How a PON works: TDM, ranging, DBA, activation, OMCI

A passive optical network has one transmitter talking to up to 128 receivers through glass and splitters, and up to 128 transmitters sharing one receiver in return. Making that work without collisions, without eavesdropping and without configuration at every home is a protocol job. This page explains the mechanisms — downstream broadcast, upstream time slots, ranging, dynamic bandwidth, ONU activation and the management channel — and ties each to what you see on an OLT and a stick.

Downstream: broadcast to everyone

The OLT transmits continuously at one wavelength (1490 nm GPON, 1577 nm XGS-PON). Every ONU receives every frame and keeps only what is addressed to it — by Port-ID in GPON GEM frames or XGEM in XG(S)-PON. Because everything is visible to everyone, the downstream payload is encrypted per ONU (AES-128 in GPON, AES-128/256 in XGS-PON) with keys exchanged during activation. Downstream also carries the bandwidth map that tells each ONU when it may transmit.

Upstream: time-division bursts

ONUs share one upstream wavelength (1310 nm GPON, 1270 nm XGS-PON) by TDMA: each transmits only in the slot the OLT assigned, as a burst with a preamble and delimiter. The laser is off between bursts — which is why an ONU's DDM Tx power is meaningless when idle and why an OLT's Rx is an average of many senders (Accuracy & limits). The OLT receiver resets its threshold between bursts from ONUs at different distances and powers.

Ranging

Slots only work if every ONU's burst arrives exactly when expected. During activation the OLT measures the round-trip delay to each ONU and assigns an equalisation delay so that all ONUs appear to sit at the same logical distance (the differential fibre distance is 20 km, optionally 40 km). Ranging repeats periodically to follow temperature-induced fibre length changes.

Dynamic bandwidth assignment (DBA)

Upstream capacity is divided among T-CONTs (transmission containers) — queues per ONU per service class:

The OLT's DBA algorithm allocates slots every 125 µs based on status reports from ONUs (SR-DBA) or on observed traffic (NSR-DBA, traffic monitoring). Upstream latency and fairness depend on this algorithm, not on the optics.

Framing and FEC

Standards detail: PON standards.

ONU activation

A new ONU goes through a defined sequence (GPON states O1–O5):

Authentication variants: serial number only, LOID + password (common with some operators, set in the stick's firmware), or registration ID. An OLT also runs rogue ONU detection — an ONU transmitting outside its slot blinds everyone on the PON, and the OLT can order it to shut its laser.

OMCI: the management channel

Once in O5, the OLT configures the ONU through OMCI (ONU Management and Control Interface, G.988): a message channel over a dedicated GEM port that creates the ONU's service model — UNI ports, VLANs, GEM ports, T-CONTs, QoS — as managed entities. An ONU that registers but passes no traffic usually has an OMCI/provisioning mismatch (service profile, VLAN, or an ONU model the OLT's profile does not fit). Sticks emulate a full ONU's OMCI so the OLT sees a familiar device.

Wavelength coexistence

GPON, XGS-PON and video overlay can share one ODN because their wavelengths differ; a coexistence element (CEx, a WDM filter at the OLT) combines them at the cost of ~1–1.5 dB. Plans and filters: PON wavelengths.

What this means for optics

• Downstream continuous, upstream burst: the OLT SFP has a burst-mode receiver (APD with fast reset), the ONU a burst-mode laser driver — both features are what makes PON modules different from point-to-point SFPs (PON transceivers).
• Budget classes (B+, C+, N1…) and splitter ratios decide how many ONUs and how far: ODN classes & budget.
• The identity a host router reads from a stick (SFF-8472) is separate from the PON identity the OLT sees (serial, LOID); CodingBox codes the former.

In CodingBox

For a PON stick or OLT module CodingBox shows the MSA identity, wavelength codes and live DDM on DDM — remembering that a stick's Tx is only valid while it bursts and an OLT's Rx is an average. PON-side parameters (serial, LOID) are set in the stick's own management interface, not in EEPROM.