EIA/TIA-568A and EIA/TIA-568B in networks

Posted on 2005-04-29
Last Modified: 2008-02-01
Hi All,

Somewhere I got it drilled in my head that a 568A terminated cable will not work in a 568B network.
The other IT guy argued with me for a little bit and we decided to bet a couple of doughnuts on who was right.
We created a 568A cable and plugged into a computer and a hub, which connected to a switch with the rest of the 568B terminated network.  All cables but this new one was 568B.
Well, two doughnuts and my confusion later I have yet to figure out how I got that stuck in my head.  We had no problems talking to any computers on the hub or over the switch.
I read that if you terminate a Cat5 it doesn't matter how the colors or wires are as long as they are the same on both ends.  However, their is performance lost and you may not achieve the speeds the cable can provide if you don't use 568A or B.   I know a cable terminated with both 568A and B is a crossover cable.  I just can't figure out where I got it that A and B are not compatible on the same network.

I guess my question is:  Is their any harm in mixing an A and B network?  If not, then what about a cable with a random pinout on both ends?

Question by:maotx
    LVL 27

    Accepted Solution

    You can mix and match 568A and 568B.  However, doing so you are going to cause yourself lots of troubleshooting headaches.  

    Ethernet 10 and 100baseT transmits and receives on pins 1,2,3, and 6.  

    On 568A, this is 1 and 2 are the green pair, and 3 and 6 are the orange pair.
    On 568B, thiis is 1 and 2 are the orange pair, and 3 and 6 are the green pair.

    So, if you have an A patch cable from the computer to the wall, and the wall is B for instance, let's say a signal travelling down pin 1 hits the white with green wire.  It travels through the patch cable where it hits the other end and then transitions to the B pin out wire in the wall, but still it transitions from pin 1 to pin 1 and from the white with green to the white with orange.  You see where there's no problem?  It's still on the pin and wire it's supposed to be on, although they change colors.

    The problem arises when one end is A and the other end is B, because now the signal has CROSSED from a transmit to a receive, or vice versa.

    Now, you might say, well I can have any colors I like, as long as they're the same on both ends, right?  NO WRONG.

    If you do a straight color pin out, like a telco color code (I forget what it's called) white blue, blue, white orange, orange, white green, green, white brown, brown - on both ends, you will create what is called a SPLIT PAIR.  

    If you could do this, no one would have bothered wiht the weird 568A and B in the first place!  A SPLIT PAIR is when you've moved the signals out of their pair and now you've increased electrical problems the pairs and the twists are designed to avoid.  The twists are all different so there isn't cross talk - electrical signal bleed between pairs.

    Hope this helps.
    LVL 2

    Expert Comment

    Hi Mark

    You have a good name as my name is mark, firstly you have placed this question under the hardware/router section but that doesn't matter as we want to help.

    To answer your question there is NO problem mixing A and B terminated cables as well as making your own random pinout in a network as long as its the same on both sides as it doesn't matter what colour cable inside your UTP/STP uses the TX and RX, but my question is why? If your job is to look after the network of an organisation, then one of your roles would be to standardise as much of the network as possible and i think that cabling is just as important as group polices.
    LVL 1

    Author Comment

    Hi pseudocyber,

    You said, "The twists are all different so there isn't cross talk - electrical signal bleed between pairs."

    Do you have a link that you can reference me to that talks about that?  I know your right I just want to learn a little bit more about how (EM field but I wouldn't mind seeing a bit more) the twists prevent cross talk and why certain twists of colors reduce it.  Very helpful post and answered/confirmed my questions/thoughts.



    Hi Mark


    Yeah I understand the the importance of having standards with cables.  My question was just more of a hypothetical "what-if" for the purpose of satisfying my curiousity.  Also, I had an idea that making a straight through cable of any color will cause problems, as pseudocyber pointed out.  I believe I read on another form somewhere that it will greatly reduce the reliability of the transfer of data if you do that.

    Thanks again.

    LVL 27

    Expert Comment

    LVL 1

    Author Comment

    Ahhh...thanks pseudocyber.  Points to you.

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