The T568B order
Hold the plug with the clip facing down and the wires towards you. From left to right:
| 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 |
Straight-through vs crossover cables
- Straight-through: the same order on both ends (T568B and T568B). This is what you use 99% of the time: PC to wall jack, patch panel to switch.
- Crossover: T568B on one end, T568A on the other. It used to connect two devices directly. Most modern devices switch automatically (Auto MDI-X), so you'll rarely need one.
Why the order matters
The wires come in twisted pairs, and each pair protects its signal from interference. Swap two wires from different pairs and the cable might work but run slow and drop out, or not work at all. 100 Mbps Ethernet uses pins 1, 2, 3 and 6; gigabit uses all eight.
How to test a cable
- Plug one end into the tester's main unit and the other into the remote unit.
- Start the test and watch the lights: each number that lights on the main unit should light the same number on the remote.
- A number that never lights on the remote = a broken wire. A different number lights = two wires swapped. Two numbers light together = two wires touching (a short).
The most common mistakes
- Stripping too much jacket, so bare wires leave the plug with nothing to hold them, and they pull out and break.
- Not pushing the wires all the way to the end of the plug before crimping, so some never touch the contacts.
- Swapping white-green and white-blue (pins 3 and 5), which look alike in poor light.
- Fitting an old cable from a drawer without testing it. A minute of testing saves you a second ticket.
The most common network cable faults
The mistakes above happen when a cable is made. But even a well-made cable fails over time, usually from everyday life in the office. These are the most common faults, and how to recognise each one from its symptoms:
| Fault | What you see | The fix |
|---|---|---|
| A broken wire (open) | The port's link light is off, or a number never lights on the tester | Replace the cable, or re-terminate the plug if the break is there |
| Two wires touching (short) | No network, and two numbers light together on the tester | Re-terminate the plug, or replace the cable |
| A split pair | Works but slow and drops out; a basic tester may say it's fine | Re-terminate both ends in the correct T568B order |
| A broken plug clip | The network drops when someone moves the PC, and the plug works loose | Cut off the plug and fit a new one |
| A crushed or kinked cable | Connects and drops, especially when a chair or desk moves | Replace it, and route it away from chair wheels and doors |
| Beside a power cable | Slowness and errors with no clear cause, especially when big equipment starts | Move it away from power cables, air conditioners and lights |
| Longer than 100 m | Poor speed, or no link at all | Break the run with a switch in the middle |
| An old cable (Cat5) | Links at 100 Mbps instead of 1 Gbps | Replace it with Cat5e or Cat6 |
And sometimes the cable isn't the problem at all: it's in the wall's TEL port instead of DATA, or the switch port itself is off. So try the cable on another PC or port before you judge it.