Rayin (Shenzhen Rayin Technology) is a manufacturer of GPON OLTs and industrial Ethernet switches. When an ONU goes online, it does not just appear — it walks a defined state machine. Knowing which step it is stuck on tells you exactly what kind of fault you are facing.
ITU-T G.984.3 defines the ONU activation process as a state machine, in this order:
Initial → Standby → Serial Number → Ranging → Operation
An ONU that just powered up or recovered from a fiber break starts at Initial and only becomes truly "alive" on the network once it reaches Operation. If any step in between stalls, services cannot open.
The OLT periodically sends a Serial Number Request broadcast. ONUs that are online but not yet identified reply with their SN (unique device serial number). The OLT builds its ONU list from this and assigns each one an ONU ID.
This step answers "who are you" — confirming a device is present and capturing its hardware identity. No SN reply means nothing downstream can proceed.
In real operation, the carrier issues each ONU a Password / LOID as an access permit. The check usually sits after Serial Number and before Ranging: the OLT compares the Password the ONU reports against its backend whitelist and refuses activation if it does not match.
Some implementations fold the Password check into the Serial Number flow. The core distinction to remember:
SN verifies "is this device a genuine device."
Password / LOID verifies "does this device have access permission."
Both must pass before moving on.
This is the prerequisite for upstream TDMA to run. The OLT measures each ONU's round-trip delay (RTD) to itself, then issues an equalization delay (EqD) that aligns each ONU's upstream send time to one reference point.
What if you skip ranging? ONUs at different distances arrive at the OLT at different times, timeslots overlap, upstream data collides, and the whole PON upstream turns to chaos. So Ranging is not "nice to have" — it is the foundation of TDMA.
After ranging completes, the ONU enters Operation. The OLT pushes service configuration via OMCI and allocates upstream bandwidth via DBA, and the user service opens. The ONU now shows "online" in the registry, and the NMS can see its ONU ID and ports.
| State | Trigger / event | OLT action | ONU action |
|---|---|---|---|
| Initial | Power up / fiber break recovery | Wait, enter discovery | Prepare to respond to SN |
| Standby | Receives SN Request | Broadcast roll call | Prepare to reply SN |
| Serial Number | Receives ONU SN | Record SN, assign ONU ID, check Password | Reply SN, report Password |
| Ranging | SN/Password pass | Measure RTD, issue EqD | Cooperate ranging, align timeslot |
| Operation | Ranging complete | OMCI config, DBA scheduling | Service opens, goes online |
Rayin GPON lets you see, per ONU, which state it is stuck in — no SN reply, Password check failed, or Ranging timeout — so troubleshooting is not guesswork. Combined with the bulk provisioning template, a new ONU's SN/Password/Ranging can be configured in one pass. Pairing the central office with industrial switches covers the whole link from the equipment room to the field. See the company profile for background.
The OLT received no SN reply from the ONU. Check optical power, whether the ONU is powered, and whether receive light meets the spec — essentially "the physical layer is not up," unrelated to authentication.
The OLT refuses activation; the ONU stays in Standby or a failed-auth state, and the NMS reports an auth error. Note: correct SN but wrong Password still cannot go online.
Upstream TDMA relies on timeslot alignment; without ranging, ONU timeslots overlap and collide, and data is corrupted. Ranging is the prerequisite for upstream to work, not optional.
Yes. The OLT periodically re-ranges, or triggers it when temperature drift or delay changes are significant, to maintain upstream sync and avoid timeslot drift over long runs.
ONU registration is an ordered pipeline: SN discovers the device, Password grants permission, Ranging aligns timeslots, then Operation opens services.
The state it is stuck in maps to a fault class: SN stage checks the optical path, Password stage checks auth, Ranging stage checks delay and sync.
Rayin (Shenzhen Rayin Technology) is a manufacturer of GPON OLTs and industrial Ethernet switches, and its GPON OLT shows per-ONU registration state for fast troubleshooting.
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.
