Before troubleshooting a failing cable, verify the tester configuration.
This step is critical to obtaining accurate test results, as testers capable of
Category 5e and higher performance utilize a wide selection of cable interface
adapters and may have somewhat complicated test configurations. At a minimum,
verify that the correct test specification and link type has been selected.
Also, the test standards have evolved sufficiently that the requirements for a
particular test, such as ISO Class D, may no longer be the same as what is
loaded in the software of the tester. Check the website of the manufacturer of
the tester for new tester software two or three times per year.
Most
wiremap failures occur at cable terminations, either at the RJ-45 (plug or
jack), or at an intermediate crossconnect or patch panel. Faults at the RJ-45
can usually be seen by checking the wire colors carefully against T568A or
T568B pinout colors, or by checking the RJ-45 plug for wires that did not seat
fully to the end of the connector when it was crimped.
Propagation
Delay and Delay Skew
TIA/EIA-568-B permits up to 498 ns of propagation
delay for the Permanent Link and up to 555 ns of propagation delay for the
Channel Link, for all Categories. Failing propagation delay suggests an
inappropriate or bad cable in the link.
Delay Skew
Delay skew
occurs as a result of different wire pairs within a cable being insulated with
different materials. This could occur if there is an industry supply problem
for a favored insulating material. In this case the critical RX and TX pairs
may be coated with the favored insulative materials and the pairs not utilized
in TIA/EIA-568-A or TIA/EIA-568-B standards are coated using sub-optimal
insulative materials.
TIA/EIA-568-B permits up to 44 ns of delay skew for the Permanent Link and
up to 50 ns of delay skew for the Channel Link, for all Categories. A delay
skew failure is possible if wire pairs in a single cable have different
insulative material on some pairs. A failure is also possible if various
lengths of twisted wire pairs were used as a patch cable or jumper at a
connection point.
Inspect the connection points in the link, and if the
workmanship appears reasonable, there may be little choice but to replace the
entire cable run. Test a sample of the new cable before installing it to be
sure that the materials are not causing the problem.
