EIA/TIA 568A and EIA/TIA 568B UTP Color-Code Standards

Perhaps the widest known and most discussed feature of TIA/EIA-568-B.1-2001 is the definition of pin/pair assignments for eight-conductor 100-ohm balanced twisted-pair cabling, such as Category 3, Category 5 and Category 6 unshielded twisted-pair (UTP) cables. These assignments are named T568A and T568B and they define the pinout, or order of connections, for wires in 8P8C (often incorrectly referred to as RJ45) eight-pin modular connector plugs and sockets. Although these definitions consume only one of the 468 pages in the standards documents, a disproportionate amount of attention is paid to them. This is because cables that are terminated with differing standards on each end will not function normally.
TIA/EIA-568-B specifies that horizontal cables should be terminated using the T568A pin/pair assignments, "or, optionally, per [T568B] if necessary to accommodate certain 8-pin cabling systems." Despite this instruction, many organizations continue to implement T568B for various reasons, chiefly associated with tradition (T568B is equivalent to AT&T 258A). The United States National Communication Systems Federal Telecommunications Recommendations do not recognize T568B.

The primary color of pair one is blue, pair two is orange, pair three is green and pair four is brown. Each pair consists of one conductor of solid color, and a second conductor which is white with a stripe of the same color. The specific assignments of pairs to connector pins varies between the T568A and T568B standards.

Mixing T568A-terminated patch cords with T568B-terminated horizontal cables (or the reverse) does not produce pinout problems in a facility. Although it may very slightly degrade signal quality, this effect is marginal and certainly no greater than that produced by mixing cable brands in-channel.

The TX (transmitter) pins are connected to corresponding RX (receiver) pins, with plus to plus and minus to minus. A cross-over cable must be used to connect units with identical interfaces.

When straight-through cables are used to connect Ethernet devices, one of the two units must, in effect, perform the cross-over function. This is the reason that straight through cables work directly between hubs or switches and NIC cards.... the Hub or Switch is designed so that their RJ45 Jacks are pre-wired with the transmit and receive pairs already reversed.

There are two colour-code standards in common use: EIA/TIA 568A and EIA/TIA 568B. These standards derive from TELCO usage and the pairs shown correspond to four phone lines, each with its own line pair. This same wiring was adopted for LAN standard Ethernet RJ45 wiring as well. RJ45 receptacle wiring for both standards are shown below:

If we apply the 568A color code and show all eight wires, our pin-out looks like this:

Note: Only pairs 2 and 3 are used for Standard Ethernet wiring. Pairs 1 and 4 can be used for other purposes such as telephones or even a second separate, complete Ethernet connection.

Pins 4, 5, 7, and 8 and the blue and brown pairs are not used in either standard. Quite contrary to what you may read elsewhere, these pins and wires are not used or required to implement 100BASE-TX duplexing - they are just plain wasted.

However, the actual cables are not physically that simple. In the diagrams, the orange pair of wires are not adjacent. The blue pair is upside-down. The right ends match RJ-45 jacks and the left ends do not. If, for example, we invert the left side of the 568A "straight"-thru cable to match a 568A jack--put one 180° twist in the entire cable from end-to-end--and twist together and rearrange the appropriate pairs, we get the following can-of-worms:

Source: Wikipedia