Fiber Cables – CompTIA Network+ N10-006 – 1.5

| March 31, 2015


Our long-haul networks rely on fiber optic technologies to get our signals across the wide open spaces. In this video, you’ll learn about the structure of fiber optics, the differences between multimode and single-mode fiber, and a comparison of UPCs and APCs.

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When we’re communicating over fiber, we’re obviously using light instead of electrical signals to get information from one side of the network to another. This is something you can visibly see. There is light going through that connection.

That makes it very difficult, of course, for somebody to tap or get into that connection. That’s why on very private networks, especially in government networks that have a lot of security, they’ll use fiber instead of copper because it’s so difficult to get into that connection to monitor or tap it without somebody noticing that you’re there.

The signal also is very slow to degrade. We’re talking about like going from one side or the other. We don’t have to worry about interference, and we can go very long distances over fiber. It’s also immune to any type of radio frequency interference. So we can go places that copper cables could never go by simply running a fiber through that particular area.

We sometimes forget just how small these fiber optic strands are. This is a picture of a connector, and you can see these very thin strands coming out the back. They don’t have the larger coating around them, so you can get an idea of the size. And generally we see them with this protective coating so that we can extend them over long distances without worrying that we’re going to damage the fiber optics inside.

Here’s a cutaway picture of just the fiber piece, not including the connector on the end. You’ve got the fiber optic in the middle and the light going back and forth over that connection. There is a cladding that goes around that core. And you’ll generally see another coating around that so that you’re able to protect it no matter where you might be moving that fiber.

If you do have a fiber optic connector lying around, you might want to look at the end of it. Make sure it’s not plugged in to anything at the time. We don’t want any laser or very bright LED lights getting into your eyes.

But if you look very closely at the end of it, there’s a tiny little dot right in the middle. That’s the fiber core. This is a ferrule that’s around it. It’s a piece of ceramic that’s very hard that’s designed to protect the fiber optics as you’re inserting it and removing it from your devices.

We generally have two different kinds of fiber optics that we might use on our networks. The first is multimode fiber. This is fiber that’s designed for short-range communication. You might use it inside of a data center. You might extend it to a distance not exceeding about two kilometers. And it’s really something that you very commonly use because this is what you’re connecting your servers and your switches up when they’re all in the same room. This also uses a relatively inexpensive light source like an LED to be able to put that light into the fiber. That’s because you’re not really going a very long distance. You don’t need a high powered laser if all you’re doing is transmitting to information that might be in the same room.

The multimode fiber is notable because the light going through the fiber is actually bouncing around and off the cladding as it’s going through the fiber itself. There are many modes that it is taking to get from one side or the other, thus the term multimode fiber. And on the other end, the light may be coming off at a different angle when it finally gets to the other side. As we’ll see in a moment, this is very different than if we’re using something like single-mode fiber. The multimode fiber is designed to work this way. And the devices that are on the end of this fiber are designed to receive the signal coming across that multimode fiber, regardless of the angle that’s coming in or going out.

If you need to go very long distances, you’re probably going to use a single-mode fiber. This is designed for distances up to 100 kilometers in some instances. And you can go a very long way without having to regenerate that signal and then continue the path on its way. As you might expect, this is a little bit more of an expensive endeavor than using multimode fiber. Not only is the fiber itself more expensive, but the devices on the ends are more expensive because they need to send a much stronger light through that fiber. So you’re usually going to use lasers to send that. So obviously the cost is going to be a bit more expensive than using LEDs.

As you recall with multimode fiber, there were multiple paths the light took to get to the other end. It would bounce off the cladding and you would have multiple modes as it was going through that connection. With single-mode fiber, it’s a single path. It’s a single mode to get from one side to the other. You don’t have a lot of bouncing off the cladding to get there. It’s a single path, and that’s one of the reasons we’re able to get such long distances by using these single-mode fibers.

If you’re using fiber optics, one of the things that you will always be concerned about is the amount of light that you are losing through this connection. Obviously there are many places where you might lose light. Every time you connect a fiber together, that connection loses a little bit of that light. As you are coupling fibers together, every couple, every connection, every tap you make is going to take a little bit more light away from that connection. So we need to think about how we’re managing our light. We have a certain budget of light and we need to be able to make sure that the light goes all the way through to the other side and that it is received properly by the device on that other connection.

One of the things we’re concerned about is something called return loss. We want as much light to go all the way down that fiber, but on some connections we will have some of that light reflected back. And if we can minimize the amount of reflection, we can obviously allow more of that light to go through to the end of the fiber.

To help manage this reflection, we have two different kinds of connectors we might use on a fiber optic. One is a UPC and the other one is an APC. UPC stands for Ultra-Polished Connectors. That means that the ferrule that’s on the end of the fiber is polished at a very flat zero degree angle to the other connector. This also is a relatively high return loss, so we may not want to use this particular connector if we’re trying to extend that light and get as far as possible down the fiber.

The other type of connector is an APC. APC stands for Angle-Polished Connectors. In this particular case, we don’t have two ferrules connecting to each other flat. They’re at a bit of an angle. And although this does minimize the amount of return loss or reflection back to us, it does create a little bit more of an insertion loss on that connection itself. It requires a little bit more precise measurement to get the exact angle on both sides.

Let’s look at the difference between a UPC and an APC connection. Here’s a breakaway view of a UPC connector. You have the two connectors on both sides. Here is the ferrule that’s in the middle. There’s a zero degree angle.

And of course, right in the middle is the fiber optic itself. And you can see with this zero degree angle you get a direct reflection back. We’re sending the light from the left side to the right side and as it hits that connector, you have the zero degree bounce that’s bouncing right back to us and affecting how much loss we’re going to have over that connection.

The APC looks very similar to the UPC. We’ve still got the two connectors on the left and the right. And we’re still sending the signal through the link. But notice in the middle instead of having the zero degrees, we’ve now got an eight degree angle.

As the light’s going through, we will still have reflection because you are hitting one of these connections. But instead of being reflected directly back to us, it’s being sent off at an angle. Because of this, you’ll find the APC is going to minimize the amount of reflection and you’ll have this smaller amount of loss because of that signal being sent off instead of being sent directly back to your equipment.

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Category: CompTIA Network+ N10-006

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