GPON vs XGS-PON
GPON built most of the FTTH in service today; XGS-PON is what operators are upgrading to. Here is what actually differs on the fiber, and what stays exactly the same.
Side by side
| GPON | XGS-PON | |
|---|---|---|
| ITU-T standard | G.984 | G.9807.1 |
| Downstream | 2.488 Gbit/s | 9.953 Gbit/s |
| Upstream | 1.244 Gbit/s | 9.953 Gbit/s (symmetric) |
| Downstream wavelength | 1490 nm | 1577 nm |
| Upstream wavelength | 1310 nm (narrowed to 1290–1330 nm where XGS-PON shares the fiber) | 1270 nm |
| Optics classes (ODN loss) | B+ 13–28 dB, C+ 17–32 dB | N1 14–29, N2 16–31, E1 18–33, E2 20–35 dB |
| Fiber, splitters, connectors | The same single-mode fiber, PLC splitters and APC connectors | |
The speeds are line rates; the service a customer buys is lower after overhead and sharing. Every ONT on a splitter shares that PON's capacity, so the split ratio decides how much each home can get at the busiest hour.
Why both can run on one fiber
GPON and XGS-PON use different wavelengths in both directions, so they do not interfere. Operators upgrade by adding XGS-PON alongside GPON on the same splitters and drops: a coexistence element (a WDM filter at the central office or cabinet) combines the two, or a combo OLT optic carries both. Customers move to XGS-PON one ONT swap at a time while their neighbors stay on GPON. RF video, where it is still used, sits at 1550 nm alongside both.
What that means in the field
An upgrade does not mean new fiber, new splitters or new drops. It does mean every connector has to be clean and every splice good, because the XGS-PON optics have to fit inside the same loss budget at different wavelengths, and a coexistence filter adds its own loss: about 1 dB, and up to about 1.5 dB on the XGS-PON port in some datasheets. A link that barely passed on GPON may not pass after the upgrade.
Loss budgets: will the old ODN pass?
GPON class B+ allows up to 28 dB; XGS-PON N1 allows 29 dB and N2 31 dB. Because XGS-PON was designed to reuse GPON plant, an ODN built and documented to B+ usually fits N1 or N2, but the coexistence filter's loss comes out of the same budget. Rerun the numbers in the splitter loss calculator with the XGS-PON class selected and the filter added as an extra connector pair or two.
Testing
- Use a PON power meter that reads 1490 nm and 1577 nm separately. A standard broadband meter adds them together and tells you nothing useful on an upgraded network.
- Test at the wavelengths the network will carry. The 1577 nm downstream is close to 1550 nm, so it is just as sensitive to tight bends.
- Clean and inspect every connector before you blame the optics. See how to clean fiber connectors.
What comes after XGS-PON
25GS-PON (an industry multi-source agreement rather than an ITU standard) and 50G-PON (ITU-T G.9804) use yet more wavelengths, and are designed to coexist with GPON and XGS-PON on the same fiber in the same way. The plant you build today is the plant those will run on.