Most common network problems can be attributed to cable problems involving media, connectors, and patch panels. These are Layer 1 issues, which cannot be overlooked. For example, multimode and single-mode fiber cables are often used for ATM, FDDI, Fast Ethernet, and Gigabit Ethernet . When troubleshooting problems with fiber-optic cables, an important consideration is asymmetric connectivity problems. An asymmetric connectivity problem occurs when one side of a transmit/receive cable pair fails, but the remaining cable continues to forward frames. Asymmetric connectivity can impair spanning-tree loop avoidance. Many things can also go wrong with copper UTP cables . For example, cable that is exposed to high-traffic areas can be smashed, bent, or pulled out of the jack causing connectivity problems.

When troubleshooting cabling from a device or between devices, ask the following questions:

• Are the cables the correct type for this installation? Category 3 is for 10BaseT only. Was a Category 3 cable installed instead of a Category 5 cable?
• Is the Category 5 cable installed correctly?
• Is the cable a crossover or a straight through? Which type should it be? Compare the RJ-45 connector wiring at both ends of the cable if unsure.
• Is there a broken wire at either end of the cable? Cables that are installed too tightly or bundled together tightly with a tie wrap may have broken wires in the connector. Cables that are pulled through a plenum can have broken wires and exhibit intermittent open-circuit conditions.
• Is the cable longer than the 100 m (328 ft) specification? A time domain reflectometer (TDR) can display the length of the cable, including all wiring closet connections.
• Is the punchdown wiring correct? Are there any missing, loose, or broken wires on the punchdown block ?
• Is the network adapter card/interface port at the user end functioning properly?
• Is the device connected to the correct port? Is the port active?
• Is a transceiver used to convert media? Is it functioning properly?

One method that can be used to test installed cabling is to replace the entire cable run with an external cable. If there is a segment of tested Category 5 cable, run the cable between the two devices to test connectivity. This test will eliminate any uncertainties about plant cables or punchdown connections. This can also be verified with a cable tester.

Hubs are still used in many LAN environments. Ensure that they are operating properly by checking the link/status light for the port as well as the unit status LEDs.