If you’re installing a lot of ethernet connections, or in my case a single ethernet connection, and you want to be sure that you’ve wired it properly on both sides, you may want to have some way to test. It’s common to use a TDR or specialized wire mapping device to be able to ensure that when you put on that RJ45 connector that none of the wires got swapped around before you were able to crimp it onto the end. Or perhaps you get a good cable person. Having someone on site and working with you all the time can help, especially if you’re doing a lot of moves and adds and changes to your networking infrastructure.
One of the important standards when we’re working with our ethernet networks is which wires we’re going to connect to which pins inside of the cable. This is standardized through something called the EIA/TIA-568-B standard. And it determines exactly which color wire is going to connect to which pin inside of our RJ45 connector. There are two standards that we need to be concerned with. One is called the 568A, and the other one is called the 568B. And there are slight differences between what color wire is connecting to which pin inside of our wiring connector.
Generally the 568 was designed for horizontal cabling when we’re working inside a single floor, but generally organizations tend to use 568B for everything. It really depends on how it’s configured in your environment and what you’ve all decided that you’re going to use when you start punching down all of your cables.
One important thing to remember is that especially in gigabit networks you cannot punch down one side with 568A standard and the other with a 568B and expect it to work, because it’s not. The wires are going to be going to completely different pins on the other side, so you want to be very careful about which standard you’re using and make sure that it is identical on both sides of the wire.
Here are the differences between the 568A and the 56B, and we’re referencing these pin numbers based on an RJ45 connector. If you look at the back of it, pin one through pin eight is what we’re looking at, you can see there are very similar pieces between A and B. For instance pins four and five and pins seven and eight are identical between the two. The real difference comes with pins one and two and three and six. You can see it alternates between white and green, white and orange, and those two pairs are swapped between the A and the B standards.
If we’re connecting ethernet devices inside of our network, we’re generally going to use a patch cable, and this patch cable is simply a straight-through cable. That means that pin one on one end of the cable is going to connect to pin one on the other end of the cable. Pin two is going to connect to pin two, pin three to pin three, and so on.
This is the most common ethernet cable that you’ll find, and you can easily put the cable ins next to each other and look at them and see that all of the pins match on both sides of the ethernet connection. If we were going to look at the 568B standard, the straight-through cable, as you can see here, all of the wires are going from the same pin on one side to the same pin on the other side straight through the cable.
We use the straight-through cable when we’re connecting an MDI device to an MDI-X device. MDI stands for media-dependent interface, and the MDI-X means that it is a crossover of the the media-dependent interface. An MDI device is something like an end station or an ethernet card that’s inside of our computer or our laptop. An MDI-X device is one that is generally an infrastructure equipment, like a router or a switch or a firewall.
This is set up so that when we send something out a transmit wire, on the other side it’s going to be a receive wire. And this is how these MDI and MDI-X devices are related to each other. Obviously we wouldn’t be able to send a transmit into a transmit. It has to be transmit on one side and receive on the other. This is a straight-through through for a 10BASE-T and 100BASE-T.
So if you’re running those now slower speeds of ethernet, notice this you only need two pair to make that happen. Pins four and five and seven and eight aren’t even used. And if you do have a cable that has extra wires in it that’s fine. But if you’re going to be running 10 and 100 megabit, there’s certain wires that you’ll never use between those two connections.
If you’re running a gigabit connection, you’re going to be using all four pair. This is a very similar design. You can see, in fact, the colors are exactly the same, but notice that we’ve added pins four and five and seven and eight, and here they’re designated as data a, data b, data c, and data d. But again it’s the same idea that one side is the MDI and the other side is the MDI-X or the [? MDIX. ?]
What if it isn’t a situation where you have an MDI on one side and an MDI-X on the other? What if it’s an MDI to MDI or what if it’s an MDI-X to MDI-X? Well, in those situations you need a crossover cable so that we can change where the pins are going so that we can still have transmit talking to receive and receive talking to transmit. That’s why whenever we’re connecting two computers to each other or two switches to each other we’re going to use something like a crossover cable. And you can see in this design exactly the way the crossover cable looks. And if you were to look at the cable you would see that on both ends of those they are wired very differently so that you’re able to cross the signal over between transmit and receive.
Here’s a diagram of two MDI devices. So you might have a laptop and a computer, and now instead of going from pin one to pin one, where this would be in this case transmit to transmit, we’re now crossing over from transmit and going down to receive. And that’s why cable is so important, so that whether you’re running 10 or 100 megabit or even the gig connections, you’re able to communicate between those two devices.
These days, you may find that most of the devices you have are using something called autoindex, which means that if you are connecting two devices that are similar to each other with a straight-through cable, it will electronically shift around where it is sending and receiving information. That way, if you find that you don’t have a crossover cable but you still want to connect two similar devices to each other, you can use a straight-through cable and the autoindex capability of one of the devices is going to perform the crossover in the electronics of the device itself.
If you’re working on the configuration of a Cisco switch, a Cisco router, or some other type of infrastructure device, they may not use ethernet as the primary connection. It may use a serial connection, and because of that it may not be a straight-through or crossover cable that allows you to communicate serially to that device. In those cases, it may be something like a rollover cable.
This rollover cable, just as the name implies, rolls over pin one to pin eight, pin eight goes to pin one, and everything in the middle is rolled over to the other side in that opposite fashion. This is a standard that was created by Dave Yost, and that’s why you’ll sometimes see it referred to also as a Yost cable instead of a Cisco console cable or a rolled cable.
Again, this is one used generally for serial communications. So you’re connecting a laptop or mobile device into a serial port on one of these infrastructure devices, generally to connect to it out of band so that you can make configuration changes without going through an ethernet connection or an IP connection on that device.