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[NOC Tutor] Fiber Optic Media - Part Two

Once you’ve determined if you need single mode or multimode, loose tube or tight-buffered,
indoor or outdoor cable, fiber-optic cables still have a variety of options from which to choose.
When buying fiber-optic cables, you will have to decide which fiber ratings you want for each
type of cable you need. Some of these ratings include the following:
? Core/cladding sizes
? Number of optical fibers
? LAN/WAN application

Core/Cladding Size
The individual fiber-optic strands within a cable are most often designated by a ratio of core size/cladding size. This ratio is expressed in two numbers. The first is the diameter of the opticalfiber core, given in microns (?). The second number is the outer diameter of the cladding for that optical fiber, also given in microns. For example, a cable with a 10-micron core with a 50-micron cladding would be designated as 10/50.

Three major core/cladding sizes are in use today:
? 8/125
? 50/125
? 62.5/125

8/125
An 8/125 optical fiber is is almost always designated as single-mode fiber because the core size is only approximately 10 times larger than the wavelength of the light it’s carrying. Thus, the light doesn’t have much room to bounce around. Essentially, the light is traveling in a straight line through the fiber.As discussed earlier, 8/125 optical fibers are used for high-speed applications, like backbone fiber architectures such as FDDI, ATM, and Gigabit Ethernet.

50/125
In recent years, Corning, as well as other fiber manufacturers, have been promoting 50/125 multimode fibers instead of the 62.5/125 for use in structured wiring installations. It has advantages in bandwidth and distance over 62.5/125 with about the same expense for equipment and connectors. ANSI/TIA/EIA-568-B.3, the fiber-optic-specific segment of the Standard, recognizes 50/125 fiber as an alternate media to 62.5/125.

62.5/125
Until the introduction of 50/125, the most common multimode-fiber cable designations was 62.5/125 because it was specified in earlier versions of ANSI/TIA/EIA-568 as the multimode media of choice for fiber installations. It has widespread acceptance in the field.  The 62.5/125 optical fibers are used mainly in LAN/WAN applications as a kind of “generaluse” fiber (if there really is such a thing).

Number of Optical Fibers
Yet another difference between fiber-optic cables is the number of individual optical fibers
within them. The number depends on the intended use of the cable and can increase the cable’s
size, cost, and capacity.
Because the focus of this book is network cabling and the majority of fiber-optic cables you
will encounter for networking are tight buffered, we will limit our discussions here to tightbuffered
cables. These cables can be divided into three categories based on the number of optical
fibers:
? Simplex cables
? Duplex cables
? Multifiber cables
A simplex fiber-optic cable has only one tight-buffered optical fiber inside the cable jacket. An
example of a simplex cable was shown earlier in this chapter in Picture-1. Because simplex
cables only have one fiber inside them, usually a thick strength member and a thicker jacket
make the cable easier to handle.

Duplex cables, in contrast, have two tight-buffered optical fibers inside a single jacket (as
shown in Picture-2). The most popular use for duplex fiber-optic cables is as a fiber-optic
LAN backbone cable, because all LAN connections need a transmission fiber and a reception
fiber. Duplex cables have both inside a single cable, and running a single cable is of course easier
than running two.

Note: One type of fiber-optic cable is called a duplex cable but technically is not one. This cable
is known as zipcord. Zipcord is really two simplex cables bonded together into a single flat
optical-fiber cable. It’s called a duplex because there are two optical fibers, but it’s not
really duplex because the fibers aren’t covered by a common jacket. Zipcord is used primarily
as a duplex patch cable. It is used instead of true duplex cable because it is cheap
to make and to use. Picture-2 shows a zipcord fiber-optic cable.

Picture-2  Zipcord Duplex Cable and Round Duplex

Finally, multifiber cables contain more than two optical fibers in one jacket. Multifiber cables
have anywhere from three to several hundred optical fibers in them. More often than not, however,
the number of fibers in a multifiber cable will be a multiple of two because, as discussed
earlier, LAN applications need a send and a receive optical fiber for each connection.

LAN/WAN Application
Different fiber cable types are used for different applications within the LAN/WAN environment.
Table 10.1 shows the relationship between the fiber network type, the wavelength, and
fiber size for both single-mode and multimode fiber-optic cables.

Note: The philosophy of a generic cable installation that will function with virtually any application
led the industry Standard, ANSI/TIA/EIA-568-B, to cover all the applications by specifying
50/125 multimode or 62.5/125 multimode as a media of choice (in addition to singlemode).
The revised Standard, ANSI/TIA/EIA-568-B.3, continues to recognize single-mode
as well because it also effectively covers all the applications.