Ethernet Cables

Categories of twisted pair

Throughout this information technology era, we've had to increase the amount of data that goes through our systems, and with that, physically, we've had to find better ways to transmit this data. That's what these categories are all about. We have different categories of cables that we use that have improved over time.

We started out with category 3, or what we call cat 3 cable, and that cat 3 cable had a much lower frequency capability than the cables nowadays, and only could support certain speeds across it. As we progressed, we found better ways to improve the frequency and speed of these different cables, getting faster and faster connections. So that's where you see cat 3, cat 5, cat 5e, cat 6, cat 7, cat 7a, cat 8. There's actually a lot of categories in between these; I've just picked out some of the big ones here.

One of the big improvements with all of these: there are adjustments that we've made in how we separate these out and limit how much EMI comes in, how much cross talk. But one of the big differences with all of these different categories is how many twists there are in the cable. What we found is, in the cat 3 we just loosely twisted the cables, and we found that there was a lot more cross talk. The more we twisted it, the less cross talk there was. So cat 5 has a lot more twist to it, and so on and so forth. So although know that there are other differences between these cables, the biggest one is how many twists per inch that we have with these cables.

Pairs

Here we have eight conductors. If I were to circle all of these lines here, these are the eight different conductors we have for data. We call each of those that are linked together and of a similar color a pair. So we've got the orange pair, we've got the green pair, we've got the blue and the brown. So we've got eight conductors and four pairs.

Within each one of those there is the conductor, and then there is the insulator that protects it. The pair is twisted together, and that twist allows the cross talk to be minimized between these different lines and other cables that are running next to it. So we've got this twist along it.

The blue pair is what we consider pair number one. The orange pair is pair two. The green pair is three, and then the brown pair is four. So these are the different pairs.

Now, in a lot of our Ethernet standards, we only actually use two of the pairs, and so the two pairs that we typically use are pairs two and three. That's because some of our jacks will still use pair one for phone systems. Typically a lot of our standards — not all of our standards, but a lot of our standards — use pairs two and three.

RJ45 connectors

So typically we're not taking these wires and wiring the equipment directly. Most of the time we're plugging in this equipment with some sort of connector on the end, and more common than not, that connector is an RJ45 connector. RJ, once again, stands for registered jack, and 45. There's different ones out there: RJ11 is another really common one. But mostly when we're talking about data, we're talking about RJ45 connectors.

So we see this plastic housing here, and this right here is usually the clip. When we plug this in, we don't want that jack to come back out, and so there's a click that happens, a clip that will click into place. And then to get that back out, we have to depress that clip and pull it back out, so that is what's being represented by this area right here.

And then each one of these are the conductors. So when this slides into a piece of equipment, then those conductors make contact with conductors of the network interface card, and therefore we complete the whole circuit.

T568A and T568B

What we do is we have to line up the data lines on each side so that way this communication can happen between these two devices. They have to know which pairs they're communicating on. And so that's where we have a couple different standards: the T568A and T568B are different pin outs for these RJ45 connectors. So T568A and T568B are those different connection standards. What it essentially tells us is what should be pin one, what should be pin two, what should be pin three.

So on our jack, we actually have pins on here, and there's a total of eight pins, and it's the same for each one of these. And we have to associate that with the different conductors on the cables.

So then we can see that really these are mostly the same, except for the blue is in the same place on both of these different termination standards. The brown is in the same place on these different termination standards. What we see the difference is, though, is the green and the orange is swapped. So the green takes pin one and two on this one, versus it takes pin three and six on this one. And the orange, one two on the T568B, is pins three and six on the T568A. So those two swap on these different connectors. We're going to talk more about the termination standards, but just realize that we've got those two swapped on these two different termination standards.

Observations, and why pins three and six are split

I want to make a couple observations of this connection just to point out some fast ways to recognize these connections. So first of all, we can see that it alternates between the solid color and the striped color. So we see a striped and then solid, stripe, solid, stripe, solid, stripe, solid. So it alternates between those.

But what else is interesting about this is that we see that all of the colors are essentially paired up together. So the greens are together, the blues are together, the browns are together, and the oranges are together. So they're together with the exception of pin three and six, which are separated out. And so I just want to explain a little bit about the background of why that's the case.

