Additional Designations of Fiber-Optic Cables

Once you've determined whether you need single-mode or multimode fiber strands, loose tube or tight-buffered cable types, and indoor or outdoor cable capability, you still have a variety of fiber-optic cable 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 are:

• 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 optical-fiber core, given in microns (μm). The second number is the outer diameter of the cladding for that optical fiber, also given in microns.

Three major core/cladding sizes are in use today:

• 8.3/125
• 50/125
• 62.5/125

We'll examine what each one looks like as well as its major use(s).

Note

Sometimes, you will see a third number in the ratio (e.g., 8.3/125/250). The third number is the outside diameter of the protective coating around the individual optical fibers.

8.3/125

An 8.3/125 optical fiber is shown in Figure 1. It is almost always designated as single-mode fiber because the core size is only about 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.

Figure 1: An 8.3/125 optical fiber

As discussed earlier, 8.3/125 optical fibers are used for high-speed long-distance applications, like backbone fiber architectures for metro, fiber-to-the-home, transcontinental, and transoceanic applications. Single-mode fibers are standardized in ITU, IEC, and TIA.

50/125

In recent years, several fiber manufacturers have been promoting 50/125 multimode fibers instead of the 62.5/125 for use in structured wiring installations. This type of fiber has advantages in bandwidth and distance over 62.5/125 fiber, with about the same expense for equipment and connectors. ANSI/TIA-568-C.3, the fiber-optic-specific segment of the standard, recommends the use of 850nm laser-optimized 50/125 fiber instead of the alternate 62.5/125 type.

62.5/125

Until the introduction of 50/125, the most common multimode-fiber cable designation 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 had widespread acceptance in the field. A standard multimode fiber with a 62.5 micron core with 125 micron cladding is shown in Figure 2.

Figure 2: A sample 62.5/125 optical fiber

The 62.5/125 optical fibers are used mainly in LED-based, lower-transmission-rate, FDDI LAN/WAN applications. However, as speeds have migrated to Gigabit Ethernet and above, 850nm laser-optimized 50/125 micron multimode fibers (commonly referred to as OM3 per the ISO 11801 Ed 2 cabling standard) have become more common.

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 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 tight-buffered 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. Because simplex cables only have one fiber inside them, only aramid yarn is used for strength and flexibility; the aramid yarns along with the protective jacket allow the simplex cable to be connectorized and crimped directly to a mechanical connector. Simplex fiber-optic cables are typically categorized as interconnect cables and are used to make interconnections in front of the patch panel (also known as "in front of shelf" connections).

Duplex cables, in contrast, have two tight-buffered optical fibers inside a single jacket (as shown in Figure 3). 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.

Figure 3: A sample duplex fiber-optic cable

One type of fiber-optic cable is called a duplex cable but technically is not one. This cable is known as zip cord. Zip cord 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. Zip cord is used primarily as a duplex patch cable. It is used instead of true duplex cable because it is cheaper to make and to use. Most importantly, however, it allows each simplex cable to be connectorized and crimped directly to a mechanical connector for both strength and durability. Figure 4 shows a zip-cord fiber-optic cable.

Figure 4: A zip-cord cable

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. Six, twelve, and twenty-four fiber cables are the most commonly used for backbone applications. These cables are typically used for making connections behind the patch-panel (also known as "behind the shelf" connections).

LAN/WAN Application

Different fiber cable types are used for different applications within the LAN/WAN environment. Table 1 shows the relationship between the fiber network type, the wavelength, and fiber size for both single-mode and multimode fiber-optic cables. Table 2 shows the recognized fiber and cable types in ANSI/TIA-568-C.3.

Table 1: Network-Type Fiber Applications 

Network Type	Single-Mode Wavelength/Size	Multimode Wavelength/Size
10 Gigabit Ethernet	1300nm–8.3/125 micron	850nm–50/125 micron-OM3 (preferred)
	1550nm–8.3/125 micron	1300nm–62.5/125 or 50/125 micron to 220m (using –LRM)
Gigabit Ethernet	1300nm–8/125 micron 1550nm–8.3/125 micron	850nm–62.5/125 or 50/125 micron 1300nm–62.5/125 or 50/125 micron
Fast Ethernet	1300nm–8.3/125 micron	1300nm–62.5/125 or 50/125 micron
Ethernet	1300nm–8.3/125 micron	850nm–62.5/125 or 50/125 micron
10Gbase	1300nm–8.3/125 micron 1550nm–8.3/125 micron	850nm–62.5/125 or 50/125 micron 1300nm–62.5/125 or 50/125 micron
Token Ring	Proprietary–8.3/125 micron	Proprietary–62.5/125 or 50/125 micron
ATM 155Mbps	1300nm–8.3/125 micron	1300nm–62.5/125 or 50/125 micron
FDDI	1300nm–8.3/125 micron	1300nm–62.5/125 or 50/125 micron

Table 2: ANSI/EIA-568-C.3 Recognized Fiber and Cable Types 

Optical Fiber and Relevant Standard	Wavelengths (nm)	Maximum Cable Attenuation (dB/km)	Minimum Overfilled Modal Bandwidth (MHz · km)	Minimum Effective Modal Bandwidth (MHz · km)
62.5/125μ micron Multimode TIA 492AAAA (OM1)	850 1300	3.5 1.5	200 500	Not specified Not specified
50/125μ micron Multimode TIA 492AAAB (OM2)	850 1300	3.5 1.5	500 500	Not specified Not specified
850nm Laser-Optimized 50/125μ micron multimode TIA 492AAAC (OM3)	850 1300	3.5 1.5	1500 500	2000 Not specified
Single-mode Indoor-Outdoor TIA 492CAAA (OS1)TIA 492CAAB (OS2)3	1310 1550	0.5 0.5	N/A N/A	N/A N/A
Single-mode Inside plant TIA 492CAAA (OS1) TIA 492CAAB (OS2)3	1310 1550	1.0 1.0	N/A N/A	N/A N/A
Single-mode Outside plant TIA 492CAAA (OS1) TIA 492CAAB (OS2)3	1310 1550	0.5 0.5	N/A N/A	N/A N/A

Note

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