Passive optical network (PON)
The fiber-access architecture in which one head-end transceiver serves dozens of subscribers through a purely passive optical splitter. No powered equipment sits between office and user. The physical layer of most FTTH.
A PON connects one OLT (optical line terminal, at the operator's office) to 32–64 (up to 128) ONUs/ONTs (at subscribers) through a single feeder fiber and a passive splitter tree. Between the two ends there is nothing to power, cool, or maintain. That property makes fiber-to-the-home economics work.
Traffic runs single-fiber bidirectional on separated wavelengths. Downstream is broadcast: every ONU receives everything, extracts its own timeslots, and encryption handles privacy. Upstream is TDMA: the OLT grants each ONU transmission windows, and each ONU's laser bursts on and off, silent except when granted. Two component consequences define PON optics: ONU transmitters must rise, lock, and emit within burst overhead times, and the OLT receiver must recover amplitude and clock from consecutive bursts arriving at wildly different power levels (near versus far subscribers). The latter is the burst-mode receiver problem. Ranging measures each ONU's fiber delay so bursts interleave without collision across up to ~20 km of differential reach.
The generations in deployment:
| Standard | Down / Up | Wavelengths (nm, down/up) |
|---|---|---|
| GPON | 2.5G / 1.25G | 1490 / 1310 |
| XG(S)-PON | 10G / (2.5 or 10G) | 1577 / 1270 |
| 25GS-PON / 50G-PON | 25–50G | new bands per standard |
Generations coexist on the same splitter tree by wavelength separation, which is how operators upgrade without touching outside plant.
Physical-layer discipline: power budgets are quoted in classes (e.g., 28–35 dB) that must absorb the splitter's brutal arithmetic. A 1:64 split alone costs ~18 dB before any fiber or connector. The plant standardizes on APC connectors throughout, because broadcast architectures with many open ports tolerate no reflections.