One thing that has happened in the past is that we've used some of these same connections for phone systems, and the phone systems traditionally just need two conductors, one pair, to operate. And so that typically was this pair one right here, the first pair, which was the blue cables, the white blue and blue. So that is pin one, and so those took the first place. And then what they did is they lined up the rest of the pairs. Pair two came next, and then pair three needed to be split, so you see pair three is split here, and then pair four is on the end. And so it's a little different. And sometimes, if a phone had two lines, what it would use is it would use the center two and then the two outside from that. So that's another reason for this. So it kind of stemmed from the phone systems and the way the phone systems are set up.

And in fact, if you look at RJ11, which are used for phones, the RJ11 looks very similar to the RJ45 except for it's just a little bit smaller, and that's because it only uses six conductors. RJ11 is six conductors, and you can actually still use all of your Ethernet lines from your patch cables out and stuff, and you can plug in RJ11s into them. So you can use this either for data or for phone. And so that is one of the reasons why the odd space right here in the middle of these. But other than that, you can pick out how there are some similarities with these.

Straight-through cables

I'm going to jump into a few different cable types, and why we would choose certain cable types, and why we would use certain connection standards on each of the ends.

So I've got two devices I'm connecting together. On this side, a computer, and this side, a switch. These each have a pin out that's a little different. I'm going to label all of the conductors on this side if I were to look at that network interface card, and then same thing over here — this is the pin outs over here, five, six, seven, eight for each of the conductors.

If you notice, the pins on a computer, pins one and two, are the transmits, versus on the switch, pins one and two are the receive. Pins three and six on the computer are the receive, and pins three and six on the switch are the transmits. This is intentionally so, so that when we connect a cable between them, what's happening is the transmits are sending data to the receives of this equipment, and vice versa: the switch now is transmitting to the receives of this computer, and everything lines up. This is what we call a straight-through cable. So we want a straight-through cable when we're connecting a computer to a switch.

Now, where does that come into? We talked a little bit about our different pin outs that we have, and our T568A and T568B. Which one do we use? Well, in this case it really doesn't matter, as long as we use both on the same side. So if I put A on this side, I would want a T568A on that side. If this was a T568B on that side, then I'm going to want a T568B on that side. So it really doesn't matter what those pins are on each side, because the transmits are going to line up with the receives. So that is the straight-through cable: it's when your standard that you use is going to be the same on each side.

Crossover cables

So when is it different? When do we use what's called a crossover cable? Here in this example I'm connecting a computer to a computer. In this case, if I had a straight-through cable, it would be the transmits would be connected together. So in other words, they're both trying to talk but no one's listening. And then the receives are connected together, so now they're both listening on this line, but no one's talking. Obviously, communication is not going to happen.

So what we need to do is we need a crossover cable. We need the transmits to come down and hit the receives on this side, and the receives to hit the transmits, for communication to happen. And then the rest of these cables — because a lot of these cables, we don't only use two pairs — the rest of these we are fine if we go straight through or not, so we just cross that over.

So a crossover cable is when we have a T568A on one side and a B on the other, and the transmits and receives are flipped around. So now these two devices that are similar can talk to each other. So that's where you use a crossover cable.

Rollover cables

Now I'm going to cover a rollover cable, even though it's really not an Ethernet standard. I've not seen an Ethernet cable as a rollover cable. I have seen RJ45Ns, and I've seen twisted pair that are set up this way, but it's not really an Ethernet cable.

A rollover cable is kind of similar, I guess, to a crossover cable, except for pin one goes to pin 8, pin two goes to seven, three, six, four, five, five, four, six, three, 7, 2, and 8, 1. So essentially all those pins just get rolled over, and then the pin outs of those two different devices on each side just need to make sure that it can accept whatever transmit or receive.

So I've got an example of a computer and a switch here, but once again, it's not really Ethernet that connects these two. But what would be in this case right here is if I'm consoled into a device, then sometimes I'll use this crossover cable to communicate from one device to another via that console cable.

Naming the categories

One last thing on the categories: although we typically don't say category three, category five, we say cat 3, cat 5. So that's the industry standard, is just to call it cat.