Port works with some devices, not others: help to diagnose/fix?

Discussion in 'Home Networking' started by Henry Law, Dec 3, 2014.

  1. Henry Law

    Henry Law Guest

    I have a strange network problem at the charity for which I am
    IT-chief-cook-and-bottle-washer. I'd like the group's opinion on
    whether I've diagnosed it correctly, whether there's anything I've
    forgotten, and what I can about it.

    Problem: one of the network ports in the office "doesn't work".


    * Correct; it doesn't work. None of the devices in the office can
    connect, (with a known-good cable).

    * Not correct; it does work with my laptop. Gets an IP address, can
    browse the web etc.

    * But laptop takes a long time to get connected; at least twice as long
    as usual.

    * Connections in the cable are good, using a cable tester. LEDs flash
    1-8 in the proper order. (I should say that when I looked at it a while
    ago I found that the cable is wired up all wrong at the inbound end;
    lights flashed 6-4-8-2-7-1-5-3. I scrambled the connections at the
    socket the opposite way so that the result is 1-2-3-4-5-6-7-8)

    * Interrogating the switch via its web management service I see that the
    port I'm connected to (which is set to auto-negotiate) is running at
    10MB. (Yes, ten).


    * There's a fault in the cable (which is laid in the ceiling). It
    passes DC but attenuates RF.

    * The network card on my laptop is (for reasons known only to itself)
    prepared to negotiate down to 10MBit/sec and gets a connection. The
    other machines in the office give up when they can't get 100.

    * The downward negotiation to 10MB is responsible for the slow
    connection of my laptop.

    Now what?

    * Are there other tests I could conduct on the cable, to see whether and
    in what way it is damaged?

    Looking at the connection at the patch panel end isn't easy,
    unfortunately; the people who installed the network didn't leave any
    cable slack so that the panel could be pulled out for maintenance.

    * Is there any alternative to replacing the cable? (That's not at all
    easy and not going to happen any time soon).
    Henry Law, Dec 3, 2014
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  2. Henry Law

    cl Guest

    The scrambled connections will slow it down unless they're correctly
    paired at least. This would possibly not be noticeable on a short
    cable but if it's many meters then there will be a problem.

    Does that apply if you connect at the switch end?
    cl, Dec 3, 2014
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  3. Henry Law

    cl Guest

    It also just occurred to me that maybe the non-working devices can't
    auto-negotiate the direction whereas your laptop can. Are the
    non-working computers fairly old? It might be worth getting a
    cross-over cable and trying that with one of them.
    cl, Dec 3, 2014
  4. Henry Law

    Rob Morley Guest

    But are the pairs preserved?
    That would be my first guess.
    Rob Morley, Dec 3, 2014
  5. Henry Law

    Henry Law Guest

    Excellent comments so far.

    Chris Green wrote, and Rob Morley echoed the same thought:
    It's probably around 50-60 meters; not just across the room. And from
    memory they're not correctly paired. Good point.

    Chris also wrote
    Not sure what you mean; are you thinking that the switch port is u/s,
    rather than the cable it's patched to? I think I chose the switch port
    fairly randomly (to get a neat arrangement of patch cables), and every
    device works in every other port, as far as anyone knows.

    Carrying on in his creative thread, he finally said
    Again you're ahead of me and I don't understand. Of the devices that
    don't work on that port, one is a fairly elderly Lenovo desktop, but the
    other is a swish MacBook, fairly new by the look of it. I have one of
    those RJ45 connector thingies, which is marked "crossover"; I can try it
    with one of the existing cables (I don't have a crossover cable handy),
    if that's what you mean.
    Henry Law, Dec 3, 2014
  6. Henry Law

    Rob Morley Guest

    That probably excludes the "can't auto-negotiate a slow or crossed/non-
    crossed-cable connection" theory, as the Apple should be smart enough
    if it's fairly recent (is it an Intel one?).
    But error handling could easily differ between the Apple and your
    machine (I can't remember which OSI level that belongs to).
    Rob Morley, Dec 3, 2014
  7. Henry Law

    cl Guest

    Yes, at that sort of distance incorrect pairing would have a
    significant effect I would think.

    No, I meant if you connect a computer at the switch end of the cable
    in the socket where the 'bad' cable is (instead of the cable) do you
    get full expected speed.

    Or does swapping the 'bad' cable to a different socket on the switch
    have any effect?

    Modern switches and computers auto-negotiate the direction of each
    pair in the cable. Older ones didn't and you have to use a crossover
    cable to get them to work computer<-->computer or switch<-->switch. I
    was just wondering if this might be happening.
    cl, Dec 3, 2014
  8. Henry Law

    Henry Law Guest

    Thanks again for this, Chris.

    I'll try tomorrow. But when I went to the wiring cabinet this morning to
    plug in my tester I found that someone had un-patched the cable
    completely (no bleedin' wonder it didn't work, I thought), and I patched
    it pretty randomly.
    I'll take my crossover thingie with me tomorrow and give it a whirl.

    Otherwise I think I'm into getting a tiny network switch to provide an
    extra socket in the office for the time being, and trying to find
    someone who knows how to get at the cables in the ceiling ... urgh.

    Thank you all for your help. This is a great community.
    Henry Law, Dec 3, 2014
  9. Henry Law

    cl Guest

    Switches are cheaper than cables!

    It's what I do for my 'off site' backups in the garage where there are
    now two backup NAS systems, adding a cheap switch at the end of the
    single Cat5 cable strung through the trees was much easier (and
    cheaper) than running a second cable.

    Unless you're using all the cable capacity of course.
    cl, Dec 4, 2014
  10. Henry Law

    Rob Morley Guest

    On Thu, 4 Dec 2014 10:03:37 +0000
    Just hope you don't get any nearby lightning strikes.
    Rob Morley, Dec 4, 2014
  11. Henry Law

    cl Guest

    It hasn't happened yet! :) It has been there for several years.
    We're surrounded by mostly taller trees so a really nearby strike
    isn't all that likely. If it happens (unless it's really close) it
    will probably just fry two switches.
    cl, Dec 4, 2014
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