Testing Adapters (Network Troubleshooting Tools)

3.2. Testing Adapters

While most problems with adapters, such as Ethernet cards, are configuration errors, sometimes adapters do fail. Without getting into the actual electronics, there are generally three simple tests you can make with adapters. However, each has its drawbacks:

If you have some doubts about whether the problem is in the adapter or network, you might try eliminating the bulk of the network from your tests. The easiest approach is to create a two-computer network using another working computer. If you use coaxial cable, simply run a cable known to be good between the computers and terminate each end appropriately. For twisted pair, use a crossover cable, i.e., a patch cable with send and receive crossed. If all is well, the computers should be able to communicate. If they don't, you should have a pretty clear idea of where to look next.
The crossover cable approach is analogous to setting up a serial connection using a null modem. You may want to first try this method with two working computers just to verify you are using the right kind of cable. You should also be sure IP numbers and masks are set appropriately on each computer. Clearly, the drawbacks with this approach are shuffling computers around and finding the right cable. But if you have a portable computer available, the shuffling isn't too difficult.
A second alternative is to use the configuration and test software provided by the adapter's manufacturer. If you bought the adapter as a separate purchase, you probably already have this software. If your adapter came with your computer, you may have to go to the manufacturer's web page and download the software. This approach can be helpful, particularly with configuration errors. For example, a combination adapter might be configured for coaxial cable while you are trying to use it with twisted pair. You may be able to change interrupts, DMA channels, memory locations, bus mastering configuration, and framing types with this software.
Using diagnostic software has a couple of limitations. First, the software may not check for some problems and may seemingly absolve a faulty card. Second, the software may not be compatible with the operating system you are using. This is particularly likely if you are using something like Linux or FreeBSD on an Intel platform.
The third alternative is to swap the card for one that is known to work. This presumes that you have a spare card or are willing to remove one from another machine. It also presumes that you aren't having problems that may damage some other component in the computer or the new card. Even though I generally keep spare cards on hand, I usually leave this test until last whenever possible.


3. Connectivity Testing		3.3. Software Testing with ping