Cisco Networking: Physical Layer Troubleshooting
Do not overlook the physical layer when you troubleshoot. Yes, troubleshooting the physical layer is not as sexy as say, troubleshooting on a Cisco Adaptive Security Appliance (ASA), but it is just as useful. For a user having connectivity issues, look around her system. If the problems are wireless, look for sources of interference that may be nearby. If the user is wired, look at the network cable.
Networking devices are often put in substandard locations because they do not usually beep if they get hot. Switches well above their rated operating temperature have been found, jammed into the tops of closets during a hot summer. In another type of hardware abuse, a user had his data cable looped around his desk with the cable acting as a speed bump for his chair. It's surprising that the cable was even in one piece, much less still passing data.
Rather than spending hours looking for the problem where it does not exist, check the physical items close to the network devices and between locations. In one example, a coil of thin coaxial cable in a ceiling was cut out and stolen, which left network users without connectivity. This physical problem would not have been found without tracing the physical connection from end to end.