Running an Ethernet cable from the router to the study or the living room is the most reliable way to get a stable connection. Once the cable has been pulled through the conduit, though, it has to be terminated: with a loose RJ45 plug, if the cable goes straight to the device, or on a keystone wall jack. It is simple manual work, but a single wire out of place is enough to get a slow connection or none at all.
Solid or stranded cable
First of all, you need to know which cable you are holding:
- solid cable (single-wire conductors): used for fixed installations inside walls and conduits. It is terminated on keystone jacks or patch panels, by punching down;
- stranded cable (multi-strand conductors): used for patch cords, the short cables between wall jack and device. It is terminated with crimped RJ45 plugs.
There are RJ45 plugs designed for solid cable too, but the practical rule stands: solid cable and jacks inside the wall, ready-made or hand-terminated stranded cords outside it.
The T568A and T568B schemes
A network cable contains four twisted pairs of wires, each with a solid colour and a matching white wire. There are two standard schemes for arranging them in the connector. In homes T568B is used almost everywhere:
| 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 |
Both schemes work the same way: what matters is using the same scheme at both ends of the cable. Mixing them creates a crossover cable, which modern devices usually handle, but which makes documentation confusing. Gigabit and faster speeds need all four pairs: the old trick of using only two limits the speed to 100 megabits.
The tools
A few tools are enough to do the job well:
- a cable stripper for network cable, which scores the outer jacket without damaging the conductors;
- a crimping tool for RJ45 plugs;
- a punch-down tool for keystone jacks, although many modern jacks close by hand without tools;
- a continuity tester for network cables, which checks every conductor.
Terminating an RJ45 plug
- Remove about 3 centimetres of outer jacket and cut the rip cord, if there is one.
- Separate the pairs and straighten the conductors without untwisting more than necessary.
- Arrange the wires in T568B order, side by side and flat.
- Cut the ends straight, leaving about 1.2-1.5 centimetres of exposed conductors.
- Push the wires all the way into the plug: looking from above, you should see the copper tips against the front wall.
- Check that the jacket goes inside the plug, so the strain relief clamps it.
- Crimp firmly, in a single movement.
The most common mistake is leaving the conductors too long: the jacket stays outside, the cable bends on the wires and over time the contacts loosen.
Terminating a keystone jack
On a keystone jack the colours are printed next to the contacts, often with both schemes shown. Pick the T568B row and press each conductor into its slot, keeping the twist up to a few millimetres from the contact. Then press with the punch-down tool or close the cap, which trims the excess and pierces the insulation. Here too the jacket should reach as close to the jack as possible.
Checking your work
The tester has two parts, one for each end of the cable. When switched on, the LEDs light up in sequence from 1 to 8 on both sides. An LED that stays off means a broken wire; two LEDs out of order mean swapped conductors. Cheap testers check only continuity, not signal quality, but they catch the great majority of mistakes.
The final test is to connect a device and check the negotiated link speed: if a computer with a gigabit port shows 100 Mbit/s, a pair is almost always not making contact. In my work this check reveals more faults than any visual inspection.
In short
- Solid cable terminated on keystone jacks goes in the wall; stranded cords with RJ45 plugs go outside.
- Use the same scheme, usually T568B, at both ends of the cable.
- The jacket must go into the plug and the twist must be kept up to the contact.
- Check every cable with a tester and look at the negotiated speed.
- A gigabit port linking at 100 Mbit/s almost always points to a faulty wire.
