Optical Fiber and Broadband Access Technologies
This domain tests fibre construction and loss budgeting, passive optical networks, and the copper broadband technologies that still serve many subscribers.
Core rules
- A fibre has a core, cladding and coating; light is guided by total internal reflection.
- Single-mode fibre has about a 9 µm core and uses 1310/1550 nm for long distance. Multimode fibre has 50 or 62.5 µm core and uses 850/1300 nm for shorter links.
- Loss is stated in dB/km; splices and connectors add loss; dirty connectors and tight bends cause excessive loss.
- FTTH uses an OLT in the exchange, a passive splitter (1:8, 1:32) and an ONT at the home. GPON provides 2.5 Gbps downstream and 1.25 Gbps upstream, with downstream at 1490 nm and upstream at 1310 nm.
- ADSL/VDSL run over copper twisted pair; PPPoE carries the session; speed falls as loop length rises.
- Test with an optical power meter, VFL and OTDR.
Worked example
A 10 km single-mode link has fibre loss 0.35 dB/km × 10 = 3.5 dB. Two connectors at 0.5 dB each add 1.0 dB, giving 4.5 dB total. With 0 dBm launch power the receiver sees −4.5 dBm. If ONT sensitivity is −27 dBm, the margin is 22.5 dB, which is healthy; the answer is not −3.5 dBm because connector loss must be included.
Traps
Using multimode for 20 km; mixing the roles of 1310 and 1550 nm; looking into a fibre to check light; ignoring connector loss; bending fibre tighter than its minimum radius; confusing splitter loss with fibre attenuation.