Information Technology Reference
In-Depth Information
balun transformer
to minimize unwanted reflection and power loss caused by the
impedance mismatch.
If a network with 100-ohm station cable connects to user equipment with 150-
ohm interfaces, a special cable with a built-in balun, called a
media adapter
, must
be used for proper operation. STP-A is typically rated at 300 MHz, in contrast to
the 100 MHz rating of normal Category 5e and lower cable.
If you are familiar with telephone circuits, you may know that such circuits are
generally characterized for operation at 600 or 900 ohms. This load impedance is
designed to minimize current flow to subscriber telephone instruments, in addition
to other factors. It is not representative of the characteristic impedance of the wire
pair, which is not as significant at voice frequencies.
Why is the characteristic impedance important to LAN wiring? The reason is
twofold. First, the maximum amount of power is transferred when the impedances
of the source and load are matched. A greater power transfer extends the usable
cable distance. Second, mismatches cause reflections of the signal, which can dete-
riorate the quality of the received LAN signal and cause unwanted emissions. At
high LAN data rates and long cable distances, this might become critically impor-
tant to performance.
As we will see later, a cable's characteristic impedance may vary over the fre-
quency range of the LAN signal. The standards specify a maximum variation that
must be met for the cable to meet minimum performance criteria.
The resistance and capacitance of a twisted-pair cable cause an attenuation of the
LAN signal that is proportional to length of cable. This signal attenuation is expressed
in decibels (dB), indicates a power ratio, 10-log (
P
in/
P
out), of the input versus output
power expressed as a logarithm. The typical attenuation of a cable is given in dB per
foot or meter so that you can calculate the total attenuation of a cable run by multi-
plying its per unit value by the total length. Fortunately, a standards-compliant instal-
lation will ensure that the total attenuation (including cable and connectors) is within
the maximum limits necessary for proper LAN operation. The attenuation of a typi-
cal cable run is enough below the maximum allowed that an attenuation measurement
usually reveals bad connections or partial cable breaks, rather than cable deficiencies.
The capacitance of twisted-pair wiring is the factor that causes attenuation to
increase steadily with frequency. The attenuation caused by capacitance is propor-
tional to 1/(2
f
). Thus, the greater the frequency, the less the parallel impedance
between the wires, and the more the signal attenuation. Although this attenuation
factor is linear, in practice, twisted-pair cables exhibit peaks and valleys that can
affect transmission performance at specific frequencies. For this reason, the new per-
formance measurement standards for installed cable require numerous discrete
measurements over the entire frequency range of a cable category.
