Troubleshooting Network Cabling – CompTIA Network+ N10-006 – 4.4

Once your cables are in place, it’s important to wire all connectors properly. In this video, you’ll learn how to recognize cabling issues, how to troubleshoot split pairs, and reversed transmit and receive.
<< Previous: Troubleshooting Signal LossNext: Troubleshooting Fiber Issues >>

If you’re having a problem with your copper cabling, then you might be suffering through a number of different kinds of symptoms. Maybe you’re getting no connection at all, you’re not able to see the network, and, obviously, not able to connect to anything over the network. Maybe you have a connection, but the data that you’re sending is very slow. You’re getting very bad throughput when you’re trying to transfer information through the network. Or maybe your connection is telling you that you’re connected, and then it’s telling you that you’re not connected, then it’s telling you you’re connected again. This is obviously indicative of some kind of cable problem.

If we’re not connected at all, then we need to look at what the cable situation is. Do we have a crimp in the cable? Is there a link light that we’re able to see? Is it punched down incorrectly? Do we have the wires in the wrong place inside of the connector? This is especially common if it’s a new installation. Maybe it would be better to swap out all of the cables that we have, it’s certainly a very quick check to see if the problem might resolve itself that way.

If we’re getting very slow throughput, that means that we’ve got a link light, we might even see some activity lights that are flashing, but we’re still not getting very good throughput. So we may want to check to see if the cable might be damaged. There might be a crimp, or a bend, in the cable itself. And if you are using fiber connectors, or using things like SFPs or GBICs, you may want to pull out those connectors and replace them with a brand new set.

If the connectivity is connected and then not connected, and you’re having this intermittent connectivity issue. Then, it’s probably best that you swap the cable completely and throw the old cable away. It’s a very quick fix, and it might be able to resolve these intermittent problems.

Sometimes the problem might be more difficult to identify. This may be the case with something like a split pair. It’s a wiring problem, and it’s a very common mistake, but one of the challenges is that if you were to do a simple wire map, it would look like it was perfectly normal. Pin 1 would connect to pin 1, pin 2 would connect to pin 2, pin 3 to pin 3, and so on.

But you’re going to have problems, because there will be a lot of Near-End Crosstalk, and this is why you will get bad throughput, even though a simple cable map is showing you that everything is wired properly. It’s all about the twist of the twisted pair. We want to make sure the pairs of the twisted pairs are all kept together. If we’re splitting off any of these pairs, we’re going to have performance problems.

This is visually what a split pair might look like. We can see that a white and green pin 1 is connected to white and green pin 1. A solid green pin 2 is connected to a solid green pin 2. A white and orange pin 3 is connected to white and orange pin 3, and so on. Where you run into a problem is down here on pin 4. Pin 4 should be a solid blue connecting to a solid blue, but instead, we have split the pairs between the brown and the blue, and we have a white and brown connected to pin 4, and a white and brown connected to pin 4. And then, a white and brown is now the solid blue. This is where we’ve done the split.

So everything matches up from pin to pin, but you can see that there are differences in splits with these particular pairs. That’s why we always want to keep our brown, our blue, our orange, and our green pairs together always. If we start splitting them, we’re going to have problems with the throughput on this connection.

If you have a cable and you have reversed the transmit and the receive for Ethernet, you’ve effectively created a crossover cable, but this may not work in all environments. It is a problem with the ends of the cable, or the punchdowns that we’re using with the cable. It’s very easy to find this with a wire map. You would troubleshoot by performing a simple wire map, and you’ll see that 1 is equal to 3, 2 is equal to 6, you don’t have a 1 to 1 mapping through this particular patch cable, and that makes it very easy to identify, and ultimately, very easy to fix.

This is, effectively, an Ethernet crossover. So you may find that a device that supports Auto-MDIX might automatically use the transmit/receive reversal with no problem whatsoever. If that’s the case, you can continue to use that cable with the knowledge that this is not a standard straight through patch cable. The reversal on this patch cable is often done at a punchdown block. Sometimes it’s done at a patch panel. It might also be done at the connector that we’ve crimped. So we need to check every single one of those to find out where we have reversed the transmits and the receives on this cable.