Information Technology Reference
In-Depth Information
Bandwidth is also limited by chromatic dispersion of the transmitted signal, which
increases linearly with distance along the fiber. Short distance transmission, such as
in the horizontal link, greatly minimizes this effect.
When a fiber link is used for low-bandwidth LANs, such as Ethernet 10BaseF,
its bandwidth is not significant, relative to the allowed transmission distances.
However, beginning at 100 Mbps, the bandwidth of the fiber starts to have a limit
on transmission that is independent of the needs of the LAN technology. And at
Gigabit Ethernet speeds, the fiber bandwidth cuts the usable distance of some types
of multimode fiber to only a couple hundred meters.
So-called fiber bandwidth is actually a bandwidth-distance product that can be
roughly translated into usable distance, simply by dividing the specified number by
the distance of the link. Thus, the bandwidth distance product of 500 MHz-km
yields an estimated usable bandwidth of 500 MHz at 1 km, 1000 MHz at 500 m,
and 2000 MHz at 250 m.
Fiber-Optic Insulation/Jacketing
Insulation/jacketing material types are roughly the same for fiber-optic cable as for
metallic cable, but the rating codes are different. The ratings include plenum use,
general purpose, and conductive cable types. Conductive fiber-optic cables contain
noncurrent-carrying metallic strength or vapor barrier components. A listing of the
fiber-optic cable types is shown in Table 5.7. For more information, see Section 770
of the NEC.
From a flame-resistance standpoint, the requirements for the use of fiber-optic
cable are identical to conventional cable. Plenum-rated fiber cable must be used in
air plenums and riser-rated cable must be used in vertical riser shafts. In general, a
plenum-rated cable may be substituted for riser use or general use. Likewise, a riser-
rated cable may be used for general use. Ports and pathways for fiber optic cables
should meet the appropriate flammability and firestop requirements, as with metal-
lic cable.
Fiber-optic cables that are intended for outside use or for direct burial should
be appropriately rated by the manufacturer. Because fiber-optic cables do not carry
voltages, the cables technically do not need to meet the
NEC
requirements for direct
burial; however, the resistance to moisture, chemicals, and abrasion should be the
same.
Remember that a cable that runs any length inside a plenum must be appropri-
ately rated, even if the cable originates outside the plenum space. An exception to
this rule is an outside cable at an entrance facility. See the
NEC
for details. Fiber-
optic cable with UV-resistant jackets and armoring is also available.
