Identifying Physical Layer Problems
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 cables should be tested or exchanged with a known-good cable. 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.

Check for crossover cables or hub and switch ports that are configured as a crossover with a simple button press. Many new switches and all 1000BASE-T links are able to compensate for crossover cables used instead of straight cables.

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.