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| 3.4 |  | Identifying Physical Layer Problems |
| 3.4.2 |  | Cabling faults – CAT5 |

Many problems can be corrected by simply reseating cables that have become partially disconnected. When performing a physical inspection, look for damaged cables, improper cable types, and poorly crimped RJ-45s. Suspect drop cables should be subject to a simple cable test, and exchanged with a known-good cable. Do not assume that just because a cable is new, just out of the package, that it will work. Test it first. Cables made on site should also be tested before attempting to troubleshoot with them. Anyone can have a bad day and miswire the termination. Also test for simple cable faults such as shorts, opens, and split pairs.

Determine whether a station connected by UTP or fiber is successfully linking to the network at the MAC Layer. Use the command ipconfig /all for Windows NT/2000/XP-based systems and ifconfig –a for UNIX/Mac OS platforms. Test UTP for voltage levels, and the presence and polarity of link pulses. Many new switches and some NICs have auto-sensing ports that compensate for polarity faults on a copper cable. Moving a station from a newer switch which compensates for some types of cable fault to an older hub or switch may give a mistaken belief that the older device is at fault, when in fact the newer switch was compensating for a pre-existing cable fault.

Copper links between one hub and another, between two switches, or between two PCs require crossover connections. Stations are connected to Layers 1-2 devices with cables that are not crossed over, often called straight cables. Check for crossover cables or hub and switch ports that are configured as a crossover with a simple button press. Many new switches, as well as all 1000BASE-T links, are also able to compensate for crossover cables used instead of straight cables. Some simple hubs use a single connection internally to service a crossover port and an adjacent normal port, only one of them may be used for RJ-45 jacks. This configuration is usually indicated beside the jacks for the shared port.

Split pair cables will either operate poorly or not at all, depending on the Ethernet speed used, the length of the split segment, and how far it is located from either end. The further the split is located from a transmitter, the less effect it will have due to signal attenuation. If the split is short, like a patch cable, and it is located midway between the two ends of a long run, 10BASE-T may operate mostly error free. Even a short split cable anywhere along a 1000BASE-T link will likely disrupt traffic, and may even prevent the link from coming up.