What is TIA-568?
TIA-568 is a wiring standard for network cabling and is usually associated with patch cords, but it applies to "structured cabling" generally.
Whats the difference between T568-A and T568-B?
It's all about the terminations… Basically the difference is that pairs 2 and 3 are transposed between the two standards, the pair "number" does not correspond to actual pin numbers on the 8P8C connector itself.
As you can see the by the diagram above that T568A type has the green pair (pair 3), connected to pins 1 and 2, and the orange (pair 2), to pins 3 and 6.
However T568B has the green pair (pair 3), which is connected to pins 3 and 6 and the orange pair (pair 2), which is connected to pins 1 and 2.
The important point to remember is that the color to pair designation stays the same between the two types… Orange is always pair 2 and Green is always pair 3.
Are the two types compatible?
As described above you can see the only termination differences are the colors of the pairs, electrically the end to end connections are the same. It can get a little confusing when you are dealing with structured cabling is wired to T568A and you have patch cords are T568B, you just have to remember where you are if you are doing any testing or repairs. If you are looking at a patch cord the easiest way to tell which standard you have is to look at the first two pins, if they are green it is T568A and if they are orange you have T568B. But always check both ends, if one end is T568A and the other is T568B then you have a crossover cable!
So why is there a standard?
The standard exists for a very good reason, especially relative to termination. The idea of the standard is to enforce the color scheme for installers, manufacturers and end users alike. One of the popular problems that we have seen with end users that make their own cables is that they put the wires "in order" of color. But as you can see in either revision of the standard pair 1 has the solid color before the striped wire, which is different than the other pairs.
Additionally it is surrounded by pair 3, putting the colors in order will mean that the signal that traverses pair 3 will go across one wire of different pairs! This causes a crosstalk nightmare, especially on longer cables where you will see errors or no connectivity at all.
Everyone is hung up on the termination type
A dramatic statement, but nonetheless true. Out of the entire standards document the termination documentation takes up one page. There are many more attributes and guidelines within the standard, but the two standards are usually talked about in the context of the termination type.