Telecom Transmission Media and Cabling
Telecom Transmission Media and Cabling
This domain tests selection, installation and testing of copper and fibre cabling. Expect questions on twisted-pair categories, characteristic impedance, attenuation, crosstalk, connector types, bend radius and distance limits.
Core rules
- UTP is inexpensive and balanced; STP/FTP adds shielding. Cat5e supports 1 Gbit/s to 100 m; Cat6/6A support higher rates with stricter installation.
- Characteristic impedance matters: twisted pair is about 100 Ω, coaxial cable is 50 Ω or 75 Ω, and fibre is matched by core size and wavelength. Mismatches cause reflections.
- Attenuation is loss in dB per unit length; crosstalk is unwanted coupling between pairs. NEXT and return loss are common test parameters.
- Fibre uses total internal reflection; single-mode has a small core and long reach, multimode has a larger core and shorter reach.
- Respect bend radius, pulling tension and connector cleanliness; poor termination is a common fault.
Worked example
An 80 m UTP link has cable loss 0.20 dB/m, two connectors at 0.50 dB each, and a required margin of 3 dB. Cable loss = 16 dB; connector loss = 1 dB; total = 17 dB. If the transmitter launches 0 dBm and the receiver sensitivity is −20 dBm, the link budget is 20 dB, leaving 3 dB margin exactly. If an extra patch panel adds 1 dB, the design fails the 3 dB margin and needs rethinking.
Traps that cost marks
Using Cat5e beyond 100 m; ignoring patch cords in length; swapping Tx and Rx fibre strands; bending fibre tighter than its minimum radius; and assuming shielded cable removes the need for grounding. Also do not confuse characteristic impedance with DC resistance.