# Why ethernet cables must be ended with specific arrangement

I just accepted that ethernet cables CAT 5 and more must be ended with specific arrangement.

I learned when I ending my cables to take attention that either end must be in same arrangement(568A or 568B ).

Sometime I get stacked with my fellow servant that they claim that Cable should work if just arrangement at both side are same even if it is not in 568A or 568B layouts.

My experience said that it is not true, but I am now looking for some technical argument to prove that.

• Just to bring up a point, but not all Ethernet is over UTP (unshielded twisted pair). (For example 10base2, and Fiber are Ethernet). Jun 16, 2010 at 20:26
• @Zoredache Yes but no one of those you mentioned are using CAT5 cable Jun 16, 2010 at 21:32

The number of twistings for each color pair in an Ethernet cable is different, and this helps avoiding crosstalk between them; an Ethernet cable can (and often will) work if you swap colors (as long as you do it consistently), but it will work better if you stick to the standards, especially on long distances and/or where there is a lot of electromagnetic noise.

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

• You mean, a blue cable is twisted differently than the orange one, for instance? Jun 16, 2010 at 9:39
• the blue pair vs the orange pair, that is Jun 16, 2010 at 9:48
• @Hannes: yes, exactly. Jun 16, 2010 at 9:52
• I had no idea about that! I thought all pairs were twisted the same number of times... I'll edit my answer as well. Jun 16, 2010 at 10:05
• This is more then just about twists though. For example pins 1&2 should use the same pair, and 3&6 should use the same pair. This way the TX+ and TX- are on the same pair, and RX+, and RX- use the same pair. The point of having a TX+/- and RX+/- on a pair is so that any interference can be factored out since it should typically affect both the + and - wire the same way. Jun 16, 2010 at 20:35

If you have the same order of cables on both ends, it's a straight through cable. If you use A on one side and B on the other, it's a crossed cable (although the order is hardly random, obviously, but I suppose you knew that). The point is, the cables inside your UTP cable are nothing but copper, it doesn't matter what color the isolation is. The electrical signals passing through don't care about colors.

It will work just fine, but there is a standard in place to easily recognize what kind of cable you're dealing with and to make sure you don't mess up when making crossed cables. I'm sure there's more complicated reasons as well though, but the RFC or what not should help you out if you want to know more. :-)

EDIT: Massimo replied that it might in fact make a difference after all: the 4 pairs of cables are, according to him (I did not know this before and didn't doublecheck) twisted differently. This twisting makes sure the electromagnetical interference is reduced to a minimum (also the reason you want to make the untwisted part when making the cable as short as possible).