Content

| 3.4 |  | Identifying Physical Layer Problems |
| 3.4.4 |  | Hardware |

When Layer 1 or Layer 2 hardware components fail, a system will experience a sudden loss of physical connectivity. There are various occurrences in frames transported over shared access media that indicate a faulty NIC or interface in Layers 1-3 equipment.

Check for link lights at both the station and the hub or switch end. However, due to increased software control the presence of a link light is not a guarantee that the port works. The absence of a link light is still a fairly reliable indication of a problem.

Disconnect the problem station or the station identified as the source of errored frames from the network. Attach a monitoring tool in its place. Be sure to use the original cable of the problem station at this time, not a known-good cable.

Test also from the network side back toward the suspect station. Use a spare cable to attach the test tool to the station and watch the power-up process. Without familiarity with low level operating system commands it is often necessary to cold-start the station in order to get it to speak on the network. Once a station has completed the initial boot-up process it may not speak on the network again without these special commands. Observe the link process and the protocols that the station is sending. Ensure that the station is communicating, as many problems relate to station configurations not recognizing or attempting to utilize the network adapter. Look for signal strength and other physical layer parameters.

Not all network adapter faults will be exhibited during testing. If the fault appears to be intermittent it may be appropriate to replace the network adapter and driver software as part of the diagnostic process. If that solves the problem then try exchanging just the network adapter to see if the fault was associated with the network adapter or the driver software. If the network adapter was inexpensive enough, it may be more expedient to simply discard the suspect adapter if the user later reports that the problem has been resolved by replacing it.

Some portable PC network adapter cards ship with a special power saving feature enabled. This feature causes the NIC to listen for link pulse, but it will not transmit anything at all (including link pulse) until it hears a signal on the receive circuit. To conserve power the entire transmit circuit in the NIC is shut down until the receive circuit indicates that a transmission is warranted. This feature has been known to cause link problems with some other network devices. Try disabling this feature in the software configuration of the NIC, or use a test tool to see what signals are offered by the NIC.