568a or 568b

Two part question here.

1) does motorola recommend either 568a or 568b for sm to surge.

2) any difference between the terminations besides colors?

I have read many different explanations to this question and am looking for “Motorolas” answer to this. To me there should be no difference as long as both ends are the same.

I have done a couple hundred of these installs with great luck until last week when one of our techs changed from 568a to 568b on an exsisting install and the customers service has become more stable since then???

I would invite everyones opinion on this subject as well.


The short answer:

ANSI/TIA/EIA wiring standards T568A and T568B specify only color-coding for RJ-45 jacks and plugs. The standards do not cover any electrical or performance specifications. Your conclusion is correct that there’s no difference as long as both ends are the same. In fact, you could wire a T568A cable from Canopy SM to 300SS, and T568B from 300SS to power supply; this might be confusing, but it would work just fine. If you found switching standards at a particular installation improved its reliability, it’s likely you had simply replaced a faulty connection.

The long answer:

Historically, T568B has been more prevalent because it is compatible with an old AT&T standard, 258A. Today, however, T568A is preferred and is considered “standard”. I’ve read that T568A is a requirement for federal contracts.

The reason both standards exist is a bit hard to imagine when, today, RJ-45 connectors – either jack or plug – are nearly always attached to both ends of every 4-pair cable run or patch cable. This has not always been the case. In the past, when one end of a cable was an RJ-45 jack, the other end most likely connected to a type-66 punch block. The color-coding on blocks and within multi-paired cables was the following:

Primary colors: White, Red, Black, Yellow, and Violet.

Secondary colors: Blue, Orange, Green, Brown, and Slate.

The sequence of cable-pairs is: Wh/Blu, Wh/Org, Wh/Grn, Wh/Brn, Wh/Slt, Red/Blu, Red/Org, Red/Grn… and so on through Vio/Grn, Vio/Brn, Vio/Slt. This only covers the sequence for cables up to 25 pairs, the type of cable terminated with an RJ-21 (commonly called “Amphenol”) connector. Within each pair, the Primary/Secondary-colored wire is first, followed by the Secondary/Primary-colored wire; for example, Wh/Blu, then Blu/Wh. (I believe this sequence within a pair coincides with Tip and Ring.) Color sequence, therefore, specifies pair numbering in the larger scheme, and this sequence is significant all the way to an RJ-45 jack.

The reason both T568A and T568B exist is because the two standards specify different pair numbers for the RJ-45 pins. Colors are simply used to identify the pair number.

We have always used 568b. I think this is what Motorola recommends as well.

I will back up Teknix on this. The two standars are functionally the same. The only difference is the color coding. if one stadard was better than the other, then why would a crossover cable be A at one end and B at the other? So one end would perform better than the other? Nope, they are the same, just different colors on different pins.


Careful! An ethernet crossover cable is not T568A at one end and T568B at the other. Yes, the orange and green pairs are reversed in a crossover cable, but the polarity of each pair is also reversed.

If a crossover cable is T568A or B at one end, the other end then conforms to neither standard. A crossover cable is simply a bastardized wiring scheme used to create a useful tool within the IEEE twisted-pair Ethernet standards specifying RJ-45 pinouts.

This is why – when attempting to connect two PCs back-to-back with a straight-thru Ethernet cable, and it doesn’t work, of course, and it has just occured to you why, so you reach for the crossover cable but it’s lying just out of your reach – the proper expression to the person hovering over you – the person wanting to help but not knowing how, or knowing how but you’re too stuborn to get out of the way – is… “hand me that bastard, please!”

If you look at the T568A and B stadards, you will see that the polarities are NOT changed when switching between the two. Yes, the pairs move, but the conductors in each pair stay the same with reference to one another. Therefore the polarities stay the same.

In a full PoE cross-over cable, the same is true. However, pairs 1 and 4 are also crossed over, causing a change in Polarity since the majority of PoE devices use pair 1 for +V and pair 4 for -V.

Also, I prefer to shout out "throw that F**king cable over here!"


The change in polarity I was referring to was within the crossover cable, not the TIA standards. But I was still wrong. I don’t know when I was mis-taught – or more likely mis-learned – but an Ethernet crossover cable should not have the polarity reversed within any pairs. The reason I’ve gotten away with my mistake is because the chips implementing the twisted-pair Ethernet physical interface are made to auto-polarize.

You are correct when you state a crossover cable should be wired T568A at one end and T568B at the other. While this may simply be a coincidence, it provides an easy way to remember how to built one. Thanks for pointing out my error.

I am confused, however, about your statements concerning “full PoE” crossover cables. I’ve found no documentation stating or suggesting the unused Ethernet pairs (1&4) should be swapped in a crossover cable. I have, on the over hand, found manufacturers specs for PoE power supplies – for the power-consuming end – that list “polarity-reversal protection” as a feature. It’s not clear to me whether this protection allows the unit to operate with reversed polarity, or if it simply prevents destructive failure of the device.

I also don’t know if the Canopy units will operate with pairs 1&4 swapped at one end; the possibilty of damage to the Canopy makes me unwilling the test this. The Canopy units, however, are not IEEE 802.3af-compatible. Making comparisons between Canopy and standards-based PoE may be misleading.

I will clarify also - I have some short cables that have pairs 1 and 4 crossed over as well. I do not know who makes them, they came with some RoIP equipment from Vega. They do say “Cat6.” on one side of the connector and “HUB” on the other (connector manufacturer I would guess). Since pairs 1 and 4 are not used in ethernet, but are in PoE, I assume they could only be for PoE crossover. :?: Just a guess at this point.

Also, I believe the polarity reversal protection prevents damage to the equipment when this occurs. I two guys that, for some reason, tried hooking them up backwards (I don’tthink they meant to, just did their connectors wrong, one guy at each end) and the radio would not power up. Correct the connections and everything was fine. The hardware side of Canopy seems very robust.


I know that Canopy units from as recently as ayear ago died with reverse polarity on power leads. They were talking then about diode protecting them. I assume they have by now.