Straight-through cable: Tips on UTP for the pros, lesson 2

Straight-through UTP cables and CAT5 networks are discussed in this tip.

This part of our series will focus on the wiring of CAT5e cable because it is the most common type of unshielded twisted pair (UTP) cable in the world. It is flexible, easy to install and very reliable when wired properly and can be deployed either as straight-through cable or crossover cable as circumstances warrant. We'll also cover wiring classic CAT1 phone cables. It is very important to understand UTP cabling standards and how to correctly terminate them. Cabling is the foundation for a solid network, and implementing it correctly the first time will help avoid hours of frustration and troubleshooting. On the other hand, if you are dealing with a poorly cabled network, this knowledge will help you to find the problem and fix it more efficiently.

Wiring UTP cables

We are now going to look at how UTP cables are wired. There are two popular wiring schemes that most people use today: the T568A and T568B. These differ only in which color-coded pairs are connected -- pairs 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 okay, but if you mix A and B in a cable run, you will get crossed pairs.

UTP color codes
RJ-45 connector

UTP cables are terminated with standard connectors, jacks and punchdowns. The jack plug is often referred to as a RJ-45, but that is really a telephone company designation for the modular eight-pin connector terminated with the Universal Service Order Codes (USOC) pinout used for telephones. The male connector on the end of a patch cord is called a "plug" and the receptacle on the wall outlet is a "jack."

As mentioned, UTP has four twisted pairs of wires. The illustration shows the pairs and the UTP color codes they have. As you can see, the four pairs are labeled. Pairs 2 and 3 are used for normal 10/100 Mbps networks, while pairs 1 and 4 are reserved. In gigabit Ethernet, all four pairs are used.

The picture below shows the end of a CAT5e cable with an RJ-45 connector. These are used by all cables to connect to a hub or to your computer's network card. The picture beneath it shows a stripped CAT5e cable and identifies the four twisted pairs.

CAT5 connector
CAT5 cable with RJ-45 connector
CAT5 cable stripped
Stripped CAT5 cable

UTP cables are now available in a variety of colors, making it possible to have different colored cables for different applications.

Colored UTP cable
UTP cable color options

RJ-45 wiring standards: T568A and T568B

Ethernet is generally carried in eight conductor cables with eight-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.

The eight-conductor data cable contains four 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 (about 1 cm). The pairs designated for 10 and 100 Mbps Ethernet are orange and green. The other two pairs, brown and blue, are used when gigabit Ethernet is supported, or they can be used for a second 10/100 Ethernet line or for phone connections. It should be noted that running a second Ethernet or phone line over an existing UTP cable is not recommended as UTP wasn't designed for such applications.

There are two wiring standards for UTP cables, called T568A (also called EIA) and T568B (also called AT&T and 258A). The only difference between the two standards is the wiring of two out of four pairs, which are swapped, as shown below.

T568A is supposed to be the standard for new installations, while T568B is an acceptable alternative. However, most off-the-shelf data equipment and cables seem to be wired to the T-568B specification. T568B is also the AT&T standard. It's perfectly acceptable to use either wiring standard; however, special consideration should be taken so that the same standard is used throughout the whole cabling infrastructure. For existing installations, it's best to first check which of the two standards is being used and continue with that standard.

Pin number designations for T568B

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

T568B

Here we break out the color codes for T568B:

Pin 1: white/orange (pair 2) TxData+
Pin 2: orange (pair 2) TxData-
Pin 3: white/green (pair 3) RecvData+
Pin 4: blue (pair 1)
Pin 5: white/blue (pair 1)
Pin 6: green (pair 3) RecvData-
Pin 7: white/brown (pair 4)
Pin 8: brown (pair 4)

T568B pin color code

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 center pins; this pair translates to the red/green pair for ordinary telephone lines that are also in the center pair of an RJ-11 (green = white/blue; red = blue).

Pin number designations for T568A

The T568A specification reverses the orange and green connections so that pairs 1 and 2 are on the center four pins, which makes it more compatible with the telephone company voice connections. (Note that in the RJ-11 plug at the top, pairs 1 and 2 are on the center four pins.) The illustrations show the order of colors in T568A.

RJ-45A
T568A color code

Pin color codes for T568A in writing are as follows:

Pin 1: white/green (pair 3) RecvData+
Pin 2: green (pair 3) RecvData-
Pin 3: white/orange (pair 2) TxData+
Pin 4: blue (pair 1)
Pin 5: white/blue (pair 1)
Pin 6: orange (pair 2) TxData-
Pin 7: white/brown (pair 4)
Pin 8: brown (pair 4)

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

T568A and T568B pinouts

Where are they used?

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

CAT5e straight-through cable - PC to hub

The picture above shows a standard CAT5e straight-through cable used to connect a PC to a hub or switch. 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 will automatically crossover the cable by using its internal circuits. The result is that pin 1 from the PC (which is TX+) connects to pin 1 of the hub (which connects to RX+). This happens for the rest of the pinouts as well.

If the hub didn't crossover 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, no matter what we do with the hub port (uplink or normal), the signals assigned to the eight pins on the PC side of things will always remain the same. The hub's pinouts, however, will change depending whether the port is set to normal or uplink.

Next Steps

Catalog of all 10 cabling lessons

Lesson 1: Network history and fundamentals
Lesson 3: CAT5 UTP crossover cable
Lesson 4: 10Base-T/2/5/F/35 - Ethernet

This was last published in June 2016

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