Information Technology Reference
In-Depth Information
the stranded construction and “design differences.” This is one of the reasons that
the lengths of the patch and user cords are limited by the standards. The attenua-
tion limits vary for the three categories of performance and are prorated from the
values per 100 m (328 ft). The allowable attenuation values for patch cords are
shown in Table 8.1. If you purchase preassembled patch cords, you should be sure
that the cords are certified by the manufacturer to comply with TIA-568-C for the
appropriate category of performance. Certification testing by an outside agency,
such as UL, is a good indication of quality and assurance.
The testing of patch cords presents somewhat of a dilemma, both for the man-
ufacturer and for you. The standard contains detailed performance requirements
for the cable and the connectors that are used to make up patch cords, but there
are no specifications for assembled cords. In addition, certain tests, such as NEXT,
are known to be unreliable for links less than 15 m, because of a phenomenon
called
resonance
. Many testers will not test a cable that is less than 6 m, except for
wire map.
Because of these factors, the only assurance of patch cord performance you will
have (until the cord becomes part of a larger link) is the use of quality components
and cable, and good workmanship. Workmanship is important because the cable
pairs must be untwisted before the modular plug can be attached. If the amount of
untwist is not minimized, the plug will contribute even more degradation to the link
than is inherent in its design. By the way, the unavoidable amount of untwist in the
modular connector design is one of the reasons that these connectors are not looked
upon favorably for high-frequency performance. Annex B of TIA/EIA-568-B.2
shows a detailed procedure for assembling modular cable ends. Although this pro-
cedure is for the preparation of connectors that are to be used in the testing of con-
necting hardware, it represents an ideal method to maximize the performance of
patch cord plugs.
Screened Twisted Pair (ScTP) Patch Cords
Screened twisted pair (ScTP) patch cords are a new addition to the TIA-568 stan-
dard (as of the -B revision), and are described in the normative Annex E, which
means the annex sets mandatory performance and installation requirements when
ScTP is used in lieu of UTP. As frequencies rise on network cabling, the use of ScTP
is increasing to minimize interference emissions and to decrease susceptibility to out-
side interference sources. TIA-recognized ScTP consists of four twisted pairs with an
overall shield.
The cable impedance is specified at 100 ohms, and either AWG 24 or AWG 26
plastic-insulated, stranded conductors may be used. The standard allows a 50%
