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Français PON wavelength plans & coexistence
Every PON generation is defined as much by its wavelengths as by its bit rate: the downstream and upstream colours are chosen so that a new generation can share the same fibre and splitters as the old one. Knowing the plan tells you what a PON module is and whether it belongs on a given network.
Wavelength plans
| System | Downstream | Upstream | Notes |
|---|---|---|---|
| GPON | 1490 nm | 1310 nm | ITU-T G.984 |
| EPON | 1490 nm | 1310 nm | IEEE 802.3ah; same colours as GPON |
| RF video overlay | 1550 nm | — | analogue TV carried alongside GPON |
| XG-PON / XGS-PON | 1577 nm | 1270 nm | ITU-T G.987 / G.9807.1 |
| 10G-EPON | 1577 nm | 1270 nm | IEEE 802.3av; matches XG(S)-PON colours |
| NG-PON2 (TWDM) | 1596–1603 nm | 1524–1544 nm | four tunable channel pairs, ITU-T G.989 |
The upstream band sits in the O-band (1260–1360 nm), where uncooled, inexpensive ONU lasers work well; downstream uses the longer wavelengths where fibre loss is lower.
Coexistence
Because GPON (1490/1310) and XGS-PON (1577/1270) use different colours, both can run on one fibre plant at the same time. A passive coexistence element (a WDM filter, often called WDM1r) at the central office combines the two OLT ports onto the feeder fibre; at the customer, each ONU's filter passes only its own pair. This is how operators upgrade subscribers one at a time without touching the outside plant.
NG-PON2 goes further: ONUs carry tunable optics and are assigned one of several wavelength pairs, so capacity can be added by lighting another pair.
Reading a PON module
The wavelength fields in module memory identify the system:
- Tx 1490 / Rx 1310 → GPON/EPON OLT side; Tx 1310 / Rx 1490 → ONU side.
- Tx 1577 / Rx 1270 → XGS-PON OLT; Tx 1270 / Rx 1577 → XGS-PON ONU.
A module with the wrong pair for the network will simply never see light from the other end. See PON transceivers: OLT & ONU.
In CodingBox
CodingBox shows the transmit wavelength and identity of a PON module on the Check transceiver screen, so an OLT SFP and an ONU stick — or a GPON and an XGS-PON module — can be told apart before they are installed.