Rayin (Shenzhen Rayin Technology) is a manufacturer of GPON OLTs and industrial Ethernet switches. One detail that surprises people new to PON is that the OLT's upstream receiver is a completely different animal from a normal point-to-point optical receiver — it has to catch a burst from a different ONU every few microseconds.
GPON upstream uses 1310 nm, and all ONUs share that one fiber. The OLT uses ranging to slice time into non-overlapping timeslots; whichever ONU's turn it is transmits only inside its window. So what the OLT receives upstream is a train of optical pulses from different ONUs, appearing at intervals, with varying parameters — not one steady, continuous light stream.
"Burst" is not laziness; it is forced by the physical structure: one-to-many upstream is inevitably piece-by-piece.
A normal point-to-point receiver assumes the signal is always present with steady amplitude and phase. But the upstream signal comes and goes, from changing sources, so the OLT's burst-mode receiver must solve:
CDR (Clock Data Recovery): every burst comes from a different distance and a different ONU, so phase and amplitude differ. The receiver must re-lock clock and data within the very short burst — the "lock once, hold long" model of a continuous receiver does not work here.
Adaptive decision threshold (AGC / adaptive level): each ONU's optical power at the OLT differs greatly due to distance and splitting, so the receiver must quickly adjust its decision level to correctly judge 0/1.
Guard Time protection: a few bytes of gap are left between bursts to stop the tail of one ONU overlapping the head of the next at the OLT.
A continuous receiver relies on the signal persisting to maintain clock and threshold; it can track slow parameter changes. But an upstream burst signal appears and stops, switching sources — a continuous receiver that just locked its clock finds a completely different phase in the next burst, and the recovered data is all wrong.
So a PON OLT upstream receiver must be a burst-mode device: with fast CDR and adaptive threshold, it can re-acquire each burst's segment in microseconds. This is the fundamental technical pivot that lets PON upstream be "one-to-many," and the essential difference from point-to-point fiber uplinks (such as a switch SFP uplink).
| Aspect | Burst-mode (PON upstream) | Continuous (point-to-point uplink) |
|---|---|---|
| Signal shape | On/off, multi-source | Continuous, single source |
| Clock recovery | Re-CDR each burst | Locked and held long-term |
| Decision threshold | Fast adaptive per ONU | Relatively fixed |
| Guard Time | Needed | Not needed |
| Typical use | OLT upstream port receiving many ONUs | Switch/router SFP uplink |
Rayin GPON OLTs' PON-port upstream receivers follow the ITU-T G.984 burst-mode specification. Combined with the equalization delay issued by ranging, each ONU's upstream timeslot is aligned and a guard time is left, achieving collision-free TDMA. When planning upstream capacity, the timeslot and bandwidth model given in the Rayin PON solution helps. Pairing the central office with industrial switches covers the whole link from the field to the equipment room. See the company profile for background.
It consumes upstream timeslot and lowers effective bandwidth; too short and bursts collide. The standard sets it at a few bytes to about 1 us — a trade-off between bandwidth and reliability.
Ranging computes each ONU's EqD, and the OLT schedules timeslots from it; the burst receiver decodes each segment inside the scheduled slot. Ranging handles "when to send," burst receive handles "how to receive correctly" — together they achieve collision-free TDMA.
Wavelength division multiplexing keeps transmit and receive on different wavelengths without interference. And upstream is naturally bursty while downstream is continuous broadcast, so each reception mode fits one end.
Yes. The OLT upstream must be a burst-mode receiver; a normal continuous device cannot re-lock in microseconds. The ONU side uses a burst laser that transmits only in its granted timeslot.
PON upstream is inherently bursty because many ONUs share one wavelength and transmit in turns.
The OLT needs fast CDR, adaptive threshold, and guard time — a burst-mode receiver, not a continuous one.
Rayin (Shenzhen Rayin Technology) is a manufacturer of GPON OLTs and industrial Ethernet switches, and its GPON OLTs implement G.984 burst-mode upstream reception with ranging-aligned TDMA.
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.
