Straight-Through and Crossover Cables – N10-008 CompTIA Network+ : 2.3

The wiring of an Ethernet cable is critical. In this video, you’ll learn about straight-through and crossover cables, and we’ll discuss a common myth regarding 568A and 568B wire colors.

When we’re connecting ethernet devices, we’re commonly using a cable that is a straight-through cable. This means that the wire on pin 1 on one side is connecting to the wire on pin 1 on the other side, pin 2 to pin 2, pin 3 to pin 3, and so on. You’ll sometimes hear this referred to as a patch cable, because this is the cable we use inside of a wiring closet to patch from a patch panel into an ethernet switch.

This is a very common Ethernet cable. If you were to look at the Ethernet cable being used on your computer, it’s probably a straight-through cable, and it’s most commonly used to connect workstations to network devices, such as switches. This is a wire map of 10BASE-T and 100BASE-T straight-through cables. You can see that there are only two pair being used here, because only two pairs of wires are used for 10-megabit and 100-megabit networking.

This block on the left is a network interface card. This is the adapter that’s inside of your computer, and it’s technically referred to as a media-dependent interface, or an MDI. You can see that pin 1 is a transmit plus, pin 2 is a transmit minus, pin 3 is a receive plus, and pin 6 is a receive minus. You can see that, with 10 and 100-megabit Ethernet, we don’t use pins 4, 5, 7, and 8.

On the right side is a network switch. This is a media-dependent interface crossover, or MDI-X. Sometimes we’ll refer to this as “em-dix.” This is a receice plus and receive minus on pins 1 and 2, and transmit plus and transmit minus on pins 3 and 6. This means that pin 1 transmit on one side is connected to pin 1 receive on the other side.

If we were to look at the traffic flows across this 10 or 100-megabit Ethernet cable, the network interface card would be transmitting to the receive pins and receiving traffic from the transmit pins on the other side. This traffic flow changes a bit when we get into gigabit networking and higher. With a gigabit straight-through or 1000BASE-T straight-through connection between a network interface card or MDI communicating to a network switch or MDI-X, you can see that there is no longer a transmit and receive side.

That’s because, with gig communication, there’s both transmit and receive on every single wire used. And we’re using all four pairs of wires in a single Ethernet cable. In this diagram I’ve labeled the pairs of wires as data A, data B, data C, and data D. This is why, whenever we’re connecting a gigabit Ethernet cable, we have to make sure that all eight wires are connecting on both sides of the cable.

If we’re connecting two devices to each other that are both MDI devices– for example, two separate workstations to each other– or we’re connecting MDI-X devices to each other– for example, connecting two switches to each other– we would not use a straight-through cable. Instead, we would use a crossover cable. This is the crossover cable pin out for 1000BASE-T based on the 802.3 standard.

You can see that pin 1 goes to pin 3, pin 2 to 6, 3 to 1, 4 to 7, 5 to 8, 7 to 4, and 8 to 5. Because all eight of the wires are used in a gigabit communication, we would also need to cross over all eight of those wires to create a crossover cable. One nice feature on most modern Ethernet devices is that it can automatically identify when a crossover cable is needed and implement what’s known as Auto-MDI-X, or Auto “em-dix.” this means that both devices will identify that they’re both MDI devices or both MDI-X devices, and internally automatically crossover the communication without needing a separate physical crossover cable.

This certainly simplifies things if you’re connecting to a network, because you can plug in a single straight-through cable, and the devices themselves will determine if a crossover is needed or not. One common misnomer with crossover cables is that one side of the crossover cable is wired to be TIA 568A colors and the other side is wired to be TIA 568B colors.

Well, as you can see, with this crossover cable for 1000BASE-T, that’s actually not the case. One side is configured in this case for 568B, but the other side is neither 568A or B colors. The determination of what colors are used on each pin come from the TIA 568 standard, but the determination of what makes up a crossover cable comes directly from the IEEE 802.3 Ethernet standard.

This confusion with a crossover cable being 568A on one side and 568B on the other probably started when we only had two pairs of wires that we needed to worry about. And with 10 and 100-megabit Ethernet, you could see that the wires on one side and the wires and the other side do have a similarity to A and B wiring. Obviously, we’re not using pins 4, 5, 7, and 8.

This definition files apart, though, when we get into gigabit communication, because obviously, a 1000BASE-T crossover is not 568A on one side and 568B on the other. I realize there are many books and many websites that will tell you that a crossover cable is 568A on one side and 568B on the other, but as you can see, that isn’t actually the case when you get into gigabit networking.

So if you’re connecting devices together, do you use a straight-through cable or do you use a crossover cable? If you’re connecting a workstation to a switch– this would be an MDI device connecting to MDI-X– we would use a straight-through cable. If you’re connecting a router to a switch, it’s a similar configuration with a straight-through cable. If you’re connecting a switch to a switch, those are both like devices, which means we would not use a straight-through cable. We would use a crossover cable.

A router to router is also a scenario where both devices are identical. We would use a crossover cable with those as well. And the same applies when connecting a workstation to a workstation. Because those are the same device, we would use a crossover cable. A workstation to a router looks like these would be two separate devices, but both a workstation and a router are both MDI devices, so you would use a crossover cable with that configuration as well.