In cable termination, the end of someone's cable is connected ....?
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Terrible Terminations
There are at least two good ways to make a bad Ethernet cable. One can start with badly-made bulk cable -- this seems to be the usual explanation for Chinese-made Ethernet cables -- or one can take good cable and do a terrible job of putting connectors on it. Some types of connectors -- a crimp BNC, for example -- are very easy to put onto a cable if someone has shown you how and you've got the right tools for the job, but the 8-pin Ethernet connector is another matter entirely. Bad technique, applied to perfectly good cable and connectors, can give you an apparently good connection, but horrible loss characteristics.
A Customer, A Problem:
Last week (this article was written February 2015) a customer wrote to us with a problem. He's dealing with network performance issues at a small company, and has come to suspect that bad patch cords, made by an electrician, account for some of the problems he's seeing. He asked if we would be willing to test one of his patch cords to see how it performed, and we were happy to oblige.
The Cable Under Test:
The patch cord arrived in the mail. It was about 7 feet long, bore branding from a major company dealing in industrial electronics, had a "Cat 6" designation on the jacket, and was terminated with fairly typical field-installable connectors. We plugged it in to our Fluke DTX tester, dialed up the 2.5 metre patch cord test limits, and tested it to ISO Cat 6 standards. The test came back absolutely awful, failing the Cat 6 crosstalk limits by over 10 dB. Stepping the protocol down to Cat 5e, we tested it again, and it still failed, though by a more modest 1.6 dB. Here are the test reports.
Now, at this point there are really only two possible causes, and this quite spectacular failure could be the product of one or both of them. First, there is the possibility that the bulk cable from which this patch cord was made is no good. Second, there is the possibility that the connectors are causing the problem, either because they're not very well designed or because they've been badly applied to the cable.
A Picture Worth a Thousand Mbps:
A quick look at the connectors confirmed that all was not right. This cable was put together using a pass-through type plug, where the eight conductors are lined up in order, threaded through the plug, and then cut off after crimping. These plugs are not the best performers electrically, but it is possible to get good results with them, so the plug was not likely to be the cause of such a large-scale failure. Looking through the plastic at the wires, however, it became clear that not a lot of care had gone into termination. There's a lot that could be said, and more than one thing wrong here, but one picture in particular illustrates the sort of thing we saw:
Note the blue wire, and its blue/white companion. This was a 568B termination, which means that these two wires are headed for adjacent pins (4 and 5) on the connector. Note that the blue goes off to the left, while the blue/white goes off to the right -- the blue-white is basically wrapped right around the orange pair, which sits between it and its mate.
Other things are not well in these connectors. Conductors are sharply bent, probably due to some awkward shoving-in to the connector body. Pairs are inartfully routed to their destinations, resulting in their being squished hard into other pairs. And then, in a move that neither helped crosstalk nor mechanical stability, one of the connectors was crimped on downstream of the end of the cable jacket -- so instead of gripping jacket, the back-crimp of the connector was smashing pairs into pairs without any cushioning or ability to slip around in-jacket.