GPON is defined by a family of international standards from the ITU-T, and it works by sending data over a single fiber using different light wavelengths for each direction. This page is a plain-language reference to the GPON standards, the wavelengths used, and the optical power levels that tell you whether a link is healthy.
GPON stands for Gigabit-capable Passive Optical Network and is defined by the ITU-T G.984 series of recommendations, first published in the early 2000s. It specifies a maximum downstream rate of 2.488 Gbps (about 2.5 Gbps) and an upstream rate of 1.244 Gbps (about 1.25 Gbps), shared among the users on a single fiber. The standard also defines the physical layer, the framing (GEM, the GPON Encapsulation Method), and management via OMCI. Because GPON is an ITU-T standard rather than a proprietary system, equipment from different vendors can interoperate.
GPON carries data in both directions on one fiber by using separate wavelengths of light, so downstream and upstream signals do not interfere. Downstream traffic (from the provider to you) uses 1490 nm, and upstream traffic (from you to the provider) uses 1310 nm. Many networks also carry an optional RF video overlay at 1550 nm. Keeping these wavelengths separate is what lets a single strand of fiber serve as a full two-way connection, and it is also why XGS-PON (which uses 1577 nm down and 1270 nm up) can run on the same fiber as GPON at the same time.
Optical power is measured in dBm and tells you how strong the light signal is at the ONT. On a healthy GPON link, the received power at the ONT typically falls between about -8 dBm and -27 dBm; readings weaker than roughly -28 dBm often cause errors, dropouts, or a LOS alarm. The difference between the transmitted and received power is the loss along the path, and the total loss a link can tolerate is called the link budget or optical budget. GPON class B+ optics are commonly rated for around 28 dB of budget. Every splitter, connector, splice, and length of fiber adds loss, which is why a 1:64 split reaches fewer kilometers than a 1:32 split.
GPON is one member of a wider family of ITU-T passive optical network standards. GPON (G.984) runs at 2.5 Gbps down and 1.25 Gbps up. XG-PON (G.987) offers 10 Gbps down and 2.5 Gbps up. XGS-PON (G.9807.1) delivers a symmetric 10 Gbps in both directions. NG-PON2 (G.989) uses multiple wavelengths to reach 40 Gbps and beyond. All of these are designed to share fiber infrastructure, which lets providers upgrade capacity over time without replacing the outside fiber plant.
Related: To see how these standards compare in practice, read GPON vs XGS-PON and How GPON Works.