atcomsystems.ca/forum Forums Wiring - Telecom Carriers - Trunks & Services Cabling Telephones Systems Wiring 568a or b configuration

Modern network interfaces and switches will adapt to a crossover cable. Older network interfaces will not.

My personal preference (or policy) for equipment that requires a cross-over situation (channel banks, etc) is always to wire a pair of jacks for the "crossover" and use 2 standard A>A or B>B patch cords. In that way, thinking like a repairman rather than an IT geek, when the weakest link, the patch cord, goes wonky, it can be replaced quickly by any old body at the site, and there is no need for an expert to go out and create a crossover cord in the field.

The term for this is "auto=sensing" Only one ethernet interface needs to be auto-sensing. So if the switch has auto-sensing ports, you can plug old equipment into it, and the switch will figure out what it needs to do.

Maybe what you have is crossed pairs. This is a little harder to overcome. For instance, I once saw a cable wired white-blue, blue, white-orange, orange, white-green, green, white-blue, blue, white-brown, brown. This is entirely logical but may not work very well. The reason is that ethernet will use white-orange and blue for one pair (pins 3 and 6). This is not a twisted pair.

The only way to check this out is to open up some wall plates. Expensive cable certifiers can detect this. But a simple cheap wiremap tester will not.

You may have shorts in the cables. You can test this with a cheap wiremap tester. Shorting problems tend to be intermittent, and can be caused by someone tugging on a cable that has snagged on some metal somewhere.

I have seen a single computer bring down an entire network. Try unplugging some computers and see if the network problem goes away.

Twisted pair wiring was invented by Alexander Graham Bell in the nineteenth century to reduce electromagnetic interference from electric trams and power lines, according to Wikipedia. The encyclopedia that anybody can edit also says that twisted pair cables reduce cross-talk between pairs in a cable.



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The first telephones used a single open wire, with earth return, and it worked because no one else was "using" the earth for a ground return. Then the electrical industry started using the earth as a safety return, and electrical railways used the rails (bonded to the earth) for their return circuits. This caused loud noises and humming, to the detriment of telephony.

So, the original single strand open-wire was improved by creating a balanced pair of wires, with two strands of open-wire to every subscriber. It became apparent that parallel runs of open wire, over substantial distances, created crosstalk and other interference, so the "transposition" scheme was introduced. There are several good descriptions, with mileage calculations, available in BSP's, BSTJ articles, etc.

Once we had the capability to produce multi-conductor cables, the wires were laid up in pairs, since this was already an accepted and established engineering practice in open-wire construction.

Neither Mr. Bell, the non-inventor, nor Mr. Meucci, the actual inventor, had much to do with twisted pair cable, since it appeared later in the history of telephony.

Arthur -

How about "Phantom Circuits"? If I remember correctly these were deliberately induced circuits caused by long runs of side-by-side wire (where you got an extra circuit with fewer pairs.

I never saw one in real life, but I do remember hearing about them in class at some point (generations ago).

Sam

If someone told you that the circuits were created by some sort of induction, they were lying... er..."laboring under a mis-apprehension". There would be no way to create or control such induced signals.

The term "phantom circuit" refers to the scheme whereby two "side" circuits, each comprised of a balanced pair, and each fed via a center-tapped repeat coil, could create a third "phantom" circuit. Since the two side circuits are balanced, there is no cross talk from the derived circuit.

https://en.wikipedia.org/wiki/Phantom_circuit

See page 148 in this book:

https://long-lines.net/sources/att_principles_ocr.pdf

There is a lot of amateur wiring out there. Here are some possible problems:

More than 1/2 inch of wiring was untwisted at the terminals.

A cable was tugged too hard during installation.

A cable is not at least 6 inches from any electrical power cable or does not cross the cable at right angles.

A cable has a bend with less than a 3 inch radius.

The jacks, punchdown blocks, patch panels and patch cables are not of the same category as the cable.

Mice and rats have been chewing on the cables. Yes this really happens.

All of these things can cause intermittent problems. Just because it worked yesterday does not mean it has to work today.

Aha! Thank you, Arthur. i seem to remember the instructor sketching the cable runs and the two center tapped relays.

Aeons ago.

Sam

Once upon a time, the TV station I work for needed to extend some police scanner feeds from a remote site. They phantomed a third circuit using the two pairs they already had and a handful of 111C's.

Back to the OT, I did find at one of my older office sites, an attempt to use some CAT3 cables where the blue pair was split off for phones on a 4P4C jack, and the other pairs were landed on the 8P8C jack, in 568B style (missing the blue pair, of course.)They were wondering why they couldn't get gigabit speeds. This was good enough 20 years ago for 10-base-T...