# Calculating Signal Loss – CompTIA A+ 220-901 – 2.2

December 6, 2015

Your network signal will degrade over a long distances, when passing through patch panels, and at the connectors. In this video, you’ll learn how to calculate signal loss and recognize symptoms of signal loss on your network.

If our network doesn’t have a signal, then we’re not going to have any data go over that connection. So it’s important to understand how signal loss might affect the communication between one end of the network and the other. The signal loss is usually a gradual loss, as you’re going a longer distance, the signal begins to degrade. We call this degrading of the signal attenuation. That means that as we’re moving the signal through a medium, whether it’s a copper connection, a fiber connection, or even through the air, we’re going to have attenuation of the signal as it goes farther and farther through that medium.

Of course, it’s all of these different network types that are going to be using some type of signal. We have electrical signals through copper. We have light that goes through fiber. And we have the radio waves that are going through our wireless networks. We tend to measure signal strength in relative terms using decibels. This is a ratio of how much signal strength we have on a particular medium. And the decibel stands for 1/10 of a bell, a decibel. The B is a capital B. And it’s capitalized because it stands for Alexander Graham Bell.

When you start looking at these decibel measurements, you’ll notice that it’s not a linear scale. It’s a logarithmic scale. That means that you might look at 3 db change and that is a change of 2 times either the signal strength or signal loss. If you look at 10 db, it means there is it 10 times difference in the signal strength. If you see a measurement of 20 dB, that means there has been a 100 times difference in signal strength. And 30 dB means there is a 1000 times difference in signal strength.

You can see here that the scale is definitely logarithmic. It is not a linear change. These will change very dramatically as the decibels get larger and larger. Fortunately, the calculation of these decibels is very easy. It’s addition and it’s subtraction. Let’s take an example of how we might calculate how much signal loss might go through a fiber connection. Let’s say we have two buildings, we’re connecting them together through one kilometer of fiber. And if we talk to the fiber manufacturer, they will tell us that we can expect to get about 3 and 1/2 decibels of loss over that entire one kilometer.

We can even measure it if we’d like on both sides of the fiber to see exactly what the type of loss might be. And we’ll probably see somewhere in the 3 and 1/2 decibel loss. Now of course, every time we go through a connection, we’re going to lose some signal. So there will certainly be patch panels at the end of these two fiber connections. Each patch panel is going to be half a dB of loss, which means for both sides of this if we add up both of those patch panels, it’s a total of a 1 db loss. That means if we add up all of this, which means we’re looking at one patch panel, the entire one kilometer of fiber, and the patch panel on the other side, we can expect to lose 4 and 1/2 decibels of signal.

You’ll see these decibels reference also for copper connections. Here’s a good example. This is a copper splitter you would use on a coax connection. And you can see it’s a two way splitter, which means it’s going to cut the signal right in half. In fact, this particular splitter tells you that this particular interface is going to lose about 3 and 1/2 decibels of signal. And this one is going to use 3 and 1/2 decibels of signal. Of course, we know if we split a signal in half, we’re going to lose 3 db. And we can see this is 3 and 1/2, probably because we’re taking into account that there’s an additional connector on the splitter where we might be losing a little bit more signal.

So it’s really a total of 3 and 1/2 db signal loss on each side of this. If we’re not able to get enough signal across the network, then the other side is not going to see anything. We’re not going to have any connectivity. And there certainly won’t be any data communicated back and forth. You might have just enough signal to synchronize the connection between the two. But because it’s such a low amount of signal, you may have intermittent connectivity.

If you do have enough signal to at least get the link up and send some data, you still might see errors on that connection. So look at the interface of your network equipment, you may CRC errors or the data itself has to be re-transmitted because it’s corrupted when it comes to the other side. Ultimately, you may want to use some test equipment that can really examine what the signal loss is from one side of the network to the other. That way you can make a decision on whether this is a link you want to use for your network connection.

Category: CompTIA A+ 220-901