GPON

GPON is a point-to-multipoint fiber access architecture standardised in the ITU-T G.984 series. A single fiber leaving the OLT in the central office is divided in the field by unpowered PLC splitters, typically reaching 32 or 64 subscribers. The device at the customer end is called an ONT or ONU. The passive plant in between (fiber, splitters, splices and connectors) is the ODN (Optical Distribution Network).

GPON provides 2.488 Gbit/s downstream and 1.244 Gbit/s upstream. Different wavelengths share the same fiber: 1490 nm downstream, 1310 nm upstream, with 1550 nm optionally reserved for RF video overlay. Downstream, the OLT broadcasts the same signal to every ONU and each ONU processes only its own GEM ports (AES encryption is supported). Upstream, ONUs transmit in turn in time slots granted by the OLT (TDMA), and dynamic bandwidth allocation (DBA) distributes those slots according to the real-time needs of each T-CONT.

Because capacity is shared per port, subscriber counts and plan speeds must be planned together. Statistical multiplexing still means high-speed plans work comfortably on a port split 1:32 or 1:64. Typical reach is 20 km. The common optical classes are B+ (28 dB) and C+ (32 dB), and the class chosen sets the total optical loss the link may have.

ONUs are managed by the OLT through OMCI (ITU-T G.988). Interoperability between OLTs and ONUs from different vendors depends on compatibility at this layer and should always be tested before field deployment. GPON also allows a gradual migration to XGS-PON, which runs on different wavelengths over the same ODN.