Copper Cabling and Connectors

Why the wires are twisted

Electric signals pick up noise from anything nearby: power lines, motors, fluorescent lights. Twisting each pair of wires makes the noise hit both wires equally, and the receiver cancels it out. That is the whole idea behind twisted pair. UTP (unshielded) relies on the twist alone and is the default everywhere. STP (shielded) adds foil or braid for noisy places such as factory floors.

The plug on the end is the RJ45 (technically 8P8C: eight positions, eight contacts). The smaller phone plug with fewer contacts is RJ11.

The categories: what changes between them

Each category is rated for a higher frequency, which is what lets it carry more data. Higher categories have tighter twists and better insulation.

Category   Speed            Distance         Note
Cat 5e     1 Gbps           100 m            the minimum for gigabit
Cat 6      1 Gbps           100 m
           10 Gbps          55 m only        runs out of steam for 10G
Cat 6A     10 Gbps          100 m            the 10G answer
Cat 7      10 Gbps          100 m            shielded, odd connectors, rare in the US
Cat 8      25 or 40 Gbps    30 m             short data center hops

Every twisted-pair Ethernet run is limited to 100 meters (328 feet), and that includes the patch cords at both ends. The usual split is 90 m in the wall plus 5 m of patch cord at each end.

T568A and T568B: which wire goes to which pin

Both standards put the same eight wires on the same eight pins, with one difference: the orange and green pairs are swapped.

Pin   T568B            T568A
1     white/orange     white/green
2     orange           green
3     white/green      white/orange
4     blue             blue
5     white/blue       white/blue
6     green            orange
7     white/brown      white/brown
8     brown            brown

• Same standard on both ends: a straight-through cable. PC to switch. This is almost every cable you will ever touch.
• Different standard on each end: a crossover cable. It swaps the transmit and receive pairs, for connecting two of the same kind of device, such as switch to switch, on old equipment. Modern ports have auto-MDIX, which detects the situation and swaps internally, so crossover cables have mostly disappeared.

Coax and twinax

• Coax carries cable modem and TV signals. The screw-on F-type connector is what you see today; BNC is the bayonet twist connector from old LANs. RG-6 is the modern grade.
• Twinaxial, sold as a direct attach cable (DAC), is a short thick cable with transceivers permanently attached at both ends. It links a switch to a server a few meters away at 10 to 100 Gbps for less money than fiber optics.

Plenum and riser

The plenum is the air space above a drop ceiling or under a raised floor that the building's ventilation uses. Burning PVC cable there would pump toxic smoke through the whole building, so code requires plenum-rated cable with a low-smoke jacket. Riser-rated cable is for vertical runs between floors, where the concern is fire climbing the shaft.

How the exam asks it

• "10 Gbps at the full 100 meters": Cat 6A.
• "Cable runs above the ceiling tiles": plenum.
• "A 120-meter run links intermittently": attenuation; add a switch or repeater in the middle, or use fiber.
• "Which pins does the orange pair use in T568B": 1 and 2.
• "Connect two switches without auto-MDIX": crossover.

What to memorize

• Copper stops at 100 m. Cat 6A for 10 Gbps at 100 m.
• T568B: orange on 1 and 2, green on 3 and 6. A and B differ only by swapping those pairs.
• Plenum above the ceiling, riser between floors.