From the course: CompTIA Network+ (N10-009) Cert Prep

Testing cable

From the course: CompTIA Network+ (N10-009) Cert Prep

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Testing cable

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- Just because you've got a nice structured cabling setup installed, that doesn't actually mean that it works right. So what we have to do now is actually go through the process of testing all of these runs. So to do that, we need testing equipment, and I've got all kinds of cool testers here. But one of the things you need to keep in mind is that when you're testing a cable, there's one thing where you can test a cable that you hold in your hands and you have both ends. And then there's testing a cable where one end of the connection might be 100 meters away from the other end of the connection. So that's when we talk about with testers, we're always dealing with a remote piece and a main piece. So like with this tester here, I can break into two pieces and the reason I can do that is because I can be on one end and then on the other end that's far away and test both sides. So let's go through the process of testing, starting with wire map. Now, I've got two different testers here. Right here is a inexpensive, about a $50 tester that you can pick up at just about any home electronic store. Over here though, is a much more expensive tester. This particular tester, which is made by Fluke Corporation, goes for, right now, just shy of $700. So let me show you the difference. First of all, we'll start over here as we discuss wire map. Now, wire map simply means that all of the wires are punched into the right place on each end of the cable. So what I'm going to do is I'm going to plug into both ends. Oh, let me get some juice. And this cable's actually messed up. This is kind of cool as you watch this error here. Normally, if things are working exactly right, you'll get a very different code. Let me punch a nice pretty one in just so you get an idea of what it's supposed to look like. (cables shuffling) Now, this is a perfect wire map cable. What it's doing, it's comparing pin one and two on one end to one and two on the other, and it goes through each one. So it goes one, two, three, six, four, five, seven, eight all green. What that's telling us is that everything is in the right order. Now, now that we understand what right looks like, let's look at a bad one again. (cables shuffling) So as we watch this one march along, you'll see one and two doesn't light up. That tells us that one and two aren't even crimped in. They're simply missing their holes. Three and six is okay, but you see how it goes red from three to six and four and five, red to red? It's saying that four and five is crossed with three and six. So there's a mess up there as well. And then seven and eight doesn't light up at all. That means on one end or the other, just like with one and two, I've completely failed to get seven and eight even punched into the right spot. So that's a great example of wire map. So if wire map means each individual wire is punched down to the right location, continuity means that it's actually connected at all. As we just saw on that test, pairs one and two and seven and eight didn't even light up at all. That's a continuity problem. Let's do continuity again with the exact same patch cables, except, this time, let's use the more advanced tester. It makes a little bit more clear explanation. All right, by the way, this is actually called the MicroScanner from Fluke Corporation and it does a lot of different things. So I'm going to go ahead and just get it turned on. And let me just, it's got two holes up at the top. I'm just going to punch in both ends. And as you take a look at this, you'll see that there's one, two, three, four, five, six, seven, eight, and a one, two, three, four, five, six, seven, eight. That's telling us that not only is there continuity between each one of these connections, but everyone's in the right order. So that's what we want to see for a straight through cable. Let's go ahead and put Mr. Ugly back in and see what happens. (cables shuffling) Yipe, okay. So what we're looking at here is there's no connections on one, two, three, six, seven and eight, and pins seven and eight are incorrectly wired to pins four and five on the other end. So that's one of the reasons I like these MicroScanners. That's why they're worth the extra money to me, is that they're a little bit easier for me to read and figure out. So we've got wire map and we have continuity. There's a third thing we want to test for, and that's called, well, distance. And in order to test for distance, we need to use something called a time domain reflectometer or TDR. That's one of the reasons there's a big difference in price between Mr. $50 and Mr. Close To $700 right here, is that this guy has a built-in TDR function. The EIA/TIA 568 rules specify that an individual horizontal run can be no more than 90 meters. Now that ties in nicely with the ethernet 100 meters. The reason being is that you're going to need a patch cable on each end. So by keeping it at 90 meters, that guarantees you have enough room, up to 10 meters, for patch cables alone. So what we need to be able to do is test this guy right here to make sure that he's within the 90 meter TDR limit. So to do that, we typically do something like this. So I'm going to plug in and what I'm going to do, this is number 16, so I'll plug in with number 16 here. So I'll have that up and running. And then on this end, well, usually I'll leave the patch cable in and I'll use a remote device (device thuds) and I'll plug in with this. And that way, it creates a connection all the way, including the patch cables, from the workspace all the way through the horizontal run into the equipment room. And one of the cool things about this guy is I can actually reset this and it can give me a length. In this case, I know it's way under 90 meters, so there's not much to look at, but that's what we call a TDR. Now keep in mind that TDR is not unique to unshielded twisted pair cable. Fiber optic also uses TDRs. They call it an OTDR for optical TDR. And it works exactly the same way. You plug an optical TDR into one end, you have your big long fiber optic run, plugs into a wall outlet on the other side, and then you can do the exact same thing. Now, obviously, the distances are a lot longer, but do keep in mind that TDR is not unique just to unshielded twisted pair. You can do it with fiber optic just as well. So those are your three big tests. You want to test for wire map, you want to test for connections, continuity, and you want to run a TDR. Those are about the only tests that we do ourselves. Other tests are done. And to do these tests, you actually have to get big, very expensive, like $10,000 boxes. And even though they're expensive boxes, they're not very fancy. They plug into each end of a horizontal run. There's usually a big button called test, and you press the button and it either passes or fails. These guys do all kinds of interesting tests with names like near-end crosstalk, far-end crosstalk. Crosstalk is, if you're old enough to remember LAN telephones, when you would pick up an old telephone, you'd pick it up, you'd hear a little crackle, what's happening is you actually have interference between pairs on an individual cable. And this interference manifests as this crackling noise or what's known as crosstalk. Too much crosstalk will actually keep a cable from working at a particular level. Keep in mind, you've probably spent a lot of money to pay a cable installer to put in, for example, CAT 6A cable, which means it needs to run at 10 gigabits per second. The cable installer has to certify his runs to verify that they are capable of that. In essence, they put a little checkbox saying, we've installed everything and we guarantee you that it'll run at whatever you've paid for in terms of rating. And that's where things like crosstalk really come into play. Crosstalk's measured in decibels, so it's just a decibel value. And what will happen is these big expensive testers will have a USB connection or something, you plug it into a laptop, it makes a nice Word document, and each individual run, it shows the amount of crosstalk and everything. And that's what a cable installer will hand you to prove to you that they've done everything exactly right. So when it comes to little problems, for example, continuity, wire map and TDR, we have the tools to do that. But when it comes to the big hairy stuff, in particular, crosstalk, that's not our job. That's the job of professional installers. We just need to be able to read that piece of paper and verify, yUp, you've done a great job.

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