FTTH Network Architecture and Applications
What this domain tests
The architecture domain checks whether you can explain how a passive optical network carries service from the operator's equipment to a living unit, and whether you can prove a design closes optically. Expect PON vocabulary, topology comparisons, wavelength allocation, split-ratio-versus-reach trade-offs and the service applications that ride on the same fiber.
The ODN is three cable segments
- Feeder – OLT in the central office or headend to the first distribution point, usually a fiber distribution hub (FDH).
- Distribution – FDH to the drop terminal or pedestal near the premises.
- Drop – drop terminal to the ONT at the living unit.
Splitters sit at the FDH in a centralized design, or are cascaded closer to the subscriber in a distributed design. Centralized splitting uses one high-ratio PLC splitter and simplifies records; distributed splitting saves distribution fiber but adds loss and splice points.
Wavelengths and rates
GPON carries 2.488 Gbit/s downstream at 1490 nm and 1.244 Gbit/s upstream at 1310 nm, with 1550 nm reserved for an RF video overlay. XGS-PON runs 10 Gbit/s symmetric with 1577 nm downstream and 1270 nm upstream. Single-mode attenuation is roughly 0.35, 0.25 and 0.20 dB/km at 1310, 1490 and 1550 nm, so the downstream direction normally sets the reach limit.
Loss budget is the design tool
ITU-T G.984.2 defines optical classes: Class B+ allows 28 dB and Class C+ allows 32 dB between OLT and ONT. Working values: 1:32 PLC splitter 17.0 dB, 1:16 13.8 dB, 1:8 10.5 dB, 1:4 7.2 dB, 1:2 3.5 dB; a mated connector pair about 0.5 dB; a fusion splice 0.1 dB.
Worked example
A 1:32 PON with 12 km of fiber, four splices and three mated connector pairs (OLT patch, FDH, ONT). Loss = 4.2 (12 × 0.35) + 17.0 + 0.4 + 1.5 = 23.1 dB. Against a 28 dB Class B+ budget that leaves 4.9 dB for repairs, temperature drift and aging - acceptable. The same PON at 1:64 (20.5 dB splitter) totals 26.6 dB, leaving only 1.4 dB, which is why 1:64 is normally paired with Class C+ optics or shorter reach.
Traps
Treating a splitter as dropping exactly 3 dB per halving (real PLC parts add 0.5–2 dB excess loss); counting connectors at only one end; using the 1310 nm attenuation figure for a 1490 nm downstream check; and ignoring return loss, which must stay at 32 dB or better for GPON.