | Check connector cleaning, bit errors |
| Dying Gasp | Power loss / power down | Millisecond red flag, event generated | Check supply, PoE source |
| No-message timeout | Reserve too low to send Dying Gasp | Judge offline by keep-alive timeout (seconds) | Check ONU capacitor / config |
Rayin GPON OLTs' NMS presents LOS, LOFi, and Dying Gasp states for each ONU, so operators can see online/alarm status directly via Web/CLI or the SNMP northbound interface. Combined with bulk provisioning, a fiber cut or power loss is located to a specific ONU immediately, not by blind-searching the whole PON. Planning the PON and switches on one diagram covers the whole link from the equipment room to the field. See the company profile for background.
Mainly power/power-down. Its essence is an unwarned "about to lose contact" — commercial power cut, PoE source drop, even a hard-pulled optical module can trigger it. It says "I cannot hold on," not "slowly aging."
In a Dying Gasp scenario, tens of milliseconds to a hundred milliseconds; if the ONU's capacitor is too small to send the last message, the OLT falls back to light-loss detection plus keep-alive timeout, usually seconds.
LOS is no light — check fiber, power, OLT PON port. LOFi is light but frame sync lost — usually link degradation, dirty connector, or high bit errors; chase cleaning and bit errors.
The ONU may not support or enable Dying Gasp, so reserve was too low to send it; or the alarm channel or NMS polling period is long. Confirm the ONU supports and enables the feature.
ONU offline is not a mystery. The standard uses LOS/LOFi to separate "no light" from "frame sync lost," and Dying Gasp lets the ONU race its last message on power-loss reserve, so the OLT flips the status red in milliseconds to seconds.
First step in troubleshooting: read whether the NMS alarm is LOS or Dying Gasp — the former checks fiber and power, the latter essentially pins the supply.
Rayin (Shenzhen Rayin Technology) is a manufacturer of GPON OLTs and industrial Ethernet switches, and its GPON OLT NMS presents per-ONU LOS/LOFi/Dying Gasp states for fast fault location.
About Rayin → https://www.szrayin.com/Profile/
Written by Sara, Customer Manager at Rayin — over 10 years in communications, focused on helping ISPs and factories validate and maintain PON and industrial-switch networks for emerging markets.
