Information Technology Reference
In-Depth Information
568-C.1 (Part 1) describes the essential wiring structures and link testing methods,
while Parts 2 and 3 cover cable and connecting hardware standards for 100-ohm
unshielded twisted-pair (UTP) and fiber-optic cabling components, respectively.
Pluggable connector components present some unusual challenges in reliability
testing, as they must be checked for connectivity degradation. Random samples of
each component are tested for degradation of contact resistance and insulation
resistance in tests for durability, vibration, stress relaxation, thermal shock, and
humidity/temperature cycle. This testing is to be done by the manufacturer to cer-
tify its parts to a recognized performance category. A cautious manufacturer might
choose to have the certification verified by an independent testing laboratory,
because a lot rides on the higher categories of operation.
Surprisingly, with regard to connector plugs and jacks, the TIA-568-C stan-
dard calls for only 200 insertion/withdrawal cycles without failure (usually defined
by excessive contact resistance). Some manufacturers may specify a usable life of
500 to 2000 cycles for their connectors, although the standard only calls for 200
cycles. The point is made that these components have a finite life. Modular connec-
tors that are used in patch panels with frequent use may be expected to exceed the
standard's parameter over normal equipment life. Moreover, connectors that are
used in test equipment, such as for cable certification, can exceed that number of
cycles in a single afternoon! The user is only cautioned by the standards to “inspect
the connectors for signs of wear.” You would be much safer either to use connec-
tors designed for high usage or to periodically replace the connectors on high-use
equipment. You would also be wise to keep a usage log of your test cables and
replace them regularly, as the modular plugs used on these cables have the worst
degradation from multiple insertions.
Connectors must also be tested for transmission performance (attenuation and
NEXT). Barring the degradation from excessive use, modular connectors have an
innate contact resistance that is not significant when compared to the bulk resist-
ance of a length of cable. The wire pins that form the connector are typically gold-
plated to minimize this contact resistance. However, the modular-type connectors
used in the current wiring standards do exhibit a significant amount of cross-cou-
pling that appears as near-end crosstalk (NEXT). All of the 8-pin modular-style con-
nectors are bad in this respect, but testing is done to make sure the NEXT is not
worse than allowed.
Table 15.1 shows the attenuation and NEXT performance parameter limits for
Category 3, 5e, and 6 components. Category 5 is no longer sanctioned, but it had
about 3 dB worse performance at 100 MHz. You can see from the values that little
contribution to circuit loss is expected from the circuit hardware, as most comes from
contact resistance. For comparison, the DC insertion resistance is only 0.3 ohms for
