Ethernet wiring: "A" or "B" configuration?

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Ethernet wiring: "A" or "B" configuration?

My previous posts concerning this problem are in
http://discussions.apple.com/thread.jspa?threadID=763090

What started out as "how do I get my PowerBook to work on the ethernet wiring in my house" has now become "how do I get my house wiring to satisfy my Powerbook and Intel Duo computer?" Let me explain.

The house was recently built, with ethernet wiring in the walls. It turns out there are two wiring configuration standards for the wall RJ45 jacks, called "A" and "B". Apparently, my electrician used "B" throughout the house.

My Intel Duo and another recent-vintage iMac seem happy with "B", but my OS 9.2 PowerBook doesn't work with "B", and does work with "A".

The electrician has agreed to change out all the "B" jacks and replace them with "A" wiring. But I'm puzzled: if this is really the case -- that B doesn't work with older computers -- then why isn't that better known? And why is B still being used?

I've learned in this discussion that newer computers can auto-detect the difference between a straight-through and crossover cable and adapt to either. Is the difference in these cable configurations the same as the difference in the jack wiring A vs. B?

One last puzzle: the electrican writes "In the project we are now doing the homeowner is a programmer from MAC and he requests that all terminals to be wired on the B Configuration." Hmmm. Can someone explain?

iMac G4 and iMac Intel Duo 24" Mac OS X (10.4.1)

Posted on Dec 14, 2006 9:54 AM

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Answer 1

Dec 14, 2006 2:18 PM in response to Paul-san

Hi, Paul -

I've learned in this discussion that newer computers can auto-detect the difference between a straight-through and crossover cable and adapt to either. Is the difference in these cable configurations the same as the difference in the jack wiring A vs. B?

That's probably the answer - that the newer machines are adapting to the wiring config they are presented with, but the older machie is restricted to just one.

Not sure what the "A" and "B" designations meay refer to - it may be an electrician-type code, or may not even be hard-coded; it may just be a choice, with the installer being consistent in connections.

In any case, I would want to be present and, after part of the wiring is changed to "A", do some testing with the electrician there. It should be nought more than how the wires are connected to the pins in the jacks, and should not entail running new cabling in the walls.

I don't recall seeing a connector/adapter that one could insert at the wall jack to flip the config, but there may be such available.

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Answer 2

eww

Level 9

53,052 points

Dec 14, 2006 2:59 PM in response to Don Archibald

I don't recall seeing a connector/adapter that one could insert at the wall jack to flip the config, but there may be such available.

Hi, Paul and Don. IF the "A" and "B" nomenclature is electrician-speak for "patch" and "crossover" (or vice versa), couldn't one invert an unsuitable A/B choice by the electrician just by using a crossover cable instead of a patch cable between the older Powerbook and the wall jack, and leaving the wiring inside the jack alone? Wouldn't that amount, in effect, to the adapter mentioned in Don's post?

Another way of posing the question: am I right or wrong in thinking that two crossover cables in series equal a patch cable? I don't know much about this stuff, and I may be all wet.

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Answer 3

Dec 14, 2006 9:39 PM in response to eww

Hi, eww -

Your thinking sounds logical, valid, and certainly worth trying.

Assuming that "A" and "B" are referring to Patch vs. Crossover, the bottom drawing on this page -
http://www.incentre.net/incentre/frame/ethernet.html
- reveals that the crossover pattern is symmetrical. That is, pin 1 becomes 3 at the other end, and 3 becomes 1; and 2 becomes 6 and 6 becomes 2. Because the crossover pattern is symmetrical, it should be reversible by adding a second crossover element (a cable) at any point, and would affect that point only.

This would not be the case if the pattern were something like 1 becomes 3, 3 becomes 6, 2 becomes 1, and 6 becomes 2.

***

This assumption is, of course, based upon "A" and "B" not being a communications industry designation standard, where "A" is the old style straight-through (T568A, the top drawing in the article I linked to) and "B" is the new style (T568B, the second drawing in the article). If the latter happens to be the case, then which way the house is wired should make no difference - straight-through is straight-through no matter the colors of the wires; that is, pin 1 remains pin 1 at both ends, as do all other wires.

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Answer 4

eww

Level 9

53,052 points

Dec 14, 2006 10:15 PM in response to Don Archibald

Thanks for your thoughts, Don. It's always comforting to hear that one's thinking, however wrong it may ultimately turn out to be, has made sense to somebody else else whom one admires.

If it turns out I was right, of course the advantage of addressing Paul's problem that way will be that the old Powerbook will be compatible with any of the jacks in the house as they are right now, saving whatever it would have cost to have the electrician require some or all of them.

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Answer 5

Ron JACKLE

Level 7

33,451 points

Dec 15, 2006 12:23 AM in response to Don Archibald

Don Archibald...

"don't recall seeing a connector/adapter that one could insert at the wall jack to flip the config, but there may be such available."

Well... If it is standard crossover wired from wall to wall another standard crossover cable should get you back to a straight wired connection. (And vice-versa)

Of course I have taken a RJ45 coupler apart and reconfigured them for what I needed at a given moment.

Now that I have the same Linksys router I never mind it at all.

+++

Paul R....

Is the technician using high quality shielded cable? Otherwise you can be fighting a lot of crosstalk should the Ethernet wiring and household wiring get a bit comfy in the walls with each other.

Another issue happens when there is too many connection points made to a single line. I got myself into trouble by having 3 couplers in a line rather than have a 30' patch cable made up.

...Ron

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Answer 6

Paul-san Author

Level 1

0 points

Dec 15, 2006 9:37 AM in response to Ron JACKLE

I think the issue is resolved. As you've noted, the choice of wiring color designation standard TIA 568A and TIA 568B doesn't matter one bit -- as long as one is consistent. What happend is that I wired the central distribution panel for the house, where the in-wall cables terminate at punch-down connector blocks (From here, the conductors are internally connected to RJ45 jacks on the block into which I patch outputs from the router). But, the electrician wired the house and the wall jacks. He never asked me which standard I wanted him to use, and just went ahead and chose "B". Turns out the central connector block I wired is compatible with "A" standard (it says so right on the block).

I pulled one of the wall jack connectors and replaced it with a new one, using the "A" color designations, and now it connects with the laptop and the OS X iMac. So it was a simple as that: uncoordinated team wiring. When the house was built (a year ago), I didn't know there were two standards. If the electrician had asked, I would have referred to the installation instructions for the distribution block, which says "use 568A wiring for jacks".

The electrician has agreed to return and rewire all the wall jacks -- at no cost to me!

The details of the A and B standards are unimportant, but it turns out that using one type at each end interchanges the orange and green wire pairs. I think (but am not sure) that this is the same difference as between straight-through and crossover cables. If so, using crossover cables from the wall jack to the computer would unscramble the mixup. But of course it's better to get the house wiring right than to compensate for it.

Thank you, all, for your continuing interest and helpful comments. Having you in this forum to talk to has made the difference between a tooth-grinding frustration and a satisfying exploration. It is satisfying to get to the bottom of this, with your help!

-Paul

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Answer 7

Dec 15, 2006 11:17 AM in response to Paul-san

Hi, Paul -

Great - glad it was that easy to recify.

The details of the A and B standards are unimportant, but it turns out that using one type at each end interchanges the orange and green wire pairs. I think (but am not sure) that this is the same difference as between straight-through and crossover cables. If so, using crossover cables from the wall jack to the computer would unscramble the mixup.

Correct on all counts, based on your previous determination in that post.

All of us got to stretch our knowledge a bit in analysing this, so, thanks go to you, too.

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