I Don't Suppose Anyone Can Explain...

... why Category 5 network cables seem to be so picky in line order?

I know the (well, a) right way to make a patch cable. The mantra of color order has been ingrained into my mind after doing it many, many times. Ultimately, though, the same color on one end matches the same color on the other end, and I always wondered if there was a real reason why the blue and green pairs get mixed up in the middle.

With some research, I've come to understand that has a little to do with compatibility with phone cables that usually use central pins as a line pair and the outer two (of four) pins for a second line. This makes some sense to me.

And yet, someone brought a cable to me today because it wasn't working on his network at home. He had attached the connectors at the end with the pairs simply wired straight through. A tester showed signals getting through on each individual line, so why would it actually matter how the pairs are ordered? Yet when I tried plugging it in, sure enough, there was no connection. Maybe this is just a minor logic error on my part, but I'm not able to visualize why that wouldn't work...

Comments

  1. northawkeNovember 20, 2008 at 10:30 AM

    I'll try to remember to ask my cable guy friends tonight and get you an answer. (I have an SCA meeting, though, so it's possible I'll forget. Remind me) If I have to listen to them ramble about nodes and load and blah, blah, blah... I might as well get useful info from them, too. :)

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  2. marinredwolfNovember 20, 2008 at 12:09 PM

    Remind, remind, remind! ;)

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  3. northawkeNovember 20, 2008 at 3:46 PM

    He says it sounds like 6&7 are flipped for crossover connections. I'm told that the guy probably has a crossover connection and be sure that both ends line up the same. (the explanation really didn't make sense to me...) (I don't know if he's so much explaining why things are, as just how to fix it - he said your friend should try the different connections to find the one that works.) Apparently the equipment or even the service could prefer one or the other. Our two main providers here, ACS and GCI, use different connectors. GCI uses straight match, ACS uses crossover connections. I don't know if this is a deliberate incompatibility or just a preference for one or the other (maybe one company started with a huge shipment of the one kind?) Apparently different machines can have preferred wiring, too....

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  4. marinredwolfNovember 21, 2008 at 7:48 AM

    Well, it wasn't wired as a crossover cable either way. I know crossover connections were used for a specific connection in networks back in the day (I want to say connecting one switch/hug to another mostly), but most current hardware doesn't seem to require that anymore. Our (admittedly cheap) tester sends a signal down one wire at a time and shows if the signal is getting through the other end with a LED readout. This cable, even when wired improperly, showed 1-1, 2-2, etc. all the way through. A crossover cable, because those wires are deliberately switched, would show 6-7 and 7-6. With a little asking here, I was told it had to do with interference - that only the central pair or two actually transfer data and the other pairs are wrapped around them over the length of the cable to block EM interference. That would mean it does matter which pairs you put in the middle, as the "wrong" ones would be more vulnerable to outside interference - something that would be increasingly a problem with a longer cable. This explanation makes a little sense to me, but it's hard to visualize as I've never stripped a cable for any length. Cut it and you get a cross section that doesn't really look like any pairs are specifically wrapped around the others, but maybe... If that makes any sort of sense. It's one of those things that's definitely easier to see in person than talk about.

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