Tip

Finding cable faults

Perhaps the most annoying type of problem to troubleshoot is the intermittent type that turn out to be a cable problem. This is often because of the way the important part of the cable, the parts that conduct, are hidden behind layers of plastic and often pass inside walls or ceilings. This makes it difficult to visually observe kinks where the bend radius has been exceeded or where the installers pulled too hard and stretched the cable or if someone bundled it too tightly and put an unseen crimp in the wire inside. Then there are the problems with improperly terminated or "punched down" cabling. The list goes on.

Tools to troubleshoot cabling issues are common, but often expensive. Many popular models of cable testers exceed $5000. So, I was pleasantly surprised to find that my recently purchased motherboard for my new PC came with a free "Virtual Cable Tester" which is mostly software, but also part of the chip built-into the on-board 10/100/1000 NIC.

I was even more surprised that in addition to standard fare like detecting impedance mismatches, pair swaps, pair polarity problems and pair skew problems, that it actually contained a bona fide Time Domain Reflectometer. The TDR is a very useful tool that can detect a short in a cable by bouncing a signal off it. The nice thing about it is that when you have a cable that goes underground or through the walls and you suspect it has a problem, the Time Domain Reflectometer will tell you where the problem

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is, rather than just that the entire cable is good or bad. This gives you the opportunity to fix the cable, instead of running a whole new cable, which may be expensive or difficult, depending on what your walls are made of.

Although there may be more models on the market, I bought an ASUS mobo, which had a 3Com NIC onboard, and included the Marvell Virtual Cable Tester software, which can detect shorts to within 1 meter of accuracy. That is pretty respectable for $130 or so.

So if you're in the market for a cable tester, before you spend thousands of dollars, consider a mobo upgrade.


Tom Lancaster, CCIE# 8829 CNX# 1105, is a consultant with 15 years experience in the networking industry, and co-author of several books on networking, most recently, CCSPTM: Secure PIX and Secure VPN Study Guide published by Sybex.


This was first published in March 2004

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