Bileog Cheat Sreangaithe RJ45 Cat6

A complete, colour-accurate reference for terminating RJ45 / Cat6 Ethernet cables: the two pinout standards, what each pair does, PoE, and the rules that decide whether the run passes a certifier.

🎨 T568A & T568B Pinouts

Both standards are electrically identical — only the colour order differs. T568B is the default in most commercial installations; T568A turns up in US government and residential specifications. Pick one and use it on both ends of every cable in the building.

T568A

T568B (most common)

💡 The mnemonic for T568B: orange, green, blue, brown — striped before solid, except that the blue pair sits inside the green pair on pins 4 and 5.

🔌 What Each Pin Does

Gigabit and 10 Gigabit Ethernet use all four pairs in both directions at once — which is why a “Fast Ethernet works, Gigabit doesn’t” fault is almost always a bad termination on pins 4, 5, 7 or 8.

🔗 Straight-Through Cable (T568B ↔ T568B)

The standard patch cable: computer → switch, switch → router, wall jack → patch panel. Both ends use the same standard, so every pin lands on the same pin.

End A (T568B)                         End B (T568B)
------------------------------------------------------------
1  White/Orange  ------------------  White/Orange  1
2  Orange        ------------------  Orange        2
3  White/Green   ------------------  White/Green   3
4  Blue          ------------------  Blue          4
5  White/Blue    ------------------  White/Blue    5
6  Green         ------------------  Green         6
7  White/Brown   ------------------  White/Brown   7
8  Brown         ------------------  Brown         8

🔀 Crossover Cable (T568A ↔ T568B)

Historically needed for switch ↔ switch, PC ↔ PC and router ↔ router links. Every device made in the last twenty years negotiates Auto-MDI/MDI-X, so this is now a legacy cable — but it is still what you grab for two ancient devices, or a lab exercise.

End A (T568A)                         End B (T568B)
------------------------------------------------------------
1  White/Green   ------------------  White/Orange  1
2  Green         ------------------  Orange        2
3  White/Orange  ------------------  White/Green   3
6  Orange        ------------------  Green         6

Pins 4, 5, 7 and 8 stay straight-through for 10/100.

A true gigabit crossover also swaps the other two pairs: 4↔7 and 5↔8.

🧵 Pair Mapping

Four twisted pairs, each a solid conductor and its white-striped partner. Keeping a pair together is what cancels noise.

⚠️ Never untwist more than about 13 mm (½ inch) at the connector, and never split a pair across two pairs’ positions — a “works but slow” cable is usually a split pair, and only a certifier will tell you.

⚡ Power over Ethernet

• Mode A puts power on the data pairs 1–2 and 3–6; Mode B uses the spare pairs 4–5 and 7–8. A powered device must accept either.
• Because 802.3bt energises all four pairs, sloppy terminations on 4/5/7/8 now break power as well as gigabit speed.
• Power raises cable temperature; in a full, bundled tray, derate the length or move to a shielded, larger-gauge cable.

📏 Categories & Limits

• Maximum channel length is 100 m: 90 m of solid-core in the wall plus 10 m of stranded patch leads.
• Use solid conductor for permanent runs and stranded for patch cords — stranded flexes, solid does not survive repeated bending; connectors are rated for one or the other.

🔧 Termination Checklist

• Strip only the jacket, never nick the insulation of the conductors.
• Keep the pairs twisted right up to the point of termination.
• The jacket must sit inside the connector’s strain-relief crimp.
• Use Cat6-rated plugs (wider conductor spacing) with a load bar for 23 AWG cable.
• For keystone jacks, follow the colour code printed on the jack and punch down with a 110 tool — do not push the wires in with a screwdriver.
• Terminate to T568B unless the site specification says otherwise, and label both ends.
• Test every run: a cheap continuity tester catches swaps and opens; only a certifier catches split pairs, NEXT and length problems.

🧯 Common Faults

📚 Resources

• TIA-568 wiring standards overview
• Power over Ethernet standards
• Ethernet physical layers and distances