Information Technology Reference
In-Depth Information
Screw terminals are also technically rejected on grounds of pair untwist, which
must remain below 0.5 in for Categories 5e and 6. The station cable must often be
untwisted in excess of those limits to reach the screw terminals. Screw terminal con-
nectors also have an unacceptable length of untwisted wire between the screw ter-
minal and the actual connector body. These factors lead to the unacceptable level of
NEXT and impedance anomaly in screw-type jacks.
You should be aware that there are some insulation-displacement connectors
(IDCs) that use similar space-wiring between the IDC block and the modular con-
nector. Both 110- and 66-type miniblocks may be found in these types of outlet
plates. These connector assemblies do not meet the needs for Category 5e/6 and
should be avoided. This problem comes not from the connecting blocks, but from
the internal outlet wiring from the jack to the block. Do not confuse these jack
plates with the acceptable style that integrates the 110 block into the connector as
part of a printed wiring assembly.
Cable used in horizontal and backbone runs must also meet performance stan-
dards. Table 15.2 shows the attenuation and NEXT performance parameters for
Category 3, 5e, and 6 cable. Note that these levels are for the raw cable, not an
installed link (see “Testing Methods” later in this chapter).
Horizontal cable must meet essentially the same requirements as backbone
cable. The attenuation and NEXT limits are calculated from a formula in the actual
standard and the values in the table merely illustrate the calculation at specific dis-
crete frequencies. Cable attenuation is known to vary with ambient temperature. The
cable must also be tested (by the manufacturer) to meet adjusted loss limits at 40 and
60°C. The maximum allowable attenuation at these elevated temperatures for
Category 5e and 6 cables is adjusted by a factor of 0.4% increase per degree Celsius.
Cable must also meet other transmission-related specifications of DC resist-
ance and unbalance, mutual capacitance, capacitance unbalance to ground, charac-
teristic impedance, and structural return loss. The resistance and capacitance
components are primarily a function of the characteristics of the copper wire and
the insulation material. These values are consistent and are easily described by a sin-
gle set of values for each category, as shown in Table 15.3.
Characteristic impedance is specified as 100 ohms
15% (or 85 to 115 ohms).
However, it turns out that it is rather difficult to make a consistent measurement of
characteristic impedance of twisted-pair wire, because of variations in the cable's
structure over its measured length. Consequently, the characteristic impedance value
is derived by “smoothing” the results of measurement of a parameter called
struc-
tural return loss
(SRL). The SRL may vary rather wildly from 1 MHz to the highest
referenced frequency (16, 100, or 250 MHz for Category 3, 5e, or 6, respectively).
For this reason, an absolute minimum limit for SRL is imposed for each category.
