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The documentation with your cable tester will provide more details in understanding and using these tests.
The better cable testers may be preprogrammed with appropriate values for different types of cable, allowing you to quickly identify parameters that are out of specification. A good tester should also
allow you to print or upload measurements into a database. This allows you to easily compare results over time to identify changes.
3.1.3.3 Other cable tests
In general, moving cables around is a poor way to test them. You may jiggle a nearby poor connection, changing the state of the problem. But if you cant afford a cable tester, you may have little choice.
If the cable in question is not installed in the wall, you can try to test it by swapping it with a cable known to be good. However, it is usually better to replace a working cable with a questionable cable
and see if things continue to work rather than the other way around. This method is more robust to multiple failures. You will immediately know the status of the questionable cable. If you replace a
questionable cable with a good cable and you still have problems, you clearly have a problem other than the cable. But you dont know if it is just a different problem or an additional problem. Of course,
this approach ties up more systems.
Remember, electrical connectivity does not equate to network connectivity. Ive seen technicians plug different subnets into the same hub and then wonder why the computers cant communicate.
[1] [1]
There are also circumstances in which this will work, but mixing subnets this way is an extremely bad idea.
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 dont, 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 isnt too difficult.
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•
A second alternative is to use the configuration and test software provided by the adapters 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 manufacturers 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 arent 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.
3.3 Software Testing with ping
