What is TIA-568?
TIA-568 is the result of more than 60 contributing organizations, including manufacturers, end-users, and consultants. That's an impressive piece of work! The very first revision (A) was published in 1991, with the second (B) in 1995.
The main goal of the standard is to define cabling types, distances, connectors, cable system architectures, cable termination standards as well as performance standards.
T568a vs. T568b. What is the difference between the two?
It's all about the termination. The difference is that colors identifying pairs two and three are transposed between the two standards.
As you can see by the diagram above that the 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 type 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 critical 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 still pair 3. The diagram also shows that the "pair numbers" do not correspond to actual PIN numbers on the 8P8C connector itself.
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 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 an excellent reason, to enforce the color scheme for installers, manufacturers, and end-users alike. One of the big problems that we have seen with end-users that make their 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 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.