Information Technology Reference
In-Depth Information
The problem is that the increasing use of this popular plenum-rated material
often outpaces the ability of the material manufacturers to produce sufficient quan-
tities for the cable industry. Additionally, the rapid growth of LANs and the ten-
dency to overwire new installations for future requirements has thus far made
increases in manufacturing capacity only a short-term cure. Of course, some dou-
ble-ordering from cable and material users has occurred, which has caused the
appearance of shortages to be even more severe.
For the cable user, the FEP crisis may mean that supplies of cable are periodi-
cally lower than normal, or that the favored manufacturer, style, or even jacket
color is not available on short notice. Deliveries may be extended, selection limited,
and prices higher.
Several cable manufacturers have substituted other types of insulation on one
or more of the four pairs in popular LAN cables to stretch the supply.
Unfortunately, they are very restricted in the types of material that can be used for
the very high-speed data rates of modern LAN systems. Common substitutions are
polyethylene, polyolefin, and some special polyvinyl chloride compounds. Some of
these materials, in pure form, would not be able to pass the UL flame-spread tests,
but are acceptable when used in conjunction with three normal FEP pairs and a
plenum-rated jacket.
Costs of plenum-rated cables of all types have increased because of these mate-
rial shortages. Some question remains whether the substitution of alternate materi-
als may affect the long-term stability of the cable. However, there is only limited
evidence of problems at this point, as the alternative polyethylene and polyolefin
materials are normally quite suitable for high frequency use in nonplenum applica-
tions. The current controversy centers around dielectric property differences that
cause a delay skew failure in a mix such as 2-2 or 3-1, and the tendency of non-FEP
plenum cables to flame up dramatically in simulations, even though they are able to
pass the smoke-tunnel tests.
Experimentation is ongoing to find other materials, such as from the chloro-
fluorocarbon family, that can meet plenum requirements and have adequate dielec-
tric characteristics for Category 5e and 6 LAN use. You will see some of these
materials on the market today.
At the current rate of growth of LAN networks, we will continue to experience
periodic material shortages for years to come.
UTP/STP Color Coding and Marking
Each insulated wire of a twisted-pair LAN cable is colored differently. The colors
form a standard code, so that each wire may be easily found and terminated prop-
