Physical Layer Standards and Media Types
This domain covers the electrical and optical rules behind the cable. For twisted-pair, know the parameters that determine performance: insertion loss, return loss, near-end crosstalk, far-end crosstalk, attenuation-to-crosstalk ratio, propagation delay, delay skew, and alien crosstalk for 10GBASE-T over Category 6A. Understand why shielding, pair balance, and twist rate matter, and how impedance mismatches cause reflections. Know the common Ethernet media designations and their distance limits for copper and fiber, and the difference between multimode (OM3 through OM5) and singlemode fiber with their typical reach and connector polish types.
Expect questions on polarity, duplex fiber orientation, and wavelength compatibility. Also understand power over Ethernet types and how higher power classes affect bundle heating and cable choice.
Traps: assuming a cable rated for a frequency supports every application at that frequency; ignoring alien crosstalk in bundled 10GBASE-T runs; mixing polish types or wavelengths on a duplex pair; and forgetting that bend radius and tensile limits are physical-layer requirements.
Rule of thumb: match the weakest link. A channel performs only to the worst component and installation condition, so a premium jack on a damaged cable still fails.
Worked example: a 10GBASE-T link runs 95 m in a tight bundle of unshielded cable. Even if insertion loss passes, alien crosstalk between adjacent cables likely fails, so specify shielded or Category 6A cable with separation and retest.
Key concepts
- Copper performance parameters
- Fiber types and polarity
- PoE classes and thermal effects
- Application-to-media matching