Understanding Network Cable Testers
Understanding Network Cable Testers
This content is from our CompTIA Network + Video Certification Training Course. Start training today!
In this post, CompTIA Network + instructor Rick Trader demonstrates common cable tester and how to use them in network configurations.
We have previously covers cables and connectors so now let’s look at some tools that we can use to test our different cables in our environment.
- Properly Wired
The first is a simple cable tester. Since you may find yourself creating your own cables such as installing our own RJ45 or BNCs ends, we want to make sure those cables have been correctly wired.
One of the things we can look at is are we properly wired using this cable tester. It also allows us to check connectivity from one end to the other end.
Let’s take a look at a simple cable tester.
These simple cable testers are inexpensive. You can pick these up at most of your computer shops anywhere you go.
The first one we are going to look at is this blue LinkMaster.
You’ll notice on the blue one, we’ve got a series of cables here and it’s got lights. It’s going to show us whether or not we’re wired correctly. It also shows us the actual T568‑A and the 568‑B, the actual way the cable should be wired.
We will plug the ends into the tester (we already know this is a good cable) but we always want to make sure we test our cable tester with a known good to make sure that we are using good cables.
Next we’ll push the button and it goes out and it tests the cable.
It’s ensuring that pin 1 is wired to pin 1 and 2 to 2, 3 are together, 4, 5 etc.
This is a good test. All it’s doing is testing the cable.
This other cable tester does the exact same thing but it just gives a little bit different of a display.
Again we’ll attach the cable on one one cable tester and notice it comes out and it shows me that pin 1 and 2, 3,4, 5, etc… all the way through to ensure they’re all wired correctly.
This is a known good cable.
Now let’s look at another good cable but now it’s actually a crossover cable. I know that by looking at the wiring cables.
We will plug this into the cable tester and turn it on.
When plugged in, it starts flashing. It says, “Hey, I’ve got an open.”
On this crossover cable, pins 4 and 5 are not wired, also notice that 1 and 3 have been swapped, 2 and 6 have been swapped, and 2 and 6 on the other end 1 and 3 have been swapped. It’s showing us that this is actually the crossover cable. It shows it as a bad cable.
If I use the original simple tester, notice it it informing us that we have a bad cable.
1, 2 and 3,6 are not detected and 4,5,6,7, and 8 aren’t even wired correctly at all.
In this particular one, the cables four and five weren’t even wired. It’s showing me that I have a miswired cable. This allows me to go again. It allows me to go in and test my cables and be out to look at things correctly.
Wire Mapper (Certifier)
A wire mapper is sometimes referred to as a certifier. What a wire mapper can do is the same thing a cable tester can do. It can test my connectivity.
For example, we can put a loopback on one end and connect the wire mapper on the other end. It will test connectivity. It’s going to test to make sure I’m wired correctly.
The other things it can do is test the wire for attenuation. That ensures that I don’t have too much signal drop from the time from one end to the other end. A lot of times if the wire goes out of specifications, by running cables longer than we are allowed to, you’ll see that attenuation drops. As a result, we’ll get inconsistent data transfer.
It also is going to check for crosstalk. Crosstalk for example can be caused by untwisted wires when the patch panel was built or bad connectors can cause crosstalk.
We can also be getting crosstalk because of the way the cable in the cable feed. So the wire mapper, is more expensive the simple testers but it also provides more testing options.
- Verify End-to-End Connectivity
The next type of device we can use is called a loopback adapter. What a loopback adapter does is it takes pin 1 through 8 and will take pin one and pin three and ties them together.
Further, it will take pin 2 and 6 and ties them together.
Finally is will take pin 4 and 5 together then 7 and 8 together.
What this is actually going to do is it’s going to establish a loopback. This is what you’ll commonly see at a store. It’s just a simple one.
If we were to unbolt this, you’d see that pin one and three are connected to each other. What we can do is plug this in and actually verify that we get a link light. It will show that our network adapter card is being useful.
Here’s one that we made.
We took an RJ45 connector with a little piece of twisted pair cabling and then we twisted with 1 and 3 being connected, 2 and 6 being connected, 4 and 5 and 7 and 8 being connected.
Again we can plug this into the back of the machine or we can plug this into a cable and establish a loopback.
Finally, some of the little more expensive ones. If you look on here, it tells you actually how it’s looped back on the sides.
This is actually designed so that we can plug into the back of the computer to provide a loopback to the network adapter card ‑or we can plug this on the end of a cable and provide a loopback on the end of the cable.
These are just different vendors, different ways they make the loopback adapters.
- Check Connectivity
- Short vs Open
The next device that we might use in our environment is called a multimeter. Traditionally, most people are going to be using multimeters to check for voltage whether it be AC voltage or DC voltage. They might be checking AC voltage and making sure that the outlet is wired correctly, or making sure that an extension cord is working correctly or a power panel is working correctly.
