Troubleshooting Signal Loss – CompTIA Network+ N10-006 – 4.4

| May 8, 2015


For devices to communicate across the network, they have to hear each other. In this video, you’ll learn about signal loss, how to calculate DB loss, and best practices for cable placement and avoiding interference.
<< Previous: Troubleshooting Copper CablesNext: Troubleshooting Network Cabling >>


Whenever you’re working with cabling, whether it’s copper or fiber, you always have to be concerned about loss of signal. We generally will have a loss of signal as the distance gets farther for both our copper and our fiber connections. We often refer to this as attenuation. This is the loss that a signal has as it moves through one of these types of mediums.

For instance, you have electrical signals through a copper connection, you have light that travels through fiber, radio signals travel through air. And as they move through these mediums, the signal will continue to degrade as the distance gets further and further away. The amount of signal strength is measured in decibels.

This is 1/10 of a bell. This bell stands for Alexander Graham Bell. So you’ll notice every time we abbreviate decibels, we have the capital B to stand for Alexander Graham Bell’s name. This is a logarithmic scale, and we use simple addition and subtraction to calculate the amount the signal strength.

Because this is a logarithmic scale, the numbers may not be what you expect. For instance, a three decibels change is twice the amount of signal change on a particular medium. A 10 dB change is 10 times the change. A 20 dB change is 100 times of a signal change. And a 30 dB change is a 1000 difference in the amount of signal on that particular medium.

The calculation of decibels is pretty simple, it’s addition and subtraction. So we may want to go through a scenario where we are calculating single loss through one kilometer of fiber. What we know as the signal is going through this fiber, through a single kilometer, that we’re going to lose about 3 and 1/2 decibels for every kilometer. So for one kilometer, we’ll add 3.5 decibels of loss.

When we are connecting these to the patch panels on both sides, that particular connector is also losing a bit of signal just by having the connector on the fiber. And on each end, each patch panel loses about half a decibel of signal, which means for both ends we’re also losing an additional one decibels. That means if we add all of this up to get the signal from one end to the other end through an entire kilometer, we have a total loss of 4 and 1/2 decibels of signal to go through that fiber.

Some of the symptoms associated with single loss on a particular wire or fiber is that you’re getting no connectivity at all. You’re going over that one kilometer of fiber, you’re plugging in, and neither side can see each other. Or maybe the connectivity is intermittent. Maybe at sometimes the link is up and running, but at other times it drops completely. This might also cause poor performance.

You’re able to get the connection up across that entire kilometer of fiber, but the signal strength is just too weak to be able to send a lot of traffic without seeing a lot of errors on the fiber. That’s why one of the best things you can do is to get test equipment that’s designed for this environment. If you’re running on a copper network, you’d have a TDR. And for our fiber networks, we would get an OTDR.

When we’re thinking about the impact of signal loss, we have to consider where the cables happened to be. From our workstation, the cables usually come out of our computer and they go into the floor underneath all of the workstations to a central closet, or they’ll go into the ceiling to get to that central closet. You’ll also have riser cables. These are cables that go between floors of a building so that you can connect all of these different closets together.

In the data center of course, you’ve got a lot of cables. And the cable management there will be critical because you want to minimize the links of these cables between all of these different pieces of equipment. For all of these, we have to think about the distances, and the standards for the type of networks that we’re running. If we have a gigabit network, then we generally have 100 meter distances. And we want to be very sure that we’re not going to exceed the standards for these ethernet runs.

Copper-end fiber cables have very different environmental requirements. You can bundle copper together. Unfortunately here, you can see someone has used cable ties. Not really the best idea on your copper networks. But you certainly don’t want to have anything like this with your fiber. Fiber is glass and we have to be very particular about how we handle these fiber connections.

You might also want to consider installing the highest category cable that you can. This of course has budgetary issues, and of course concerns around the equipment you’re using for crimping and terminating all of these wires. But it may help you down the road as this wire is in place, and now 10 years later you need to upgrade the speed of your network.

You should also think about centralizing your wiring plan. On each floor, the wiring closet should be in the center of the floor. And in your data center, you want to have everything come back to a central point.

These cables and fibers are the foundation of your network. If these are not installed properly, then nothing is going to work properly on your network. So you want to be sure to use a structured cabling system and keep everything as neat as possible. If you’re having a problem on your network, you want to be sure this problem is never happening because of your physical infrastructure.

Both copper and fiber have very specific handling requirements. You want to be sure that you’re not twisting these cables. You don’t want to pull them or stretch them as you’re putting them into a ceiling or down under a floor. You want to be very careful of the bend radius.

Even with copper, you want to be sure you’re not exceeding the bend radius for that particular wire. Do not use staples on your ethernet or your fiber. Do not use cable ties, if at all possible, because those will tend to crimp if you pull them too tightly on those cables.

Another concern that we sometimes don’t even think about are the external pieces that might cause interference. Electromagnetic interference can be caused with power cords, fluorescent lights, electrical cables. There may be fire prevention components that are putting off electrical magnetic interference. This obviously is not as big of a problem when you’re using fiber.

But since so much of our networks are running over these copper connectors, we want to be sure to avoid any cases of EMI. And if you really wanted to be sure, you attest would test it. Run your wire, add your connectors to the end, and bring in your cable tester. And it will tell you if your cables are working to specification.

Tags: , , , , , , , , ,

Category: CompTIA Network+ N10-006

Comments are closed.

X