What is the logic behind the pin diagram of Ethernet cables?

Question

Is there a reason why the pin diagram of an RJ-45 looks like this?

I understand that the orange pair is used for Tx and the green pair for Rx. Why is it that the orange pair are kept together while the green pair are separated?

What are the consequences of messing around with this order as long as it is the same at both the ends?

Answer 1

From Wikipedia, emphasis mine:

Cables terminated in registered jack connectors used in building wiring and the telephone network normally consist of twisted pairs of wires. Wiring conventions were designed to take full advantage of the physical compatibility, thereby ensuring that using a smaller plug in a larger socket would pick up complete pairs, not a (relatively useless) two half pairs. But here again, there has been a problem.

The original concept was that the centre two pins would be one pair, the next two out the second pair, and so on until the outer pins of an eight-pin connector would be the fourth twisted pair. Additionally, signal shielding was optimized by alternating the live (hot) and earthy (ground) pins of each pair. This standard for the eight-pin connector is the USOC-defined pinout, but the outermost pair are then too far apart to meet the electrical requirements of high-speed LAN protocols.

Two variations known as T568A and T568B overcome this by using adjacent pairs of the outer four pins for the third and fourth pairs. [...]

So, the center pins 4 and 5, and their neighbours 3 and 6, follow the original conventions. But pins 2 and 7, and 1 and 8, could not follow that conventions due to electrical requirements. Hence the other two pairs are wired to pins 1 and 2, and 7 and 8.

Also note that keeping twisted pairs together is important to limit crosstalk. And different pairs have different twist rates, so maybe interchanging pairs (like using brown for blue, and blue for brown, on both sides of the cabling) affects electrical characteristics too; I'd stick to the standard colour scheme.

Answer 2

General answer:

The actual reason for placement of specific signals in a connector, the design of the connector itself (and the color coding for the wires) is most likely hidden inside the technical standard(s) related to the connector and cabling.

For TECHNICAL reasons, e.g. avoiding "crosstalk", it is wise to place wiring that has signals with sharp edges (e.g. square wave) separate from signals that might get disturbed by the "noise" caused by the sharp transitions from "low" to "high" state of that OR make sure to have some type of shielding in between.

Reference:
"twisted pair" cabling (e.g. two intertwined wires) and also "balanced pair" exist as these help to mitigate trouble with disturbance (as above, and other).

Note:
"cross over"-type cables might not be needed, many network devices detect the correct polarity and adjust automatically.

Answer 3

my memory was that the 2 center pins were reserved for POTS so all computer network equipment was designed to utilize the outer 6 pins. Back when plain old telephones were still common there was a requirement that the RJ45 plugs be compatible with the plain old telephone and you could plug in a cord into the RJ45 jack and get a dialtone. Today however data networks utilize all 8 pins but to remain compatible with old networking gear we still keep with this standard.

The A/B difference is just to do with better performance. More modern and faster networking equipment requires you follow B wiring diagram as it offers better performance.

Answer 4

As a retired professional, (degree qualified), ex. BBC RF engineer the choice of ethernet RJ45 terminations and cable pairs looks to me like a typical 'design by committee/historical reasons' decision and is far from ideal technically for the following reasons:

1. Separating a data pair in the RJ45 plug, (pins 3 and 6), results in a significant and unnecessary disturbance in the 'twisted pair' characteristic impedance match which will inevitably result in data reflections that will limit the maximum transmission frequency of the cable.
2. The two types of channelised Cat6 cable that I use, (BT spec. external grade polyethylene and high quality internal spec. PVC), both have the Rx and Tx data Orange/Green channels running together in adjacent channels in the cable which I suspect may not the best choice for minimum crosstalk, but there's possibly not a lot in it - it would be interesting to measure cables that use adjacent pairs for data & compare them with cables that use opposite pairs in the cable for data. The separation problem can not be avoided, however, as that is simply down to the strange choice for the RJ45 pinout.

Answer 5

The reasons for the standard is several fold:

1. A standard across all platforms helps everyone get on the same page and cross-compatibility is a dream when dealing with the manufacturer from a customer basis.
2. As PPPD commented earlier in the Q&A, the middle pins have to do with line 1/2 for POTS systems which originally only had 4 pins (RJ11) instead of the 8 in an RJ45. The requirement that an RJ11 can plug into an RJ45 jack and function was part of the initial design. However the difference between the standards of A/B have to do with the fact that the pinout for Type B was originally the setup by AT&T and was not allowed to be used based on the TIA format of A. So when legislation came out for Type A as the standard, AT&T said they would be forced to change all of their installations and so to appease AT&T's existing installations type B was accepted in TIA.
3. In the old days, for certain transfer formats like RS-232, it required crossover cabling (TX to go to RX) as there was no handshake protocols to do the transfer. Kind of similar how polarization works for fiber optics. Today most switches can do that automatically removing the need for crossover cabling. However, it is stated by TIA and BICSI to pick a standard and stick to it. Fiber optics still require polarization considerations as a transmitter to transmitter connection will damage the devices.