Cable testers do exactly what their name says. They test cables. First, let's determine what a "bad" cable might be. Say, you have cables under a raised floor and you happen to also acquire a tiny rodent in your facility that decides to chew on one of your cables. If that rodent chews through all or any of the UTP or wires, this would be considered a cable break or a "bad
cable. Beyond that scenario, there are other things that dictate whether or not you have a "bad" cable. This list states a few.

1) A connector is crimped incorrectly.
2) The distance between a hub and a PC is too long.
3) "Good" cables run too close to electrical equipment.

All of these instances can result in a potentially "bad" cable.

Cable testers are designed to answer some or all questions. That is, depending on the amount of money you are willing to spend. A low end tester only tests for broken wires. A wire not broken can conduct electricity because it has continuity. Cheap testers, costing less than $100, are often called "continuity testers". Some of these cheap testers are also capable of testing for split or crossed pairs and shorts. These testers usually require that both ends of the cable be inserted into the tester. Naturally, this may sometimes pose a problem if one end of the cable is already in the wall. Moderately prices testers, usually less than $300, add the ability to tell the length of the cables. They can also tell the locations of a break. These are generally called Time Domain Reflectometers (TDRs). These testers have a small loopback device that gets inserted into the far end of the cable, enabling them to work with installed cables. This type of tester is idea to have in our possession.

If you're looking to start testing electrical characteristics of a cable, the price only increases, $2000 and up. These are professional testers that test the critical EIA/TIA electrical characteristics and are used by professional installers to verify installations. Given the price, you'll find that some have powerful extra, like the ability to plug into your network and literally produce a schematic of your network. This will include MAC addresses, IP or IPX addresses and so much more. These types of testers are known as protocol analyzers. They're definitely not a device that the average network tech would use.

So, when would you use a cable tester? Well, when you know that a good, quality, professionally installed cable suddenly goes bad. And, unless you have a known rodent problem, you might have a cable problem. Of course, you'll need to first assume that it's a software problem. One obvious clue that there might be a problem is when the network cannot be seen. You may receive an error message that no server is found or when exploring Network Neighborhood, no other system appears other than the system producing the error. One other thing to check is the NIC driver to ensure that it hasn't suddenly failed. Additionally, you should run a diagnostic on the internal NIC and a hardware loopback. If everything passes, you may find yourself with a cable problem. But, also check the lights on the NIC and the hub. If the light is not lit, there is a connectivity issue. Simply switching out the patch cable may resolve your issue. Try seeing whether or not others can see the and shared resources. Try logging into another computer using the same username and password to ensure that that particular user can access shared network resources. Visually check to see that the patch cable is inserted into the PC and into the drop. Lastly, if you are capable of plugging the system into a known active drop, do so to see if it works. If none of this works, them a simple continuity test may provide you with the answer to all your problems. But, then again, a bad NIC can also produce similar error messages. So, make certain that you run the NIC diagnostics.