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Cat 5e cables and networking

Discussion in 'Hardware' started by Opticreep, Apr 6, 2004.

  1. Opticreep

    Opticreep Guest

    I'm having a weird problem setting up my router.

    I have a computer, a DSL modem, and a router.

    If I hook them all up properly using a few short CAT5 cables,
    everything works fine.

    If I hook them all up using a few short CAT5 cables, and a 40-ft long
    CAT5e cable between the PC and the router... the PC LAN card doesn't
    acknowledge the connection.

    My first guess was that the long CAT5e cable is faulty. But then, if
    I ditch the router altogether, and hook up my DSL Modem directly to my
    PC with the 40-ft CAT5e cable... everything works fine.

    I don't get it. I can't figure out why the long CAT5e cable would
    allow for a direct PC-to-modem connection, but not PC-to-router
    connection.
     
    Opticreep, Apr 6, 2004
    #1
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  2. Does the 40ft cable have the same config as the shorter cable? In other
    words: is one of them a cross-over cable?

    Do the link lights come up?

    Do you have a cable tester? What does it say?

    Cross-over cables are evil.
     
    William Tasso, Apr 6, 2004
    #2
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  3. Opticreep

    Opticreep Guest

    I don't think it's a cross-over cable. If it was, it wouldn't have
    allowed connection between the DSL modem and the PC. The problem only
    occurs if I use that same cable between the router and the PC.

    The link lights come up, but it blinks all the time. My PC detects it
    as "unplugged".

    Sorry, I don't have a cable tester. I heard those things are
    expensive. I only tested the cable by using it between the DSL modem
    and PC, and it seemed fine.
     
    Opticreep, Apr 7, 2004
    #3
  4. Opticreep

    jesus X Guest

    Which means it's a crossover cable. If it worked directly between the two, then
    it's crossover, and not straight through.

    Cable testers can be as cheap as a buck for a couple parts, a LED and a battery.
    You can get a decent one for just a few dollars.

    --
    jesus X [ Booze-fueled paragon of pointless cruelty and wanton sadism. ]
    email [ jesus_x @ mozillanews.org ]
    web [ http://www.mozillanews.org ]
    insult [ As usual, you've been a real pantload. ]
    warning [ Don't touch that! You might mutate your fingers. ]
     
    jesus X, Apr 7, 2004
    #4
  5. As far as I know, DSL modems come both ways.

    -- glen
     
    glen herrmannsfeldt, Apr 7, 2004
    #5
  6. Opticreep

    Opticreep Guest


    Huh?

    I'd hate to sound so newbie-ish, but what makes you say that it's a
    crossover cable? I use that cable between the DSL modem and my PC
    (without the router). That means it's a straight-through, doesn't it?
    I thought crossover cables were only used if I was connecting one
    computer directly into another computer with nothing else in-between.
     
    Opticreep, Apr 7, 2004
    #6
  7. Opticreep

    Emil Luca Guest

    Look at the rj45 and see if the color of the cable connection inside the
    rj45 is the same on both ends.
    If it is different it is a crossover cable.




    Huh?

    I'd hate to sound so newbie-ish, but what makes you say that it's a
    crossover cable? I use that cable between the DSL modem and my PC
    (without the router). That means it's a straight-through, doesn't it?
    I thought crossover cables were only used if I was connecting one
    computer directly into another computer with nothing else in-between.[/QUOTE]
     
    Emil Luca, Apr 7, 2004
    #7
  8. Opticreep

    jesus X Guest

    While Glen is right and there are DSL adapters that come both ways, most come
    wired ready to be plugged into a hub/switch/router. Thus, if you can connect it
    to a PC directly, it's most likely a crossover cable. As Emil said, check the
    colors of the wiring in the cable ends.

    --
    jesus X [ Booze-fueled paragon of pointless cruelty and wanton sadism. ]
    email [ jesus_x @ mozillanews.org ]
    web [ http://www.mozillanews.org ]
    insult [ As usual, you've been a real pantload. ]
    warning [ Don't touch that! You might mutate your fingers. ]
     
    jesus X, Apr 7, 2004
    #8
  9. Opticreep

    Opticreep Guest

    It's straight-through.

