Most importantly, the two wires that make up the pair really just are common single-ended wires, not elaborate coax or anything else.
A single coax transmission line supporting 10Gbps Ethernet would likely be much more expensive than the little bundle of twisted pairs we typically use nowadays.
In many ways, for its applications, twisted pair and RJ45 connections are better than coax wiring with BNC.
Yeah, but mechanically, the RJ45 plug with its finicky easily breakable plastic tab can be an annoyance. And it's easy to see that the pin ordering is not ideal, with the pair in the middle splitting another pair. AFAIK, there exists a more robust connector (the M12 connector), but it doesn't seem to be that common.
Maybe M12 is that, but it looks way more expensive at first glance. Possibly more laborious to connect/disconnect, too, with its screw-locking? Seems to be better for applications where a secured connection is more important (transportation is mentioned).
And yeah, the 1000base-T pin ordering seems unusual. I'm curious about the history there, because even 10base-T (where I thought Ethernet for Twisted Pair begun) had this really weird pinout, which does not support my initial theory that it was because Ethernet kept progressively adding more differential pairs: https://www.arcelect.com/10baset.htm It may well be because they added the original two pairs to a pinout that already carried something else, but the diagrams don't say what those other lines were for, so if anyone knows...
According to those same diagrams, though, it seems to be more common to split up the pairs than not, which now makes me wonder if there is any benefit to that?
Telephones. Telephones are why. Those other two pairs were often used for voice communication. If you had four-pair station cabling, the pairs were provisioned on the modular jack from the inside out. So line one was the blue/blue-white pair on the inner pins, line two was the orange/orange-white pair on the next two pins, and so on.
Ethernet comes along and lots of places where you'd want a network connection already had a phone jack with two pairs unused, so for signal integrity reasons those are moved to the outside and used for data, leaving the inner two pairs where they were to be used for voice.
But why 4 pairs in the first place?
Just about the time that Ethernet was transitioning from coax to twisted pair, the digital PBX was taking over from key systems (1A2) and reduced the number of wires required for a business telephone from 25 pairs (or more ... secretarial sets often had 100 or more pairs) per station down to 4 (for HORIZON[0]) and later two pairs (DIMENSION and eventually Merlin, Definity, etc.). So if you're wiring a new building, you can just run one CAT-3[1] cable to each desk and use the first two pairs for voice and the second two for data[2].
[0] OK, for the pedants out there, HORIZON wasn't ever very popular and really pre-dated Ethernet, but the telecom world moves kinda slow [1] Wasn't really CAT-3 until the early 90s [2] Not on the same jack, but by using pins 1, 2, 7, and 8 for data, you can plug the wrong cable in without risk of hurting the phone or your computer's network card
> so for signal integrity reasons those are moved to the outside and used for data
I'm not sure about that bit, though. Would keeping the pair together not help with signal integrity?
But 10BASE-T uses pins 1, 2, 3, 6, thus green and orange pair.
I've seen somewhere that a pair of the two outer lines didn't have sufficient performance, so the outer pairs needed to wired side by side instead, but I don't have a reference. Also, there's a reasonable question of why use one inner pair and one outer pair, and not both inners or both outers.
That is not really the fault of the RJ45 specifications. The choice is available between cheap breakable connectors or reliable well-designed connectors: it isn’t the fault of the specification that cheap is often chosen.
> the pin ordering is not ideal
A very minor nitpick. And designed that way for specific reasons.
I like that it works well, was backwards compatible, and the connectors, wiring, and tools are cheap, available, and abundant. 1000Base-T is amazing technology (even if we are blasé about it!)
1. https://community.fs.com/blog/what-is-multigig-ethernet.html
Do you know that reason? I was wondering in my other reply.
Nothing stops you from wiring connectors different way though, to the annoyance of anybody splicing that cable in the future :)
You can only swap the wires in a pair (eg swap solid orange and striped white/orange), or swap pairs (eg blue pair with orange pair).
You must keep each twisted pair matched, so that the electrical signal travels correctly, otherwise you run into problems at length. For very short runs I would guess you could ignore pairs, even though it would increase crosstalk (between signals within a cable) and interference (aerial like transmission or reception, especially with different cables run close together).
Great picture of different twists per centimetre comparing each pair within a cable (compare orange against blue in one cable), and also shows the different twists for different cats (less for cat5, more for cat6): https://store.chipkin.com/articles/differences-between-categ... — if the twist lengths of all pairs were exactly the same within a cable, there would be a lot more crosstalk between pairs.