Wrong order in CAT5 cabling -- does it matter?

I do tech support and the argument of TIA-568A vs TIA-568B vs 'whatever, but straight through" comes up every now and again. You should use the appropriate standard for your application.

Since you're referring to Ethernet, you should use Cat5e or Cat6 cable and terminate with the TIA-568B pinout.

TIA-568A

This standard is acceptable/meant for voice communications. By voice, I mean telephony. You would probably see this on a CAT3 cable, and telecom engineers and electricians are probably familiar with it. Any analog voice systems would probably be using this standard, even if they were using a higher CAT cable like CAT5.

This pinout should not be used for data networks, though it will function to some degree; you may see errors, reduced data rate, or other less-than-desirable results.

TIA-568B

This standard should be used for data networks, packetized video, or other intensive applications (throughput or bandwidth).

I don't think there is a negative consequence to using this standard for voice, but it goes against convention, so I would not use this standard for voice communications.

Random straight through

Sometimes, someone will tell you that as long as the ends match, you don't even need a standard. That may suffice for electrical conductivity, for example if you are using some CAT cable to control a relay, but for communications (tele- or data-), you should use the appropriate, relevant, standard.

Misc notes

• The actual wires in CAT cables have different lengths and twists per turn (see Wikipedia), so the choice in pinout does matter and has real world consequences.
• 10/100 mbit Ethernet uses 2 of the 4 pairs, while 1000 mbit Ethernet (Gigabit) does use all 4 pairs of the CAT cable.
• Wikipedia may say that -A or -B is fine, but from real world experience, use -B for data networks.
• Telecom engineers, electricians, or other analog-based technicians will probably give you the most static about how the pinout doesn't matter or that "it's all the same anyways". In such a case, they do not understand the complexities of the data communications world. They may not be wrong for the tech they are used to, but data communications are different than what they are used to working with.
• TIS-568 Standards
• CAT5 Cable

First and foremost it is absolutely critical that both ends match (at least with respect to wires that are actually used). If a given color originates on pin 3 on one end that same color must be tied to pin 3 on the other end. (This is contrary to phone cables where opposite ends are mirror images.) If you use a coupler to join two cables the coupler has a built-in "twist" so that this all works out.

Second (not quite as critical at lower speeds) the pairs must be kept together. That is, there is a "send" pair and a "receive" pair, and the wires of the pairs are twisted together. If you get one wire of the "send" pair twisted with one wire of the "receive" pair then you can get "crosstalk" which will introduce errors and slow things down (if not halt things entirely).

The pairs are not assigned to pins in an entirely logical sequence. The pairs are 1-2, 3-6, 4-5, 7-8. That is, three of pairs are adjacent pins, but one pair straddles the adjacent pins of the center pair.

Otherwise, electrons are color blind, so it doesn't matter which colors you tie to which pins, so long as the pairing is maintained. (Some claim that there is an ordering/positioning of the pairs in the cable and using the wrong pair of pairs can cause problems, but this argument is tenuous at best.)

However, it is a good idea to stick to a standard color assignment scheme, to make it easier to get cables terminated the same way on both ends. Unfortunately, there are 2-3 "standard" schemes.

This is an old question, but since none of the answers are truly correct I will try to answer more clearly.

TL:DR same color pairs must be kept together. The same color wire must be on pins 1&2, 3&6, 4&5 and 7&8. Which color is on which doesn't matter as long as it is the same on both ends.

Reversing the two conductors in a pair does not matter. Ethernet signaling does not care about polarity. Whether white/[COLOR] is connected to pin 4 or 5 doesn't matter as long as [COLOR]/white is connected to the other.

Which color pair is connected to each pair of connector pins does not matter. This is why TIA-568A vs TIA-568B wired sites will both work with data. (A is designed to more closely match telephone colors where pair 1 is blue, pair 2 is orange.)

(The wires do have different number of twists per inch, but this is to minimize the crosstalk between pairs, it does not greatly effect the performance of each pair.)

10/100 used to have one pair as TX, and the other as RX (the other two are unused). Pure 10/100 devices require that pairs 1,2 and 3,6 match on both ends when connecting a switch to a PC (a crossover cable was required if connecting two same devices together). Later HP released the "Auto-MDIX" standard that allows the devices on each end to negotiate which pair will be used for which TX and RX. This eliminated the need for crossover cables when connecting two switches, or connecting two PCs.

When 1GB was released the Auto-MDIX standard was included, and expanded. Now each pair can be used for TX or RX, and which pair is pair 1, pair 2, etc is negotiated. 1GB should work as long as one pair is connected to 1&2, another pair to 3&6, pair three to 4&5 and the forth pair to 7&8. (Negotiating gets tricky and won't work for 10/100 devices if the 10/100 pairs 1&2 or 3&6 gets crossed with the additional 4&5 or 7&8 pairs. If there is 1GB equipment on both ends of the wire even crossing pairs will usually work. )