Troubleshooting Cables and Connectors

Troubleshooting Cables and Connectors

Most network problems occur at the OSI Physical layer, and cabling is one of the most common causes. A cable might have a short or a break, or it might be attached to a faulty connector. Tools such as DVMs and TDRs help search out cabling problems.

If a workstation cannot access the network, and you think the problem might be the cabling, try disconnecting the network cables and attaching them to a portable PC. If the portable reaches the network, cabling probably isn’t your problem.

When troubleshooting any network, begin with the more obvious physical problems. For example, make sure that all connectors are tight and properly connected, that ground wires and terminators are used when required, and that manufacturer’s specifications (such as cable grade, cable lengths, and maximum number of nodes) are met and are consistent with the specifications for the transmission medium.

Try the following checks when troubleshooting network cabling problems:

With 10BASE-T, make sure the cable used has the correct number of twists to meet the data-grade specifications.
Look for electrical interference, which can be caused by tying the network cable together with monitor and power cords. Outside, fluorescent lights, electric motors, and other electrical devices can cause interference.
Make sure that connectors are pinned properly and crimped tightly.
If excess shielding on coaxial cable is exposed, make sure it doesn’t ground out the connector.
Ensure that coaxial cables are not coiled tightly together.
On coaxial Ethernet LANs, look for missing terminators or terminators with improper impedance ratings.
Watch out for malfunctioning transceivers, concentrators, or T-connectors. Make sure that connectors have not been mixed up, such as ARCnet connectors used on an Ethernet network.
Test the continuity of the cable by using the various physical testing devices discussed in the previous section, or by using a software-based cable testing utility.
Make sure that all the component cables in a segment are connected. A user who moves his client and removes the T-connector incorrectly can cause a broken segment.
Examine cable connectors for bent or broken pins.
On Token Ring networks, inspect the attachment of patch cables and adapter cables. Remember, patch cables connect MSAUs, and adapter cables connect the network adapter to the MSAU.

One advantage of a Token Ring network is its built-in capability to monitor itself. Token Ring networks provide electronic troubleshooting and, when possible, actually make repairs. When the Token Ring network can’t make its own repairs, a process called beaconing narrows down the portion of the ring in which the problem is most likely to exist. (See Chapter 4, “Network Topologies and Architectures,” for more information on beaconing.)