>>> 2025-01-05 pairs not taken (PDF)

So we all know about twisted-pair ethernet, huh? I get a little frustrated with a lot of histories of the topic, like the recent neil breen^w^wserial port video, because they often fail to address some obvious questions about the origin of twisted-pair network cabling. Well, I will fail to answer these as well, because the reality is that these answers have proven very difficult to track down.

For example, I have discussed before that TIA-568A and B are specified for compatibility with two different multipair wiring conventions, telephone and SYSTIMAX. And yet both standards actually originate within AT&T, so why did AT&T disagree internally on the correspondence of pair numbers to pair colors? Well, it's quite likely that some of these things just don't have satisfactory answers. Maybe the SYSTIMAX people just didn't realize there was an existing convention until they were committed. Maybe they had some specific reason to assign pairs 3 and 4 differently that didn't survive to the modern era. Who knows? At this point, the answer may be no one.

There are other oddities to which I can provide a more satisfactory answer. For example, why is it so widely said that twisted-pair ethernet was selected for compatibility with existing telephone cabling, when its most common form (10/100) is in fact not compatible with existing telephone cabling?

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