Straight Thru UTP Cables

Introduction

We will be mainly focussing on the wiring of CAT5 cables here because they are the most popluar cables around ! You will find info on wiring the classic CAT1 phone cables as well. It is very important you know how exactly to wire UTP cables because it's the base of a solid network and will help you avoid hours of frustration and troubleshooting if you do it right the first time :) On the other hand, if you are dealing with a poorly cabled network, then you will be able to find the problem and fix it more efficiently.

Wiring the UTP cables !

We are now going to look at how UTP cables are wired. There are 2 popular wiring schemes that most people use today: the T-568A and T-568B, that differ only in which color coded pairs are connected - pair 2 and 3 are reversed. Both work equally well, as long as you don't mix them! If you always use only one version, you're OK, but if you mix A and B in a cable run, you will get crossed pairs!

UTP cables are terminated with standard connectors, jacks and punchdowns. The jack/plug is often referred to as an "RJ-45", but that is really a telco designation for the "modular 8 pin connector" terminated with a USOC pinout used for telephones. The male connector on the end of a patchcord is called a "plug" and the receptacle on the wall outlet is a "jack."

As I've already mentioned, UTP has 4 twisted pairs of wires, we'll now look at the pairs to see what colour codes they have :

As you can see in the picture on the left, the 4 pairs are labeled. Pairs 2 & 3 are used for normal 10/100Mbit networks, while Pairs 1 & 4 are reserved. In Gigabit Ethernet, all 4 pairs are used.

CAT5 cable is the most common type of UTP around the world ! It's flexible, easy to install and very reliable when wired properly :)

And to be a bit fancy, don't think that UTP CAT5 cable only comes in one boring colour... those days are over ! You get a wide range of choices today :

T-568A & T-568B 4-pair Wiring

Ethernet is generally carried in 8-conductor cables with 8-pin modular plugs and jacks. The connector standard is called "RJ-45" and is just like a standard RJ-11 modular telephone connector, except it is a bit wider to carry more pins.

Note: Keep in mind that the wiring schemes we are going to talk about are all for straight through cables only ! Cross over cables are examined on a separate page !

The eight-conductor data cable contains 4 pairs of wires. Each pair consists of a solid colored wire and a white wire with a stripe of the same color. The pairs are twisted together. To maintain reliability on Ethernet, you should not untwist them any more than necessary (like about 1 cm). The pairs designated for 10 and 100 Mbit Ethernet are Orange and Green. The other two pairs, Brown and Blue, can be used for a second Ethernet line or for phone connections.

There are two wiring standards for these cables, called "T568A" (also called "EIA") and "T568B" (also called "AT&T" and "258A"). They differ only in connection sequence - that is, which color is on which pin, not in the definition of what electrical signal is on a particular color.

T-568A is supposed to be the standard for new installations, while T-568B is an acceptable alternative. However, most off-the-shelf data equipment and cables seem to be wired to T568B. T568B is also the AT&T standard. In fact, I have seen very few people using T568A to wire their network. It's important not to mix systems, as both you and your equipment will become hopelessly confused.

Pin Number Designations for T568B

Note that the odd pin numbers are always the white with stripe color (1,3,5,7). The wires connect to RJ-45 8-pin connectors as shown below:

Color Codes for T568B
Pin color - pair name
1 white/orange (pair 2) TxData +
2 orange (pair 2) ........ TxData -
3 white/green (pair 3) ..RecvData+
4 blue (pair 1)
5 white/blue (pair 1)
6 green (pair 3) ...........RecvData-
7 white/brown (pair 4)
8 brown (pair 4)

The wall jack may be wired in a different sequence because the wires are often crossed inside the jack. The jack should either come with a wiring diagram or at least designate pin numbers.
Note that the blue pair is on the centre pins; this pair translates to the red/green pair for ordinary telephone lines which is also in the centre pair of an RJ-11. (green=wh/blu; red=blu)

Pin Number Designations for T568A

The T568A specification reverses the orange and green connections so that pairs 1 and 2 are on the centre 4 pins, which makes it more compatible with the telco voice connections. (Note that in the RJ-11 plug at the top, pairs 1 and 2 are on the centre 4 pins.) T568A goes:

Color Codes for T568A
Pin color - pair name
1 white/green (pair 3) ..RecvData+
2 green (pair 3) ..........RecvData-
3 white/orange (pair 2) TxData +
4 blue (pair 1)
5 white/blue (pair 1)
6 orange (pair 2) .........TxData -
7 white/brown (pair 4)
8 brown (pair 4)

The diagram below shows the 568A and 568B in comparison:

Where are they used ?

The most common application for a straight through cable is a connection between a PC and a hub/switch. In this case the PC is connected directly to the hub/switch which will automatically cross over the cable internaly, using special circuits. In the case of a CAT1 cable, which is usually found in telephone lines, only 2 wires are used, these do not require any special cross over since the phones connect directly to the phone socket.

The picture above shows us a standard CAT5 straight thru cable, used to connect a PC to a HUB. You might get a bit confused because you might expect the TX+ of one side to connect to the TX+ of the other side but this is not the case. When you connect a PC to a HUB, the HUB it will automatically x-over the cable for you by using its internal circuits, this results Pin 1 from the PC (which is TX+) to connect to Pin 1 of the HUB (which connects to RX+).This happens for the rest of the pinouts aswell.

If the HUB didn't x-over the pinouts using its internal circuits (this happens when you use the Uplink port on the hub) then Pin 1 from the PC (which is TX+) would connect to Pin 1 of the HUB (which would be TX+ in this case). So you notice that no matter what we do with the HUB port (uplink or normal), the signals assigned to the 8 Pins on the PC side of things, will always remain the same, the HUB's pinouts though will change depending wether the port is set to normal or uplink.

This pretty much concludes our discussion on straight thru UTP cables !