Splitting 3&6 ? 2

[lever]

Some one asked me the question, and i couldn't be 100% on this, though i think it has something to do with the twists.

This guy made up a patch cord using oranges, greens,blues &browns together in thier pairs.
Tested ok with a tester, but couldn't get a connection on his home network. It wasn't a crossover problem as i made a "proper" straight patch cord for him which worked.

So why on EIA/TIA 568B are the greens split(pin's 3&6)?
Why isn't a straight through just the colors in their pairs together, the same at each end ?

lever.

 

[daronwilson]

This question seems to pop up every couple weeks, you might search the forum for a more complete description, there are several.

In a nutshell though, there is a transmit pair and a receive pair on 10/100 ethernet. One pair is pins 1,2 while the other pair is pins 3 and 6. In order to keep the signals from interfering with each other, we use a twisted pair for each signal. The two wires twisted together do some magic to keep that signal on those two wires and away from the other pairs.

If you make the cable up with the pairs side by side, you will be 'splitting a pair' for pins 3 and 6. It will test OK with a simple continuity tester, but probably won't pass a test with a real LAN tester. A real tester will scan the pairs with a legitimate data signal and record the results. It will spot right away that the wire has too much crosstalk due to the wiring method.

Here's a simple page with the basic info:

http://yoda.uvi.edu/InfoTech/rj45.htm

Hope that helps!

It is only my opinion, based on my experience and education...I am always willing to learn, educate me!
Daron J. Wilson, RCDD
daron.wilson@lhmorris.com

 

[Bobg1]

The twist in the pairs that Daron mentioned is also different from one pair to the next. If you take a piece of cat 5 cable and strip it back you should be able to see the difference. Belden datatwist is a good one to look at, because it has the pairs bonded to help maintain the accuracy of the twist. The twist is critical in teying to keep the signals on thier own pairs, this is why you also must maintain the twist up to the termination points.

 

[lever]

I knew it was something to do with the twist on the pairs,
but it was the crosstalk that threw me there.

I couldn't find anything more detailed in the forum then what you've given me.
I'll check the web in more detail later.

Thanks guys for making it a little clearer.

lever.

 

[jimbopalmer]

By the way, if you pluged a phone into a RJ45 jack, the voice pair would be pins 4 &5, which is why they split 3&6, to leave POTS on a real pair. if it WAS 1&2 3&4 5&6 and 7&8, then whan you pluged a phone in to the 'wrong' plug it would not get a twisted pair. Using 568a or 568b you do preserve integrety for phones. (perhaps as much as 10% of my 'network' wiring provides phone service) I tried to remain child-like, all I acheived was childish.

 

[lever]

Now that i should have got. Considering most of the stuff i do is Voice.

Cheers Fella's

lever.

 

[Networker1]

So.. adding to this.... If I get an error on pin 6 (the cable is probably cut somewhere, due , I think, to a remodeling job done on the office. And I would not dare break anything up)... Can I take, say, wire 3, 5, 7 or 8 and replace 6? Making the change also on the patch panel!!

Would that solve my problem?

By the way... I have a second wire with pin 4 giving error also!

Networker1 - Rent-a-PCTech.Com
WebManager - WebLatino.Net
Servicing Puerto Rico & The Caribbean

 

[Bobg1]

No, by using a single wire you end up with a split pair.
even if you switch the wires for the pair, you then have something that is non-standard in the mix. makes troubleshooting later more difficult.