Testing Twisted-Pair Network Cables
Last updated: 5/8/04
Q. How can I test twisted-pair Ethernet cables?
A. Twisted--pair cable testers capable of
certifying twisted-pair cables for operation at Ethernet speeds cost in the
neighborhood of $3,000 to $5,000. Simple continuity testers,
which are relatively inexpensive ($30-$50, or probably less at Radio Shack,
etc.), send a tone down the line and/or perform a roundtrip DC resistance
test. They do not measure impedance/line capacitive loading at RF frequencies,
noise, and some of the more esoteric cable properties. They are not
adequate tests for certifying cables and do not guarantee, by any means,
that an Ethernet cable will operate at Ethernet speeds. Ethernet adapters
automatically perform a better test than continuity testers as they send
multiple pulses down the line and Link LEDs on them will not light-up unless
the cables are wired so that the correct Ethernet transmitter pins are connected
to the correct Ethernet Receiver pins. However, the
Link LEDs can be lit and the cable may still not work.
Short of expensive
test equipment, downloading and running Xixia's QCheck will provide a good indication of whether or not you have a marginal cable. This program is free and will measure network throughput at the tcp level. I get a throughputs of 82 to 84 Mbps between computers connected to a broadband router and total wire distances of about 120 feet. That is quite good considering the network protocol overhead. The program runs on Windows Me, NT, 2000, XP, and Linux. It must be running on both computers at the ends of the cables being tested.
Another test is to substitute a cable to be tested
for another one in an existing network and copy 100 MBytes from the hard
disk on one computer through the cable and network to the hard disk on another
computer. It should take 45 seconds to 1 minute and 15 seconds, depending on how fast the computers are, especially the hard disk drives. If it takes minutes, you most likely have a defective cable. I copy
the cab files on the Windows 98 upgrade (d:\win98\*.*) CD (after copying
them from the CD to the hard disk), which are stored on my file server. If
100 MBytes are transmitted in a reasonable time, you will have a cable that
can send many files without excessive retransmissions caused by corrupted
packets. If you don’t have a network yet, then first check the Link
LEDS, and then perform the copy test. I have done this many times. The
adapter diagnostics and c:\>net diag are other available tests.