Localize and isolate the cause

Once you have defined the problem of network, and recreated it if necessary, you should attempt to isolate that problem to a single device, connection, or software application. Reducing the scope of the problem in this way is where divide-and-conquer begins; the goal is to isolate the problem to the smallest element that could cause the problem. Test for and eliminate as many variables as possible. You may need to scan for a virus at this point.

Is there any normal function missing, or is there an abnormal response? Use the data gathered by your network monitoring tools to aid you in this process.

Determine whether anything was altered at that station or on the network just before the problem started. Often the user does not realize that changing something seemingly unrelated can cause problems on the network, such as rearranging the location of a portable heater or photocopier, or installing a new software application or adapter card. Do not discount the local environment when you are looking for change.

Temperature changes (heat is often a problem), electrical use from adjacent spaces – including nearby businesses, time of day, and influences from electronic sources. Even the passage of an elevator, or use of a cordless phone, should be noted.

Can the problem be duplicated from another station, or using other software applications at the same station?

Identify whether the problem is limited to one station, or one network resource such as a printer. Move one segment closer to the network resource and try again. If the problem goes away when you move closer to the network resource, test or replace the intervening infrastructure equipment.

If the problem affects an entire shared media segment, isolate the problem by reducing the variables to the fewest possible number. Try shortening the cable segment on a bus topology, or temporarily re-cabling a ring or star topology to create the smallest possible network for troubleshooting purposes.

Try a different switch or hub. If the problem is on the same, shared media segment as the network resource, try turning off or disconnect all but two stations. Once those two are communicating, add more stations.

If they are not communicating, check the physical layer possibilities such as the termination of the cable, the cable itself, or the specific ports used on the infrastructure equipment (hubs and switches).

If the problem can be isolated to a single station, try a different network adapter, a fresh copy of the network driver software (without using any of the network software or configuration files presently found on that station – delete them if necessary).

Try accessing the network using a diagnostic tool from the existing network cable connection for that station. If the network connection seems intact, determine if only one application exhibits the problem.

Try other applications from the same drive or file system. Compare configurations with a nearby but operational workstation.

Try a fresh copy of the application software (again using none of the existing software or configuration files).

If only one user experiences the problem, check the network security and permissions for that user.

Find out if any changes have been made to the network security that might affect this user:

• Has another user account been deleted that this user was made security equivalent to?
• Has this user been deleted from a security grouping within the network? Has an application been moved to a new location on the network?
• Have there been any changes to the system login script, or the user’s login script?

Now compare this user’s account with another user’s account that is able to perform the desired task.

• Have the affected user log in and attempt the same task from a nearby station that is not experiencing the problem.
• Have the other user log in to the problem station and try the same task.