Fiber optics fundamentals and home fiber architectures
Domain 1 - Fiber optics fundamentals and home fiber architectures
This domain tests how light behaves in fiber, which fiber types and wavelengths appear in a home FTTH network, and how the access link is structured from the provider's OLT to the subscriber's ONT. Questions are usually quantitative: splitter loss, loss budgets, wavelength pairs and the demarcation point.
Fiber type, wavelengths and topology
- FTTH drop and in-home fiber is single-mode, normally G.657.A1/A2 bend-insensitive fiber, backward compatible with G.652.D.
- GPON uses 1490 nm downstream, 1310 nm upstream, with 1550 nm reserved for RF video overlay. XGS-PON uses 1577 nm downstream and 1270 nm upstream and can coexist with GPON on one ODN.
- Multimode (OM3/OM4) belongs to the home LAN, never to the access drop.
- Common home architectures: PON with a shared splitter; point-to-point active Ethernet with dedicated fiber; MDU designs where the splitter sits in a building box.
- Typical PON chain: OLT → feeder → splitter → distribution/drop → ONT, which is the demarcation point and converts optical to electrical.
Splitters and loss budget
- GPON is commonly 1:32 (1:64 possible); XGS-PON commonly 1:32 or 1:64.
- A 1:32 splitter has 15.05 dB theoretical splitting loss; real parts add 1.5–2 dB excess loss, so budget about 17 dB.
- Splitter loss is counted once, although the path is bidirectional.
- Budget = fiber loss + splice loss + connector-pair loss + splitter loss + margin, compared with the PON class (B+ = 28 dB, C+ = 32 dB).
Worked example
A GPON Class B+ link: 2.0 km feeder + 0.2 km drop, two fusion splices, three connector pairs, one 1:32 splitter, 3 dB margin, at 1490 nm. Fiber: 2.2 × 0.25 = 0.55 dB. Splices: 2 × 0.1 = 0.2 dB. Connectors: 3 × 0.5 = 1.5 dB. Splitter: 17.0 dB. Total 19.25 + 3 margin = 22.25 dB, below 28 dB, so the link passes with 5.75 dB headroom.
Traps
- Quoting 15 dB for a 1:32 splitter and ignoring excess loss.
- Mixing wavelength pairs between GPON and XGS-PON.
- Counting connector loss per connector instead of per mated pair.
- Treating margin as a percentage instead of a fixed dB figure.