Straight-through vs. crossover cables

- [Instructor] In total, we have eight wires in an ethernet cable where each wire connects to one of eight pins in an RJ 45 connector. And when these devices communicate over an ethernet cable the eight wires in that cable do different things a cable where the wires are connected in the same color order to the RJ 45 connectors at each one of the cable, that's called a straight through cable. For example, here we have a PC connecting into an ethernet switch using a straight through cable. However notice the specific ethernet standards we're talking about here. 10 base T that only runs at 10 megabits per second, and 100 based TX which only runs at 100 megabits per second. Well, the PC is referred to as a medium dependent interface or an MDI device. The ethernet switch is a medium dependent interface crossover or MDI X device. The crossover element in an MDI X device indicates the transmit and receive wires are transposed. If the PC is using pins one and two to transmit pins, one and two on the switchboard are receiving those transmissions from the PC. And in the case of these two somewhat legacy ethernet standards we're only using four wires of the eight wires in that ethernet cable. Specifically, the PC is transmitting using pins one and two and it's receiving using pins three and six at the far side of the cable. It's the exact opposite. The switch is receiving on pins one and two and transmitting on pins three and six but this brings up all possible paradox. When connecting two ethernet switches using M D I exports specifically, we have transmit pins on one side of the cable, connecting to transmit pins on the other side of the cable, and that should not work. So does that mean we cannot interconnect to ethernet switches with a straight through cable? Not at all. We can use a straight through cable to interconnect most of our modern ethernet switches because they support a feature called auto M D I X, which automatically negotiates, which pins are going to be used for transmitting and receiving on each switch port. What if we're interconnecting a couple of MDI devices though, like a couple of PCs in that case we might need to use a cable called a crossover cable that has reversed the pin outs for the RJ 45 connectors at each end of the cable causing the transmit pins on one side of the cable to connect to the received pins on the other side of the cable and vice versa here, pins one and two on one side of the cable map to pins three and six on the other side of the cable that lets the transmit pins on PC one, talk directly to the receive pins on PC two while the receive pins on PC one connect directly to the transmit pins on PC two. And with these older ethernet standards noticed that we're only using four of the eight wires in the cable. And back in the 1990s, this shortcoming had a silver lining called a pair splitting in hopes of saving some time and money. What I would do is run one cable into an office that had two computers and I'd take four wires from the cable and connect them to an ethernet Jack on pins one, two, three, and six, the remaining four wires. You guessed it. I would connect those to another ethernet Jack using pins one, two, three and six. Then in the wiring closet at the other end of the cable I would take those eight wires. And again, split them out into two connectors plugging into two different ethernet switch ports. And I thought that was a great way to save money when wiring offices that had a couple of connections. However, that presented a major problem when ethernet speeds increased and I needed to connect devices at one gigabit per second rates in 1000 based T networks were using all eight wires in that ethernet cable. So all of those PCs that I had connected using only four wires cause a neighboring PC was using the other four wires. You had to go back and rewire all of those connections. So what I thought was a bright idea to save some cost turned out, costing more in both time and money. The big takeaway though from this diagram is I want you to understand that if we're running at speeds of one gigabit per second or higher, we're going to be using all eight wires. And this diagram is of a straight through cable, but what if we needed a crossover cable that used all eight wires just for your reference, it would look like this. And that's a look at straight through cables where the pin out on one side of the cable matches the pin out on the other side of the cable. And it crossover cables where the transmit pins on one side of the cable map to the receive pins on the other side of the cable and vice versa.