This is a cabling question. Why is it that the 568A and 568B standards used 1,2,3,&6? Why did they "jump" over 4&5? was it because telephones on Cat3 used 4&5?

Or was it because? In the begining when you plugged your computer accidently into a phone jack ? Lucky for you computers don't use pins 4&5 the phone uses them.

this question was recently answered by someone in another thread, can't remember for sure which one it was, but it was discussed with past two weeks.

It has nothing to do with phones using 4&5. When the Cat 5 standard was introduced, there were plent of systems out there that used pins 3-6 when wired to a cat 5 cable termination.

T568A is a wiring standard, and while it does not "use" any pairs, it does specify how all four pairs must be terminated. The 802.3 standards specify how those pairs are used, and over which pairs data and/or power are transmitted.

Early versions of the Ethernet standard did indeed leave pairs 1 and 4 as spares, which did protect NICs from accidental connection to the PSTN or anything else on the first pair that might cause a problem for the electronics. (Edited to add: I don't know if this was the reason for not using the first pair or a happy coincidence, but I suspect it would have at least been a factor.)

Current versions of the Ethernet standard use all four pairs, and NICs do have some protection against accidental connection to the PSTN. In general I wouldn't worry about people accidentally connecting their NIC to a phone line even with 1000Base-T. Having said that, how effective that protection is depends entirely on how the NIC was designed and manufactured. I wouldn't test it on purpose unless you don't mind frying something in the process.

The current POE+ standard is 802.3at, and it actually uses either pins 1,2,3,6 OR 4,5,7,8, not just 7 and 8.


There are standards for the physical media, wiring and termination practices, physical interconnecting equipment, network technologies, and so on. Most of these standards depend on or are dependencies for other standards. If you throw T568 out the window, that will have an impact on any standards or technologies which depend on that standard. Ignore T568A (or B) and you won't get 1000Base-T, and perhaps not even POE. If you are going to do things in a non-standard way, then you really need to make sure you understand all of the standards and how they are related. The problem is, new technologies are often developed which rely on existing standards for physical media and terminations.

We already have a topic dedicated to the "should/should not" debate, so lets try and keep this one related to the specifics of the standards.

For reference: POE whitepapers

Right now it's right below this thread. Scroll down to the bottom for a discussion about where 568B came from.

The 568A and 568B standards specify the wiring for all 4 pair. I do get your point though about why, when only 2 pair is used for ethernet someone chose 1,2,3,6 which corresponds to the W/O and W/G pairs for 568B. 568A simply swaps them.

My first guess would be that it was some IT genius who naturally had no idea there was a color code.

But there could be some credence to a concern by the same genius about the W/BL pair on 4&5 being used for phone when everything utilizes patch panels and 8p/8c jacks.

-Hal

The T568B standard was the old AT&T 258A wiring scheme and as AT&T and was heavily present on the standard committees in the beginning they insisted that their wiring scheme be an option