The most common maintenance situation for large LAN installations is to set up a help desk. LAN users who experience problems call the help disk. Usually a technician is dispatched if the problem can’t be resolved over the phone. Let’s take a look at some of the problems the technician may encounter when dispatched.
Some preliminary checks should be made first. Ensure that the workstation is plugged into electrical power. Is it turned on? Is the CRT brightness turned down? These are obvious thing that are easily overlooked and warrant a look. The network interface card should be checked for proper installation in the PC. Check the hub-to-workstation connection. Is the user’s account set up properly on the server? Sometimes passwords and accounts get deleted by accident. Check the user’s LAN connection software for proper boot-up. This software can become corrupted and may need to be reinstalled on the work-station.
Troubleshooting Cable-Based Networks
Networks that use coaxial cable as the installed base can cause unique problems. Usually this type of network exists as a bus topology, and a fault anywhere along the bus can affect all the workstation on it. some common fault associated with coaxial LANs include broken or missing terminators, improperly disconnected T junctions, and damaged cables. A missing terminator from a coax link will shut down all the workstations on that link. A properly seated T junction on the network interface card connector will guarantee a good LAN connection. Coaxial cables often get pinched, crimped, and stretched, any of which can cause trouble for the workstation and possible all the workstations on that link.
Troubleshooting Unshielded Twisted-Pair Networks
Twisted-pair networks represent a different challenge to the troubleshooter than coax. Two common problems represent a different challenge to the troubleshooter than coax. Two common problems that occur with unshielded twisted-pair wiring are that the pairs become crossed or split up. Both conditions produce data signal degeneration. Near end crosstalk (NEXT) is generated from split pairs. Next stems from interference between the twisted pairs. Let’s use the example of a signal being transmitted from the workstation to the hub. The signal is smallest (maximum attenuation) at the hub. A transmitted signal originating at the hub – a strong signal – will feed over into the attenuated weak signal. Preventing crossed and split pairs is the best insurance against NEXT. Crossed pairs are not difficult to find but usually require a certain amount of wire tracing and continuity checking. Split pairs are more difficult to find, and special test instruments should be used to track the splits down. A LAN cable meter will have several specialize test functions, and the miswired feature will be one of them.
Another common problem that happens with unshielded twisted-pair wiring is that the wrong kind of connector is used. Stranded copper conductors need piercing-type modular eight connectors, and solid core conductors use a connector that straddles the wire. Since both types of connectors look alike they can easily be mixed up and often are. When the wrong connector is used, the result is an open or an intermittent connection.
Some Cabling Tips
With a little extra precaution, many LAN problems can be eliminated. Be sure to keep wiring links short without stretching the wire stretching the wire tight. Coax or twisted pair should never be placed near ac power lines or other noise sources. Install your cable base carefully. Wiring runs should be dry. Moisture causes corrosion over a period of time. Use only good-quality connectors. Never untwist more twist than necessary when making twisted-pair connections. Finally, keep good-quality wiring diagrams of the network installation.