Information Technology Reference
In-Depth Information
Table A.1 in Appendix A shows a comparison of wire gauges to physical size.
Smaller diameter conductors correspond to larger wire gauge numbers. Thus, AWG-
26 is smaller than AWG-19. Telephone outside plant cabling is typically 24- or 26-
gauge, although in rural areas, 19- or 22-gauge may be used. The larger diameter
wires have more physical strength, which is an advantage for wires that must be run
between distant supports. Larger wires also have a lower resistance per unit length,
which reduces resistance to direct current and lessens attenuation to alternating fre-
quencies, such as voice. (At LAN frequencies, the capacitance is the primary con-
tributor to attenuation.) The greater copper content of larger wire sizes also
increases the cost of the wire.
At higher frequencies, such as those of a high-speed LAN, signal current con-
centrates at the outer diameter of the conductor, a phenomenon known as skin
effect . In a stranded conductor, the outer skin is ill-defined, because of the many
conducting strands that make up the wire, and attenuation is increased. For that
reason, standards typically limit the length of stranded wire that is allowed in a
channel. Solid copper wire must be used for the horizontal cable portion of the
channel. Stranded wire may be used for user and patch cords, where a 20% increase
in attenuation is allowed.
The common wire gauge that is specified by standards for LAN wiring use is
24-gauge solid copper wire. This wire size provides a good compromise between
attenuation and cost. It is also an easy size to work with.
UTP/STP Electrical Characteristics
Twisted-pair wire, like other types of transmission lines, has a characteristic imped-
ance. This impedance is a result of the dielectric properties of the insulation and the
closeness of the conductors. The insulation's dielectric properties are a function of
the type of material used. A variety of characteristic impedance values are available.
The wire used for LAN twisted-pair wiring has a characteristic impedance of 100
ohms, 120 ohms, or 150 ohms. Most of the UTP cable you will deal with will be
rated at 100 ohms, as that is the standard for UTP wire in most parts of the world.
Shielded or screen twisted-pair wire (STP) generally has a characteristic imped-
ance of either 100 or 150 ohms. Newer STP constructions that are in wide use in
Europe are of the 100 ohm variety. TIA/EIA-568-C specifies this 100 ohm construc-
tion for STP applications.
The specific type of 150 ohm STP that was defined in TIA/EIA-568-A is called
STP-A to differentiate it from the other types of STP cable. This STP-A is often
found in Token-Ring installations that use the IBM cabling system, as covered in
Chapter 3. To interconnect between 100-ohm and 150-ohm wire, you must use a
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