PON transceivers: OLT & ONU

PON optics are unlike point-to-point transceivers: one module talks to many, the two directions use different wavelengths on one fibre, and the upstream arrives in bursts. That shapes both the hardware and what you see in module memory.

Two ends, two very different modules

Both use a diplexer (BOSA) that separates the two wavelengths onto one fibre; GPON ONUs with RF video add a third (triplexer, 1550 nm).

Optical budget classes

The OLT–ONU loss budget defines the class of optics:

A splitter alone costs roughly 3 dB per doubling (1:32 ≈ 15–17 dB), so class matters as much as fibre length.

DDM on a PON

• OLT Rx power is per burst. A single "Rx power" figure on an OLT SFP is an average

or the last burst; per-ONU received power is reported by the OLT system, not the module.

• ONU Rx power is a normal continuous reading and is the first thing to check on a

subscriber with problems — compare with the class's minimum (about −27 dBm for B+).

• Thresholds are set by the module maker as on any SFF-8472

module; see DDM levels.

Identity and provisioning — the ONU stick

An ONU is registered on the OLT by its serial number (and, on some networks, a PLOAM password or LOID). SFP ONU "sticks" therefore carry two identities: the SFF-8472 module identity read by the host device, and the PON serial/vendor ID used by the OLT. Operators lock OLTs to approved ONU vendors, so replacing an ONU often means the new stick must present the serial and vendor the OLT already knows — the PON equivalent of vendor lock. Stick firmware exposes these fields for configuration; the module's MSA identity is coded like any SFP.

In CodingBox

CodingBox reads and codes the MSA identity of PON modules (vendor, part number, wavelength, DDM) per SFF-8472, which lets a stick present the module identity a host port expects. The PON-layer serial/PLOAM settings live in the stick's own firmware and are set through its management interface.