The installed base and the upgrade
GPON (ITU-T G.984): 2.488 Gb/s down / 1.244 up, 1490/1310 nm, ODN classes B+ (28 dB) and C+ (32 dB) — the world's installed base. XGS-PON (G.9807.1): symmetric ~10 Gb/s at 1577/1270 nm, deliberately placed so both systems coexist on the same ODN through CEx wavelength filters — the mainstream upgrade executed cabinet by cabinet without re-splitting. 25GS-PON / NG-PON2 (TWDM) extend the path for dense and business overlays.
How the shared medium works
Downstream is broadcast and encrypted per ONT; upstream is TDMA: the OLT grants each ONT transmission windows using measured round-trip ranging. Dynamic bandwidth allocation converts idle grants into throughput for active users. This is why one misbehaving "rogue ONT" transmitting outside its grant can degrade a whole PON — and why isolation procedures for rogue detection belong in operations runbooks.
Coexistence engineering
Adding XGS over live GPON requires: CEx filters at the OLT feed, verified filter insertion loss in the budget, ONT wavelength discipline, and filtered-OTDR test practice. Design documents must show both wavelength plans on one drawing.
Choosing today
Greenfield residential: XGS-class optics with GPON fallback pricing. Brownfield: XGS overlay where take-up justifies. Business overlays and 5G transport push toward NG-PON2/25GS or dedicated P2P — the architecture memo from Lesson 1 decides.