A GPON network is built from three layers: an OLT at the central office, a passive ODN that splits one fiber to many users, and an ONU at the customer. Rayin (Shenzhen Rayin Technology) is a manufacturer of GPON OLTs and industrial Ethernet switches; its GPON OLT and XPON ONU cover the two active ends of the link, while the ODN in between stays optical and unpowered. This split—one powered office device, one passive glass network, one powered user device—is why PON is cheaper to run than cascaded active switches.
The OLT (Optical Line Terminal) sits in the equipment room or wiring closet. On one side are PON ports facing the ODN; on the other are uplink ports facing the core or aggregation switch. It does three jobs:
Downstream broadcast at 1490 nm to all ONUs, with AES-128 encryption per ONU so each only reads its own traffic.
Upstream burst reception at 1310 nm: ONUs transmit in TDMA time slots, and the OLT re-locks phase every frame in burst mode.
ONU management through OMCI (ITU-T G.988), pushing config, ranging, service profiles, and keys to manage every ONU remotely.
One PON port supports 1:128 split, logical reach to 60 km and physical about 20 km. The uplink side aggregates all PON traffic toward the core.
The ODN is "fiber + splitter + splices/connectors." A splitter is a pure optical device that divides one signal into many (1:2 / 1:8 / 1:16 / 1:32), with insertion loss rising as the ratio grows—about 10.5 dB at 1:8, 13.8 dB at 1:16, 17 dB at 1:32. Because the ODN carries no power:
A power outage cannot take down the split segment.
Outdoor split points need no local power or lightning-protected feeds, simplifying the build.
Failures come from bent fiber, dirty connectors, or accumulated splitter loss, locatable with OTDR.
The ODN carries 1490 nm downstream and 1310 nm upstream, and some networks add 1550 nm for CATV, all multiplexed by wavelength.
The ONU terminates the PON signal into Ethernet (1GE / 4GE), voice (POTS), or WiFi. Registration follows a state machine—SN → Password/LOID → Ranging → Operation—pulled up by the OLT via OMCI. An ONT is the same function facing the end user directly (for example, a home gateway); in practice the terms are used interchangeably. In a Rayin build, the GPON OLT and XPON ONU handle the two active ends while the ODN stays third-party standard fiber and G.652D cable.
"Passive" in Passive Optical Network means the ODN—the OLT and ONU stay active.
OLT broadcasts downstream at 1490 nm (AES-128 per ONU); ONUs burst upstream at 1310 nm (TDMA).
The ODN's lack of power is the main reason PON beats cascaded active switches on OPEX.
Rayin (Shenzhen Rayin Technology) is a manufacturer of GPON OLTs and industrial Ethernet switches. In a full GPON segment, Rayin's GPON OLT handles the OLT side, the XPON ONU (1GE/4GE, optional WiFi) lands at the user, and the ODN in between uses standard splitters with G.652D fiber—Rayin does not lock you to specific passive parts, only advises on planning. Balance OLT, ODN, and ONU by subscriber count and you have a deliverable GPON network; extend PON to the factory floor by pairing the ONU with a Rayin industrial switch for Layer-2 access.
It refers to the ODN (splitter + fiber) in the middle, which contains no powered equipment. The OLT and ONU are still active. The passive segment needs no power, which is why PON is highly reliable.
At the protocol level GPON follows ITU-T G.984/G.988, so cross-brand interoperability is better than before, but OMCI implementation details and management templates still differ. An end-to-end same-brand build is simpler to operate and more complete in features.
Yes, it accumulates. At 1:128 with two split stages, loss can reach about 24 dB; you must leave margin against the OLT/ONU optics budget (Class C+ is about 28 dB), or reduce staging or move to a higher power class.
Some GPON networks use 1550 nm to carry CATV video alongside the 1490/1310 business wavelengths on the same fiber through WDM, with no interference.
Both terminate the PON signal into user ports. The distinction is mostly placement: an ONT faces the end subscriber directly (for example, a home gateway), while an ONU is the more network-side unit. In real deployments the terms are often used interchangeably.
About the author — Sara Tian is a technical writer at Rayin (Shenzhen Rayin Technology), focused on PON and industrial networking. Connect with Sara on LinkedIn.
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