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| 3.5 |  | Isolating Physical Layer Problems |
| 3.5.3 |  | Bad cabling |

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.

Another source of RJ-45 related problems is how well the connector 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. It is unlikely that this parameter could fail without other parameters failing as well. Failing propagation delay suggests an inappropriate or bad cable in the link.

Check the overall length of the cable. Inspect the cable closely to see if the correct type of cable was installed. If this is the only parameter that failed, it will probably be necessary to replace the cable 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. Both of these numbers are quite generous. It is fairly difficult to fail delay skew if good materials were used in the link. 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.

Varying the lengths of pairs at any point along the link probably indicates bad workmanship, as individual pairs should never be used for networking applications. This situation should cause other parameters to fail too.

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.