Fiber-Optic Cable

Fiber-Optic Cable
Because fiber-optic cable transmits digital signals using light impulses rather than electricity, it is immune to Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI).

FIGURE 1 . 1 7 A USB plug

Note:

You will find a complete discussion of these terms in Chapter 6, but you should
know at this point that both could affect network performance.
Anyone who has seen UTP cable for a network run down an elevator shaft would, without
doubt, appreciate this feature of fiber. Light is carried on either a glass or a plastic core. Glass
can carry the signal a greater distance, but plastic costs less. Regardless of which core is used,
the core is surrounded by a glass or plastic cladding, which is more glass or plastic with a different index of refraction that refracts the light back into the core. Around this is a layer of flexible plastic buffer. This can be then wrapped in an armor coating (where necessary), typically Kevlar, and then sheathed in PVC or plenum.

Note: 

For more information about fiber-optic cabling, see Cabling: The Complete
Guide to Network Wiring, Third Edition, by David Barnett, David Groth, and Jim
McBee (Sybex, 2004).

The cable itself comes in two different styles: single-mode fiber (SMF) and multimode fiber (MMF). The difference between single-mode fibers and multimode fibers is in the number of light rays (and thus the number of signals) they can carry. Generally speaking, multimode fiber is used for shorter-distance applications and single-mode fiber for longer distances. If you happen to come across a strand of fiber in the field and want to know if it’s single mode
or multimode, here are some general guidelines. First of all, if it’s got a yellow jacket, it’s probably single mode. If it’s got an orange jacket, it’s most likely multimode. Also, check the writing
on the cable itself. You’ll find a number like 62.5/125. These are the outside diameters of the core and the cladding (respectively). If the first number is a 8, 9, or 10, it is most likely a single mode. On the other hand, if the numbers read as before (62.5/125), it’s most likely a multimode strand of fiber. Use these two tips to help you identify that errant strand of fiber. Although fiber-optic cable may sound like the solution to many problems, it has pros and
cons just as the other cable types. Here are the pros:

• Is completely immune to EMI or RFI
• Can transmit up to 40 kilometers (about 25 miles)

Here are the cons of fiber-optic cable:

• Is difficult to install
• Requires a bigger investment in installation and materials

Fiber-Optic Connectors

Fiber-optic cables can use a myriad different connectors, but the two most popular and recognizable are the straight tip (ST) and subscriber (or square) connector (SC) connectors. The ST

fiber-optic connector, developed by AT&T, was one of the most widely used fiber-optic connectors. It uses a BNC attachment mechanism similar to the Thinnet connection mechanism,

which makes connections and disconnections relatively easy. Its ease of use is one of the attributes that makes this connector so popular. Figure 1.18 shows an example of an ST connector.

Notice the BNC attachment mechanism. The SC connector (sometimes known also as a square connector) is another type of fiber-optic connector. As you can see in Figure 1.19, SC connectors are latched connectors. This latching mechanism holds the connector in securely while in use and prevents it from just falling out.

Note:

If data runs are measured in kilometers, fiber optic is your cable of choice
because copper cannot reach more than 500 meters (about 1500 feet) without
electronics regenerating the signal, and that’s for the all-but-obsolete 10Base5
coaxial standard. The standards limit UTP to a mere 100 meters. You may also
want to opt for fiber-optic cable if an installation requires high security, because
it does not create a readable magnetic field. Although fiber-optic technology
was initially very expensive and difficult to work with, it is now being used in
some interesting places, such as Gigabit or 10GB Internet backbones. Ethernet
running at 10Mbps over fiber-optic cable to the desktop is designated 10Base-
FL; the 100Mbps version of this implementation is 100Base-FX. The L in the
10Mbps version stands for link, as opposed to such other designations as B for
backbone and P for passive.

SC connectors work with either single-mode or multimode optical fibers, and they will last for around 1000 matings. They are seeing increased use but aren’t as popular as ST connectors for LAN connections.

FIGURE 1 . 1 8 An example of an ST connector

FIGURE 1 . 1 9 A sample SC connector

Small Form Factor Fiber-Optic Connectors
One of the more popular styles of fiber-optic connectors is the small form factor (SFF) style of connector. SFF connectors allow more fiber-optic terminations in the same amount of space over their
standard-sized counterparts. The two most popular are the mechanical transfer registered jack (MT-RJ or MTRJ), designed by AMP, and the Local Connector (LC), designed by Lucent.
MT-RJ The MT-RJ fiber-optic connector was the first small form factor fiber-optic connector to see widespread use. It is one-third the size of the SC and ST connectors it most often replaces. It had
the following benefits:

•  Small size
• TX and RX strands in one connector
• Keyed for single polarity
• Pre-terminated ends that require no polishing or epoxy
• Easy to use

Figure 1.20 shows an example of an MT-RJ fiber-optic connector

FIGURE 1 . 2 0 A sample MT-RJ fiber-optic connector

LC
Local Connector is a newer style of SFF fiber-optic connector that is overtaking MT-RJ as a fiber-optic connector. It is especially popular for use with Fibre Channel adapters and Gigabit
Ethernet adapters. It has similar advantages to MT-RJ and other SFF-type connectors but is easier to terminate. It uses a ceramic insert as standard-sized fiber-optic connectors do. Figure 1.21
shows an example of the LC connector.

FIGURE 1 . 2 1 A sample LC fiber-optic connector