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This is a little project I'm doing at home.
I wanted to wire two rooms together (basically, the router is one room, and the switch is in the second room).
So I ran a CAT5 between the two rooms, and wired an RJ45 jack in each room.
I then hooked up the two jacks with two CAT5 cable to run it through the cable tester, and all 8 wires seem good.
Now, when I connect the switch and the router, the connection is unstable -- I ping the router and it barely holds on for two pings before it disconnects, and stays in that unstable state.

Just to make sure the router and the switch are ok, I connected them with long wire between the two rooms and the connection is absolutely stable, and pings continuously.

What could be the cause for the unstable connection? Especially that it pings a few times, so there IS a connection. But why is it unstable? And how come the cable tester says it's ok, but it's unstable?

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migrated from serverfault.com Sep 12 '12 at 22:36

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how long is the cable? –  user1008764 Sep 12 '12 at 21:36
    
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Most likely, the pairs aren't assigned to jack pins correctly. –  David Schwartz Sep 12 '12 at 22:29
    
@DavidSchwartz I would expect that to show up on a basic continuity tester (at least any decent one that shows the pinouts) –  voretaq7 Sep 12 '12 at 22:45
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@voretaq7: Continuity can be fine even if the pairs aren't assigned to jack pins correctly. –  David Schwartz Sep 12 '12 at 22:47

3 Answers 3

If I had to hazard a guess I would say you probably have a bad cable run from one of the following:

  • RJ45 Jacks not punched completely/correctly
    (Some pins didn't make it through the sheath and it's enough for your tester to work, but marginal)
  • Bad/Damaged cable run
    (Your cable is kinked, bent, or otherwise damaged)
  • Interference
    (Your cable is running next to a power line (or fluorescent light), etc -- things that introduce noise but wouldn't make a basic continuity tester fail
  • Bad jack(s)
    Make sure the contact fingers are actually down and make contact with the pins on the cable.
    Make sure you get a good solid CLICK when you plug cables in.

It's often easier to just re-run the cable in situations like this -- if you want to debug the problem more thoroughly you'll need something more than a simple continuity tester (a cable verifier, or better a cable certifier) -- These tend to be hideously expensive for home use ($500 and up, good certifiers are upwards of $1000) and I would only recommend buying one if you pull cable for a living.
The time and money lost re-running the occasional wonky cable over a lifetime is probably lower than the cost of a good quality certifier :-)

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You forgot the most likely (in my experience anyway) possibility, cable pairs not mapped properly to signal pairs. –  David Schwartz Sep 12 '12 at 22:47
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The UTP cable seems to be pinned out okay since a few pings do get responses. I'd vote for damaged cable due to improper/careless installation as a 1st guess. lanshack.com/cat5e-tutorial.aspx –  sawdust Sep 12 '12 at 22:56
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@sawdust -- You can still have perfectly good continuity and still have pairs crossed, causing crosstalk between transmit and receive circuits. –  Daniel R Hicks Sep 13 '12 at 2:10
    
@sawdust incorrectly punched cables (wrong pairs to wrong pins) can cause intermittent problems. I am assuming that BeemerGuy can read a wiring card (or manage to match up jacket colors to the little pictures on most jacks) though :-) –  voretaq7 Sep 13 '12 at 18:53

BeemerGuy, note that it's not sufficient to simply have the same color wires attached to the same numbered pins on both ends. In addition you have to have the PAIRS in the cable correctly connected to PAIRS of pins on the connector. It's been awhile, but the pinout goes something like this:

  1. Pair A wire 1
  2. Pair A wire 2
  3. Pair B wire 1
  4. Pair C wire 1
  5. Pair C wire 2
  6. Pair B wire 2
  7. Pair D wire 1
  8. Pair D wire 2

Note that this is not a "logical" arrangement -- You have a pair, then two pair with one straddling the other, then a 4th separate pair.

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Since I'm installing both ends, does it matter that they have to be in a particular order? Shouldn't I just make up an order on one end, and match it on the other end? Aren't they merely 8 wires just being relayed? –  BeemerGuy Sep 14 '12 at 20:34
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@BeemerGuy - The wires are in TWISTED PAIRS. It's very important, for noise rejection reasons, to maintain an individual circuit on a single twisted pair, vs have it split between two pairs. (It doesn't matter (much) which pair goes where, so long as each individual pair is kept together.) –  Daniel R Hicks Sep 15 '12 at 0:06

There are actually two possible wiring patterns. They are EIA/TIA-568A and EIA/TIA-568B which have backwards compatibility with RJ-11 (or not). Either wiring pattern can be used and most Cat 6 termination devices are color coded for both A and B wiring. Obviously you should consistently use A or B throughout a single installation.

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