Strange network cable problem

[WithoutMotive]

Need some ideas thrown my way regarding this one.
We have some networked portable classrooms, one of which has developed a problem.
Totally lost it's network connection - no link light on the computer, nothing.
So, I tested the various network leads at the computer end and at the cab - all fine.
Tested the cable between cab and wall socket - fine.
This lead me to believe it was a faulty network card in the machine - not so. Plugging two more machines in revealed still no link lights. Again, tested all cables and everything was ok.

So, with perfectly ok cabling along the whole route, perfectly working port on the switch, perfectly working network cards on machines, why am I still not getting a connection?

Any thoughts would be appreciated.

WM

[soveryapt]

These machines all definitely work elsewhere in the building? (I know it's highly unlikely to have 3 machines all die, but I always make sure I take a laptop with me that I know had a working point and connection at the server cab end of things if stuff is failing in one of our portable classrooms)

Also, I assume you've tested the main connection to that classroom and checked (if the switch is managed) that it's not been compromised at all and therefore had ports turned off and all that jazz?

[SYNACK]

How good was your cable tester? A cable can have electrical conductivity but still be completely unfit for network traffic. We had some ports crimped down - by a dodgey supplier - to cheap rack patching blocks with no supports. These passed most tests of condictivity to pairs but the termination was so bad that it echoed the digital signals into oblivion. It also had the nice little habbit of killing the links when expantion or contraction from heat or cold caused the links to fail.

Have you tried reterminating the ends?

[WithoutMotive]

These machines all definitely work elsewhere in the building? (I know it's highly unlikely to have 3 machines all die, but I always make sure I take a laptop with me that I know had a working point and connection at the server cab end of things if stuff is failing in one of our portable classrooms)

Also, I assume you've tested the main connection to that classroom and checked (if the switch is managed) that it's not been compromised at all and therefore had ports turned off and all that jazz?

All machines work fine (one was my laptop).
The port was the main one into the room from the cab.

[WithoutMotive]

How good was your cable tester? A cable can have electrical conductivity but still be completely unfit for network traffic. We had some ports crimped down - by a dodgey supplier - to cheap rack patching blocks with no supports. These passed most tests of condictivity to pairs but the termination was so bad that it echoed the digital signals into oblivion. It also had the nice little habbit of killing the links when expantion or contraction from heat or cold caused the links to fail.

Have you tried reterminating the ends?

Its a pretty basic tester which gives a yea or nay for connection. Nothing more.
I reterminated one end because it was crossed over. Admittedly, I haven't done the other end which I'll try.
The end I re-did was very badly done by the people who supplied the classrooms.

[sjatkn]

It also had the nice little habbit of killing the links when expantion or contraction from heat or cold caused the links to fail.

I also had a problem with an intermittent problem where an aircon unit blew on the patch lead making it contract and loose contact. Replacing the lead resolved the issue.

[soveryapt]

Its a pretty basic tester which gives a yea or nay for connection. Nothing more.
I reterminated one end because it was crossed over. Admittedly, I haven't done the other end which I'll try.
The end I re-did was very badly done by the people who supplied the classrooms.

You know what, I don't know why I didn't think of that either given the number of ends I had to reterminate .. I probably wiped it from my mind .. it was almost as if they had just shoved wires into the plugs and crimped, not check the pairing etc ..

[chazzy2501]

As a quick test, force your PC to use 100Mbit Full Duplex instead of Auto.

Does your tester test all 4 pairs or is it 100Mbit tester only? I found that when upgrading to Gigabit PCs my network cabling wasn't so good. I have is some cases cured issues with connectivity by forcing some PCs to use 100Mbit instead of Gigabit.

[WithoutMotive]

All sorted (not sure how).
Re-terminated both ends of the cable again and now I have a proper connection. I'd done this at least twice.
Oh well... done now

Thanks.

[Michael]

It could be that some pairs were OK, but not all four pairs.

Depending how good the tester is, a lot of them display information for each pair rather than a simple yay or nay

[K.C.Leblanc]

I had a similar issue with a cable run that had the same wiring error at both ends. The 4 pairs were next to each other instead of having one middle pair inside the other.

[plexer]

Both ends should match T568B or T568A depending on where you live.

http://en.wikipedia.org/wiki/Category_5_cable

@K.C.Leblanc not quite sure what you mean.

Ben