DC can be used to check the voltage of possibly your CMOS battery or a battery that you might have in your environment or even the battery from your laptop.
We can also use a multimeter to check connectivity. Commonly a multimeter is used to check if a cable is shorted. Such as when the center conductor is making connectivity with or touching my ground. We also can be checking it to make sure the wire hasn’t been cut or there’s an opening in it.
Let’s look at how this is actually used.
Here is a standard multimeter.
We use this with a coaxial cable because it’s a lot easier to test it with. Traditionally we are going to use the diode mode. (If you got a good Fluke meter or a good multimeter, they will have a diode mode.)
When working with a diode, you put the leads one way, you don’t get a sound, if you put the leads the other way you’ll get a sound that tells you that the diode does not have a short or the diode hasn’t opened or been damaged.
We can take the conductor and go to ground on the outside. Then we attach it to the center conductor in the middle.
We should not hear tone. If there is a tone it means we have a short in the cable.
This means that somewhere along the run this cable is either been crimped (for example, someone may have closed it in a door or it may be up in the ceiling and been crimped up there) but somehow, the center conductor is probably touching the outer conductor.
If we test from one end of this cable to the other end of this cable to make sure there’s connectivity. We can go back to the end of the cable and simply put a terminator on it.
Then test the other end of the cable and we want to hear a tone.
We will connect it to the outer shield and to the center. We should now hear a tone.
If we did not hear a tone that means that this cable is open. Somewhere in the middle, this cable has a break. It could be a break on the shielding, it could be a break with the center conductor, it could actually be cut. At that point now we’re troubleshooting, why we have an opening on the cable.
The last thing we can do with a multimeter is to attach a terminator to one end and check the ohms in this terminator, because remember, that when we are using a coaxial cable, we have to have a terminator on each end and the terminator has to match the cable.
Again, touch the outside of the terminator, go to the center conductor and measure the ohms in the terminator. Now, this is an actual 50‑ohm terminator. This will be bad because keep in mind, on an RG6 or an RG59 coaxial cable, we need to be running a 75‑ohm terminator, not a 50‑ohm terminator. This was actually for RG58 cable which is some of the older cables. That’s actually using a meter or a multimeter.
Tone Generator and Probe (Fox and Hound)
- Trace Cable End-to-End
The next device we might use is referred to as a tone generator. This tone generator commonly is referred to as a fox and hound. At one end of the cable we can attach what they refer to as the fox.
Then use the probe which is going to be the hound and find this cable. Let’s say, for instance, we have a cable run that has 50 cables in it. We don’t know which cable is the cable that the target but the tone generator can be used to find it.
Here is an example of a tone generator with a fox and hound.
We can connect it to one end of the cable.
Let’s assume that this cable isn’t tied together like it is in the image. We wouldn’t be able to see the other end because it could run through the wall. We now can turn on the tone generator and then use the probe or (the hound) in order to locate it.
What we will do it touch each of the cables in that cable run until you hear a buzz. The tone allows us to find that individual cable inside that big bundle of cables.
Personal story. I have a cable run that runs through my garage at home for our cable system. I didn’t run it. I took this Tone Generator and Probe home and connected it to my F connector in the garage and then went out to the cable box and used the probe and touched each cable until I found the cable that I had connected to and found that cable wasn’t connected at all. I could’ve done one cable at a time where to connect the TV until I did it that way. This allowed me to find that cable. Again, this is a handy environment especially if you are looking for a damaged cable from one end of the other.
TDR / OTDR
- Time Domain Reflectometer
- Optical Time Domain Reflectometer
- Distance of the Cable Run
The next tool is called a time‑domain reflectometer. There’s are two types. There’s a TDR for testing copper, twisted pair or coaxial, and then the OTDR is optical time‑domain reflectometer which is used for testing fiber.
A TDR or an OTDR, it’s used to determine the distance of a cable run.
If you are certifying cables, it allows us to determine the distance.
We attach a TDR to one end of the cable. Then we will tell the program what type of cable it is such as twisted pair, coaxial, or fiber. If it’s fiber, we will tell is it single‑mode fiber or is it multi‑mode fiber. Then we will plug that into the TDR until you hear a pulse. It’s either going to be a light pulse or a sound wave. It pings down the cable.
Now it goes down the cable and when it gets to the break…Think about the break, it’s kind of you’re at the Great Canyon you holler, “Hello.” You wait for a few seconds and you hear the hello back. When I get to the end of the cable, I’m going to get an echo. Remember that’s why we have terminators on our coaxial cable to prevent this echo or bounce back.
Now, when it gets back to the TDR, the TDR knows what type of cable you had so it does a computation on time. How long did it take that signal to travel down the cable and back? Then it can it compute the distance to where that break is.
This is used to find breaks in cables. It’s also used to be able to go in and certify our distance of our cables inside of our buildings.
We looked at some of those common tools that we use to test cables inside of our environment.
Until next time….
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