Troubleshooting Copper Cables – CompTIA Network+ N10-006 – 4.4

| May 8, 2015


If you have a bad copper cable, then your network performance is going to suffer. In this video, you’ll learn about opens, shorts, T568A and T568B standards, and crosstalk.
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If you look at copper cabling, you’ll notice that the copper itself is usually protected or separated from other pieces of copper in the cable. That’s because you want to be sure that no pieces of copper, especially on separate wires, are going to be touching each other. If they are, you have a short circuit. And if there is data that needs to go across any one of those wires, then you’re going to have a problem getting that through that particular cable.

An open circuit would be one where the copper is not connecting to anything. It’s completely open. If a wire was cut, for instance, you would have an open circuit. In either of these cases, you may have interruption of the signal, especially in the case of an open circuit. And with a short circuit, it may be something that’s intermittent.

So sometimes the signal might get through, and then the cable is moved and the circuit is shorted, and then you might have problems communicating. These opens and shorts can be difficult to find because it might only be open or short circuited if the cable is moved in a particular way. So you may have to wiggle the cable around a little bit and see if you can recognize when this particular signal interruption is occurring. The common repair for an open or a short will be to simply replace the cable.

You could, of course, open the cable, try to identify where in the cable the short circuit is occurring or where the open happens to be, and perform some type of repair to the copper itself. And that’s very time consuming. And it may be a much better use of your time to simply pull the cable out and replace it with something brand new. If you were interested in finding exactly where this open or short might be, then you could use something like a time domain reflectometer or a TDR.

You usually don’t have the cable all at one place like you do in this picture. Usually you would put the TDR large unit itself on one end of the cable, the other end might be on the other side of a room. So you would put the other piece of the TDR on that side and run your tests. And the TDR will tell you exactly how far down the cable you happen to have a short or an open. So at that point, you should be able to determine if you need to only simply change out a connector on the end or if the problem happens to be in the wall halfway down this run.

These cables are the foundation of your network, so it’s important that you follow the standards very closely. And if you are installing a lot of cable, it’s a very good idea to test the entire installation before you put anything online into production. It also helps to have a good cable mapping device or a TDR so that you can confirm exactly what pins are connected to what pins on the other side. And you can then verify that you’re able to get a good connection no matter what devices you have on both sides of that cable.

If you’re installing a large network, you may want to get a specialist who has done a lot of network installations, who has the right equipment and the right knowledge to install the network perfectly the first time. There are some very specific ways to wire our Ethernet networks. The assignments of the pins come from a standard called the EIA/TIA-568-B standard. And this specifies how you would connect eight conductor 100-ohm balanced twisted-pair cabling in your environment. If you look at the end of your Ethernet cables, they’re wired in one of two ways.

It’s either wired with the 568-A standard or the 568-B standard. Generally we see the 568-A being used in horizontal cabling, but some organizations will use 568-B everywhere. You generally choose the wiring scheme that you want to use in your environment and you tend to use it everywhere. You don’t want to terminate your Ethernet cable with 568-A standard on one side and 568-B standard on the other. You want both sides to be exactly the same.

If you don’t do it exactly the same, you’re not going to have a straight-through cable, and this will undoubtedly cause some problems on your network. This is a representation of the pin outs for both of these standards. You’ll notice that pins 4 and 5 are blue and white and blue. And they’re exactly the same whether it’s the 568-A and 568-B. You also see that pins 7 and 8, white and brown and brown are also exactly the same.

So really the only difference between the A standard and the B standard is where pins 1 and 2 and pins 3 and 6 may be located. You can see they’re white and green and green in the A standard and white and orange and orange in the B standard. So you want to choose which one you’re using in your environment and use that standard for all of your Ethernet connections. When you’re sending a lot of different signals down a lot of different wires, one of the things you need to watch for is crosstalk. You’ll see that crosstalk is sometimes abbreviated as XT.

Crosstalk is when signal that’s going across one circuit or wire is affecting another circuit or wire. And usually this is a bad thing because these wires should not be affecting each other at all. You can think of crosstalk as a leaking of signal between one wire and another. If you have analog information going across, you can sometimes hear the leaking of the signal from one wire to the other. You can measure the exact amount of cross talk with a good cable tester or time domain reflectometer.

You may need a little bit of training for devices like this, but it can provide you with a lot of detail of exactly how much crosstalk is in your environment. We’re often concerned with two types of crosstalk. One is the near end crosstalk. This is how much interference you’re getting between all of the wires that are closest to you on the near end. The other type of crosstalk you’ll be interested in knowing about is the far end crosstalk.

How much crosstalk is there once the signal gets all the way to the other side? Crosstalk problems are usually related to the wiring and it’s often related to the connector that we’ve crimped on to the end of the wire. The twists that are in our twisted pair cabling help minimize crosstalk. So we need to keep those twists in the wire until the last possible moment where we are either punching it down or crimping it to a connector.

You can see here, this is not what happened with this connector. We have straightening of the wires well before it gets inside of the connector itself. You might also want to consider using a cable where the wires are farther apart from each other, like category 6A. And ultimately, you want to be sure to have the right equipment so you can certify the cabling that you’re installing and you’re minimizing the amount of crosstalk on the wires.

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Category: CompTIA Network+ N10-006

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