Wiring Straight and Crossover Ethernet Cables

Although you may think it unlikely, I can guarantee that you’ll find plenty of network support people in the world who don’t know how to wire a simple patch cable properly. Understanding the roles of the wires in twisted pair cabling goes a long way towards explaining the communication process between connected systems. What we’re going to look at here is the way in which different network cables are created using Cat 5.

Even though there are 8 wires (4 pairs) in Cat 5 wiring, only 2 of those pairs are used to transmit and receive data. You may have heard the term ‘tip and ring’, especially in the world of telephony. The four wires that transmit and receive data are considered to be the tip, while the other wires provide grounding and are referred as the ring. When using Cat 5 cabling, there are two possible transmitting and receiving channels, allowing systems to communicate in full duplex if plugged into a switch. Full-duplex means that systems can both send and receive data at the same time – note that many older network cards will only support half-duplex, while newer cards almost certainly will support both. Full- and half-duplex will be looked at in detail when we discuss Ethernet.

Like just about anything else in the networking world, there are standards for wiring UTP cables. These standards are defined by the Electronic Industries Alliance/Telecommunications Industry Association (EIA/TIA) and fall into two flavors – 568A and 568B. For the most part, the B standard is more popular. However, many government contracts require that the A standard be used. To be honest, it’s well worth knowing both, as will become clear in a moment. When creating network cables, you’ll notice that they are made up of individual wires of different colors. Some appear as a solid color, while others appear white with a colored stripe. The proper terminology is to call the orange wire ‘orange’ while calling the white wire with the orange stripe ‘white-orange’ – the background color should always be specified first.

A straight-through cable has two ends that are identical. When looking at cables, compare the RJ-45 ends side-by-side with the snap-in clip facing down. If you’re looking at a straight-through cable the ends will be the same, while on a crossover cable, the ends will be different. To be even more precise, a crossover cable simply has one end wired according to the A standard, and the other wired to the B standard – that’s why knowing both standards is so useful.
The tables below outline the wiring order for 568A and 568B. I’ve included both the pin numbers and the colors to help you along. Note that the pin numbers represent the individual wires from left to right, when looking at an RJ-45 connector with the clip facing down. The leftmost will always be pin 1, and the rightmost pin 8.

Note: Some vendors engage in the bad habit of not wiring their pre-packages cables to the 568A and 568B standard. It’s important to recognize that the wire numbering is most important during the communication process, not the colors. However, for the Cisco exams you should be familiar with the wire color and numbering standards listed here.

568A and 568B Wiring tables:

1. White-Green
2. Green
3. White-Orange
4. Blue
5. White-Blue
6. Orange
7. White Brown
8. Brown

1. White-Orange
2. Orange
3. White-Green
4. Blue
5. White-Blue
6. Green
7. White-Brown
8. Brown
If you look closely, you’ll notice that only the orange and green pairs switch places between the standards.

To understand what the pins do, let’s take a look at a straight-through cable. In the example below, note that the cable is plugged into a PC at one end and a hub at the other. Note that the pin-outs on the PC and hub have different roles, and that only 4 wires are actually used in the communication process – pins 1, 2, 3 and 6:

Figure: Straight-through connection, PC to hub.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of 2000Trainers.com. He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.