Go Beyond Ping and Traceroute With Cable Testing

Thursday Aug 29th 2002 by Carla Schroder

We're all familiar with the sort of network faults caused by misconfiguration or complexity, but sometimes the culprit's in the cable. Build a low-cost cable testing kit with Carla Schroder.

Ethernet cabling is generally of high quality, network faults are more likely to lie elsewhere. Software glitches, such as configuration problems, and user shenanigans are the primary causes of network problems (insert favorite horror story here). However there are times when the cabling itself is at fault. With the right tools, testing is quick and accurate, which translates to less hair loss and lower blood pressure for the network admin.

Test Sequence:

  1. Look at stuff
  2. Continuity test
  3. Performance test

The first two are the most common testing chores. Always start with a visual inspection. You'll feel mighty foolish when your fancy tools take thirty minutes to find that User 377-B has nailed his patch cable to the wall, and a quick look-see would have found it immediately. Continuity testing tells you if signals are making it all the way down the wire, this is the most common test. Performance testing is more suited for testing a new installation.

While it is possible and fun to spend thousands of dollars on testing tools, a basic field-testing toolkit doesn't have to cost a mint. A multimeter and continuity tester will total around $125, and will handle basic testing duties. For testing copper cables, some choices are:

  • Toner generator and amplifier
  • Multimeter
  • Continuity tester
  • Lineman's test set
  • Certification field tester

Fox And Hound
This is my favorite tool. Tally ho! Stodgy technical persons call them "tone generator and amplifier pair", or tone and probe kit. Bo-ring! A fox and hound pair locate and identify cables, and check connectivity and polarity. Sure, when you get to build a network from the ground up, everything will be correctly cabled and labeled. In the real world, it's messier.

The fox and hound work together- the fox connects to a cable and generates a tone. The hound "sniffs" out the tone to identify and trace the cable. It reads the tone through the wire's insulation, and even through drywall. Some of the more expensive testers come with headphones, useful in a noisy environment, or in a quiet one where people keep shushing you. The better sets filter out AC hum and amplify the trace tone. Good sets cost $150- $250.

No cutesy nicknames, just an essential part of the network admin's toolkit. The ohmmeter function finds shorts and opens. Very low resistance = a short, very high or infinite resistance = open. A multimeter can test for continuity, attenuation, and will determine if a wire is terminated correctly.

A multimeter can be the whole cable testing kit for the frugal, a decent one costs less than $50. The specialized tools are faster and easier to use. One thing a multimeter can do that none of the cable testers can is test electrical outlets. A flaky outlet causes all kinds of weird, difficult-to-diagnose problems, always test them. For a few dollars you can purchase a dedicated outlet tester, it looks like a big three-prong plug with LEDs. Just poke it into an outlet, you'll know instantly if it is wired correctly.

Continuity Tester
This comes in two parts, a base and a remote unit. These are inexpensive, $40- $100. The base unit goes at one end of the cable, the remote unit the other, so this is limited to testing easily accessible cables. It will quickly find opens, shorts, reversed pairs, and bad terminations. A continuity tester is a nice low-budget alternative to a tone and probe set. Some continuity testers also do tone-generation, though they are not as accurate as the more expensive units.

Lineperson's Test Set
A lineman's test set looks like a weirdo telephone handset with alligator clips. The clips connect directly to a cable pair, or to a cable pair terminated on a punch-down block. All it does is look for a dial tone on voice lines, our concern is from the demarcation point to the voice switch. Voice cabling that is terminated correctly will have a dial tone. This isn't absolutely necessary for your toolkit, but it comes in handy on occasion. Just being able to identify analog circuits can save some grief: most commercial buildings these days are infested with a crazy mixture of analog, digital, copper, and fiber optic cabling. (Analog and digital circuits require different test sets.) Rule #1 of the Suitably Suspicious Network Admin: just because it has a certain type of connector doesn't mean anything. It is also useful when arguing with an uncooperative telecom, you can confidently tell them that your testing proves that the problem is on their side of the demarcation point, not yours.

Multiple-Purpose Cable Testers
Don't forget about the myriad of serial, parallel, USB, and FireWire cables making themselves at home in your network. SCSI cables are so darned expensive it definitely pays to test them, as are the newfangled IEEE 1284 parallel cables, which are required by most newer laser printers. I mean come on, over $50 for a SCSI-3 68-pin cable? Is it hand-made by Martha Stewart? Multi-purpose cable testers cost between $100-$300.

Performance Testing
This is a must on any new installation, whether it be an addition to an existing network, or brand-new from top-to-bottom. It's a lot easier to fix bad cabling before you move in and connect everything. One way to do the testing is with a sophisticated, highly accurate, multi-function programmable certification field tester. These cost several thousand dollars, and verify if the cabling will perform up to ANSI/TIA/EIA standards.

Plan B is to test only for continuity and correct terminations with lower-cost tools, such as Fox & Hound. Ideally you'll capture the installers before they make their escape, and make them do necessary fixes.

Plan C is hire someone who specializes in this sort of testing. However you do it, it will prevent many future headaches.

These are links to pictures of tools. I'm not endorsing or recommending any of them, this is a sampling to show what they look like.
Fox and Hound pair
Continuity Tester
SCSI and PC Cable Tester
Electrical Outlet Tester
Certification field tester
Twisted Pair Cabling Performance Requirements

» See All Articles by Columnist Carla Shroder

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