    Although I may have a weird theory on why I'm having problems.
    Apparently, my CAT5 cables weren't properly made. I noticed that pins
    3 & 6 on the RJ45 didn't make a twisted pair. Instead, the techie who
    installed the RJ45's for me made twisted pairs out of pins 3 & 4, and
    then 5 & 6. This probably generated a lot of noise on signals going
    through pins 3 and 6. I'm quite sure that category 5 standards state
    that pins 3 & 6 make up a pair (and so does 4 & 5).

    This oversight probably didn't affect my short 0.5 meter cables too
    much. But on a 25-meter cable, the signal-to-noise ratio might have
    become too high. At least that's my theory.

    But that still doesn't make sense. I don't think the signal-to-noise
    ratio should be affected so drastically from this one little mistake.
    And besides, why would this 24-ft CAT5e cable work between a DSL
    straight to my PC, but *not* work between a router and a PC? Maybe it
    has to do with signal strength or the different impedences, but
    thinking too much makes my head hurt.
     
    Opticreep, Apr 8, 2004
    #9
  10. Opticreep

    Rick Wintjen Guest

    The router probably has a 10/100 interface, also your NIC, so they would
    have tried to operate at 100T. The modem is almost certainly a 10T.
    Mis-matched pairs are tolerated better at the lower speeds, but you may
    have had data errors that weren't bad enough to get your attention.
     
    Rick Wintjen, Apr 8, 2004
    #10
  11. What you think has no bearing on physics. Mispairing the signals
    eliminates essentially *all* of the noise immunity that's designed in to
    Cat5 cables. This is undoubtedly the source of your problem and why we
    always recommend buying commercially manufactured patch cables rather
    than making them yourself (or having a friend make them).
    More likely it's because your DSL modem only supports 10BaseT, so your
    PC to DSL modem connection is only running at 10Mb whereas the router
    and your PC probably both support 100BaseTX and thus are trying
    (unsuccessfully) to run at 100Mb when they're connected together.

    -Larry Jones

    I hate it when they look at me that way. -- Calvin
     
    lawrence.jones, Apr 8, 2004
    #11
  12. Opticreep

    Opticreep Guest


    I switched around the mismatched pairs, and now the problem is gone :)

    Thanks to everyone for their help.
     
    Opticreep, Apr 8, 2004
    #12
  13. Opticreep

    Emil Luca Guest

    The speed setting at your computer and router should match are at the best
    be set to Auto.
    Mismatch speeds usually don't work well at all.
     
    Emil Luca, Apr 8, 2004
    #13
  14. Opticreep wrote:
    (snip)
    It isn't so much signal to noise, but the signal couples to the
    other wires. Depending on which wires, it can make a big
    difference in the signal at the other end.
    All routers I know of have 10baseT input and 10/100 output. That
    makes a big difference in how well the cable works if mispaired.

    -- glen
     
    glen herrmannsfeldt, Apr 12, 2004
    #14
  15. Opticreep

    James Knott Guest

    Anything other than the desired signal is noise. Therefor signal from the
    other wires is noise. There's even a word for that type of noise. It's
    called crosstalk.

    --

    Fundamentalism is fundamentally wrong.

    To reply to this message, replace everything to the left of "@" with
    james.knott.
     
    James Knott, Apr 12, 2004
    #15
  16. Well, it is, but neither describes this case so well.

    If you pair on (3,4) and (5,6) for 100baseTX the (3,6) signal
    will couple to wires 4 and 5. Most likely the result is that
    there isn't enough signal left at the end, even without any
    noise. The receiver requires a certain amount of signal,
    independent of the amount of noise actually present.

    Depending on the arrangement of the pairs in the cable,
    some will couple as common mode signals onto other pairs,
    again lost to the intended receiver.

    For a different example, if you manage to pair (1,3) and (2,6)
    the signal will then couple between the active signaling
    pairs. Though even in this case it should be absorbed by
    the terminating resistors on the transmitter and not affect
    the receiver much (for 100baseTX).

    Now for gigabit, where all four pairs are used in both directions,
    any coupling goes directly into the receiver of the wrong pair.
    I don't believe that the OP is using gigabit, though.

    -- glen
     
    glen herrmannsfeldt, Apr 12, 2004
    #16
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