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Straight-through UTP cables: Cabling tips for network professionals

Fahmi Rizwansyah says:

This part of our series will focus on the wiring of CAT5 cable because it is the most common type of UTP cable in the world. It's flexible, easy to install and very reliable when wired properly. We'll also cover wiring classic CAT1 phone cables. It is very important that you know exactly how to wire UTP cables. Cabling is the base of a solid network, and doing it correctly the first time will help you 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 T-568A and T-568B. 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.

RJ-45 jack and plugUTP 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 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."
UTP color codes

As I've already mentioned, UTP has four twisted pairs of wires. The illustration shows the pairs and the 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 CAT5 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 below that shows a stripped CAT5 cable and identifies the four twisted pairs.

CAT5 cable with an RJ-45 connector stripped CAT5 cable

And don't think that UTP CAT5 cable only comes in one boring color -- those days are over! You'll find a wide range of choices today, as you can see below.
UTP CAT5 cable colors UTP CAT5 cable colors UTP CAT5 cable colors

T-568A and T-568B four-pair wiring
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, 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 (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:
568B wiring method Pin number designations for 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)

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 which are also in the centre 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 4 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 4 pins.) The illustrations show the order of colors in T568A.
568A wiring method Pin number designations for T568A

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:

568A vs 568B 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 cross over 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 cross over since the phones connect directly to the phone socket.

CAT5 straight-through cable connecting PC to hub

The picture above shows a standard CAT5 straight-through cable used to connect a PC to a hub. 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 x-over 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 cross 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, 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.