Copper cabling — categories, shielding, PoE

Copper Ethernet is still everywhere — to desks, to APs, to industrial gear — even though optical dominates the backbone. Get the categories and the rules right, and you'll save yourself a lot of "why is this link only negotiating at 100 Mbps?" headaches.

Twisted pair categories

| Category | Speed | Max length | Shielding | Where you see it | |---|---|---|---|---| | Cat 5 | 100 Mbps | 100 m | UTP | Pre-2002 buildings (rip and replace) | | Cat 5e | 1 Gbps | 100 m | UTP | Most existing horizontal cabling | | Cat 6 | 1 Gbps (10G to 55 m) | 100 m / 55 m | UTP or F/UTP | Newer LAN runs | | Cat 6A | 10 Gbps | 100 m | F/UTP, S/FTP common | 10 GbE to the desk, data center copper | | Cat 7 / 7A | 10 Gbps | 100 m | S/FTP | Niche; never got proper RJ45 standardization | | Cat 8 | 25 / 40 Gbps | 30 m | S/FTP | Short top-of-rack DC runs |

The 100 m rule is the channel length: 90 m of horizontal cable + up to 10 m of patch cords. Exceed it and the link will negotiate down or just fail.

Shielding decoded

You'll see codes like F/UTP, S/FTP, U/UTP. The structure is <overall shield>/<per-pair shield> × TP (twisted pair).

• U/UTP — unshielded, unshielded pairs. The most common, cheapest, easiest to terminate. Fine for most enterprise.
• F/UTP — foil overall, unshielded pairs. A step up against ambient EMI.
• S/FTP — braided overall, foil per pair. Maximum shielding. Required in industrial / high-EMI environments and for some Cat 6A and all Cat 7/8 runs.

Shielded cable is only useful if both ends are properly bonded to ground. Half-installed shielding can actually be worse than UTP because it creates a ground-loop antenna.

PoE in one page

Power over Ethernet delivers DC power over the same cable as data. The standards keep growing:

| Standard | Year | Power at PSE | Power at PD | Pairs used | |---|---|---|---|---| | 802.3af (PoE) | 2003 | 15.4 W | 12.95 W | 2 | | 802.3at (PoE+) | 2009 | 30 W | 25.5 W | 2 | | 802.3bt Type 3 (PoE++ / 4PPoE) | 2018 | 60 W | 51 W | 4 | | 802.3bt Type 4 (PoE++) | 2018 | 90 W | 71 W | 4 |

Practical implications:

• An AP needs PoE+ at minimum. Wi-Fi 6E/7 APs need 802.3bt.
• Long PoE runs at high wattage push the heat budget — Cat 6A is preferred above PoE+.
• The PSE (switch) and PD (powered device) negotiate; you cannot fry a non-PoE device by plugging it into a PoE port.

Industrial gear (Westermo, Cisco IE4000)

In industrial sites you'll meet M12 connectors instead of RJ45 — same Ethernet underneath, ruggedized housing that withstands vibration and dust. Speeds and standards are identical; only the connector and the cabling jacket differ. The Cisco IE4000 and Westermo DDW225 you'll meet in Track E both support M12 variants.

What to remember

• 100 m is the magic number for any twisted-pair Ethernet run.
• Cat 6A is the safe choice for new installations — supports 10 GbE and high-wattage PoE.
• Shielding only helps if you ground it at both ends.
• PoE wattage matters; pair the switch's PoE budget against the sum of all PDs.