Optical Network Basics: FTTx, OLT/ONT and PON Architecture
Optical Network Basics: FTTx, OLT/ONT and PON Architecture
This domain tests the access-network chain from the exchange to the subscriber: OLT, optical distribution network, splitters and ONT, plus the meaning of FTTx and PON power budgets.
Core rules and concepts
- FTTx means fiber to the home, building, curb or cabinet. In a PON, one OLT port serves many subscribers through passive splitters; there are no active electronics in the field.
- The OLT is at the central office or exchange. The ODN includes fiber, connectors, splices and splitters. The ONT or ONU is at the customer premises.
- GPON uses 2.488 Gbit/s downstream and 1.244 Gbit/s upstream. Downstream is typically 1490 nm and upstream 1310 nm; 1550 nm may carry video overlay. XGS-PON provides 10 Gbit/s in both directions.
- Splitters are commonly 1:8, 1:16, 1:32 or 1:64. A 1:32 splitter has an ideal split loss of about 15 dB and a practical insertion loss near 16.5–17 dB.
- A power budget must cover fiber loss, splice loss, connector loss, splitter loss and margin. GPON class B+ is commonly 28 dB; class C+ is 32 dB.
Worked example
A GPON link uses 4 km of fiber at 0.35 dB/km, one 1:32 splitter at 17 dB, six connectors at 0.5 dB each and eight splices at 0.1 dB each. Fiber loss = 1.4 dB; connectors = 3.0 dB; splices = 0.8 dB. Total before splitter = 5.2 dB. Add splitter 17 dB = 22.2 dB. With a 28 dB budget, margin = 5.8 dB, which is adequate if the design margin is 3 dB.
Traps that cost marks
Do not count splitter loss as zero. Do not mix downstream and upstream wavelengths. Do not compare a 1:64 design with a 1:32 budget without recalculating. Do not forget that splices and connectors consume budget.