Communication and Data Systems
Standards and topology
The Communication and Data Systems domain tests structured cabling, testing, and Power over Ethernet. Commercial cabling follows ANSI/TIA-568: a 100 m channel is 90 m of permanent link plus up to 10 m of patch cords at both ends. Cat 5e supports 1GBASE-T to 100 m; Cat 6 supports 1GBASE-T to 100 m but 10GBASE-T only to about 55 m; Cat 6A supports 10GBASE-T to 100 m. Fiber uses OM3/OM4 multimode for building backbones and OS1/OS2 single-mode for long distances.
Testing and terminations
Field tests include wire map, length, insertion loss, near-end crosstalk (NEXT), return loss, and propagation delay. A “pass” on a cheap continuity tester is not a certification. Keep pair twists to within 13 mm (0.5 in.) of the termination, do not untwist more than necessary, and respect bend radius and pulling tension.
Power over Ethernet
- 802.3af: up to 15.4 W at the PSE, 12.95 W at the powered device.
- 802.3at (PoE+): up to 30 W at the PSE, 25.5 W at the PD.
- 802.3bt: Type 3 up to 60 W at the PSE, Type 4 up to 90 W at the PSE, with lower PD guarantees.
Worked example
A 48-port PoE switch has a 370 W PoE budget. The design uses 24 cameras at 12.95 W and 12 access points at 15.4 W. Cameras: 24 × 12.95 = 310.8 W. Access points: 12 × 15.4 = 184.8 W. Total = 495.6 W, which exceeds 370 W. The switch cannot power all 36 devices at once. Solutions include a larger PoE budget, splitting devices across switches, or using local power for part of the load.
Traps
- Measuring the 100 m channel as 100 m of permanent link; patch cords count.
- Assuming Cat 6 always supports 10GBASE-T to 100 m.
- Ignoring bundle heating and cable temperature when many PoE cables are bundled.
- Confusing PSE wattage with PD wattage; the difference is cable loss.
- Using copper where fiber is required for distance or electrical isolation.