Fiber Optic Cabling and Connectors for Ethernet

Fiber cabling, also called optical cabling or fiber optics, provides another option for cabling Ethernet. The main differences, in terms of function, between optical cabling for Ethernet and electrical cabling are as follows:

■ Longer distances supported by optical cabling

■ Greater monetary cost

■ Less magnetic interference, making it slightly more secure

■ Only type of cabling supported by 10 Gig Ethernet

For instance, network engineers might choose to use optical interfaces and cabling for Ethernet when building a campus LAN when the buildings happen to be a few miles apart because optical cables allow longer cables to be used. If the company has the right of way, it can run optical cable between the sites and still build a campus LAN.

Also, if you want to use Ethernet between two buildings that are a few miles apart but you do not have the right of way, you might be able to lease what is called dark fiber from a service provider. Dark fiber is just optical cabling run by the service provider, which it can do because it owns the right to run the cable under the streets. The service provider typically runs a lot of different optical fibers and then just leases to you the number of optical fibers you need.

In other cases, you might choose to use optical cabling to help protect highly sensitive traffic. Because of the emissions coming from electrical cabling, with the right tools, you actually can tell what signals are being sent across a cable. So, when an Ethernet cable might become accessible to someone who wants to listen in, the use of optical cabling can thrwart that person's efforts, because unlike electrical cabling, optical cabling does not emit electromagnetic radiation.

The key component of optical cabling is the fiberglass center of the cable. The devices on the end of the cable, such as Ethernet switches, generate an optical light signal. The signal travels down the optical fiber in the center of the cable. No electricity is used across the cable—just light is used.

Optical cabling can be divided into two general categories:

SM fiber uses a very small-diameter optical fiber, with MM fiber using a larger size. SM cables require more precision in the manufacturing process and more precision by the hardware that generates the light that crosses the cable, so SM cables and cards tend to be more expensive. However, SM cables typically allow for much longer distances and data rate than does MM fiber. MM cable still allows longer distances than copper cabling.

Often Ethernet cards use light-emitting diodes (LEDs) to generate light for MM cable, and other more expensive interfaces use a laser to generate the light for SM cables. The LEDs actually generate more than one wavelength of light, which, in part, is where the name multimode comes from. The actual terms multimode and single-mode refer to the fact that LEDs generate multiple wavelengths of light, whereas lasers generate a single specific wavelength.

Figure 11-10 shows a side view of an optical cable, including a view of the optical fiber itself. Figure 11-10 Components of a Fiber-Optic Cable

Yes, the center of the cable is made from glass, so it is fragile—but it is sturdy enough to work with the cable without worrying every second that you will break it. First, it is fiberglass, which does not break as easily as the glass in the windows of your house. The plastic adds some strength to the fiberglass. Most important for strength, a Kevlar coating is applied— Kevlar is the stuff that most bullet-proof vests are made from today.

The right side of the figure shows an example of how the light signal actually bounces off the interior walls of the glass fiber. The fiber has an inner part, called the core, and an outer part, called the cladding. The cladding has a different refractive index than the core, meaning that when the light hits the outer wall of the core, which is also the inner wall of the cladding, the light is reflected back into the core. This might be a bit oversimplified, but it's like having a mirror on the inside wall of the cladding so that the light keeps getting reflected back into the core—the light eventaully makes it to the other end of the cable.

Optical Connectors for Ethernet

To transmit data over a fiber cable, you need a single strand of fiber. The term strand refers to the center of the cable, the glass part, as shown in Figure 11-10. To transmit data in both directions, you need a pair of strands—one for each direction of data transmission. However, because there are no emissions to speak of, there is no need to twist the strands together. So, to connect two devices using fiber cabling, you just need two strands, or fibers, and the correct connector on each end.

A variety of connectors can be used for terminating optical cable when used for Ethernet. One type, called an ST connector, terminates each fiber strand with a barrel connector, much like a BNC connector. You place the connector onto a cylindrical male connector on the Ethernet interface (typically a switch) and twist to make the connector secure. Figure 11-11 shows another type of connector, called an SC connector.

Outer Part Kevlar Plastic of Cable Shield Shield

Figure 11-10 shows a side view of an optical cable, including a view of the optical fiber itself. Figure 11-10 Components of a Fiber-Optic Cable

Outer Part Kevlar Plastic of Cable Shield Shield

Optical Fiber

Optical Fiber


Figure 11-11 Fiber-Optic SC Connector

The figure shows two cables, each of which has a single fiber strand inside, attaching into a single connctor. Upon close examination, you can see that each strand terminates into a piece of plastic, with a larger, rectangular piece of plastic holding the two together. The larger piece of plastic holds the two internal connectors the the same distance from each other as the two receptacles on a card on a switch so that you can plug the whole connector into an Ethernet port at the same time. So, you have either both fibers connected or neither.

A newer type of connector, called an MT-RJ connector, has become more popular in recent years. The MT-RJ connector uses the same plastic mold as an RJ-45 connector, which makes it easy to install. Two fibers connect into the single connector, similar in concept to an SC connector. Figure 11-12 shows an MT-RJ connector.

Figure 11-12 MT-RJ Connector

Optical cabling costs more than copper cabling, and the Ethernet cards that can use optical cabling also cost more. However, there are several advantages to using optical cabling for Ethernet. You can have Ethernet segments that stretch over 10 km in length and have Ethernet speeds up to 10 Gbps, with little conduit space consumed due to the small diameter of the cable. Also, fiber cabling is more secure than copper wiring, because of the absence of emissions.

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