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An Ethernet connector seems to use a surprising choice of signal placement in its RJ45 connector. Does anyone know the reasoning behind:

  • The decision to make pair 2 straddle pair 1?
  • Why Ethernet uses Pairs 2&3 as in the left hand image?
  • Why Ethernet does not use E.G. Pairs 1&4 as in the right hand image?

The choice seems odd at the point it comes to lay out the PCB. If we are to make nice differential pair traces, the choice of signal placement is surprising.

Ethernet pairs

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  • \$\begingroup\$ If I remember right, something about symmetric about the middle was the existing telco convention for RJ connectors. That doesn't explain pairs 3 and 4 though. \$\endgroup\$
    – Olin Lathrop
    Commented Apr 19, 2012 at 11:54
  • \$\begingroup\$ Does Power Of Ethernet go on pins 4 and 5 now a days? I also wondered why these plugs were paired like that.. never actaully found out. Good question! \$\endgroup\$
    – Piotr Kula
    Commented Apr 19, 2012 at 12:07
  • \$\begingroup\$ It's worth nothing, but probably there is a good reason behind that...just to find what! \$\endgroup\$
    – clabacchio
    Commented Apr 19, 2012 at 12:11
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    \$\begingroup\$ Hmm, this might be a duplicate \$\endgroup\$
    – Rocketmagnet
    Commented Apr 19, 2012 at 12:31
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    \$\begingroup\$ If I remember the legend correctly, the telecommunications traditions say that on the plugs the pairs should be something like this 43211234. Originally that was the idea for Ethernet too, but they found out that the distance between the cables 3rd and 4th pairs is too big and that it would have negative results on data transfer speeds, so they decided to follow traditions for first two pairs and then put the 3rd and 4th together the way the are now. \$\endgroup\$
    – AndrejaKo
    Commented Apr 19, 2012 at 12:44

1 Answer 1

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Amongst other things, one important consideration was that a few years back office buildings had structured cabling. Same cabling and sockets were used for both analog telephony (RJ11) and Ethernet (RJ45). RJ11 fits the same socket, but it only connects the middle four pins. The problem with Ethernet on the same cabling system as a PABX is the moment when I attach your NIC (network interface card) to a socket that is patched to the PABX (telephony system). No problem so far, until ... someone calls the line and the ringing voltage (not sure of the proper english term, the voltage to make the telephone bell ring) easily blows your ethernet card because of very high voltage ( > 100V ). It fries either your NIC or your Ethernet hub.

Another consideration is not being able to accidentally making loops in the network by patching two hub ports or two NICs together. The reason why we still sometimes use cross cables.

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    \$\begingroup\$ Cross cables are needed because 10/100 devices use one pair to transmit and one to receive, like in rs422. Hubs have the transmit and receive pairs reversed so you can use a straight cable when you connect a NIC to hub. Gigabit Ethernet uses all pairs and not longer has the distinction of transmit/receive pairs, so you can use cross or straight cables connecting two NICs, they sort it out automatically. \$\endgroup\$
    – Pentium100
    Commented Apr 20, 2012 at 7:43
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    \$\begingroup\$ Many modern NIC's and switches are able to 'auto cross', not just the Gb ones. It only works for copper, not for glass ;o) \$\endgroup\$
    – jippie
    Commented Apr 20, 2012 at 7:47
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    \$\begingroup\$ Yes, but the auto cross is part of the Gb standard, while there are some 10/100 hubs/switches (usually older) that cannot do it. Also, AFAIK the common 10/100 NICs also can't do it, gigabit ones can do it for any speed though. \$\endgroup\$
    – Pentium100
    Commented Apr 20, 2012 at 7:50
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    \$\begingroup\$ Not sure what common NIC is, but in my experience as system and network administrator for many years, auto MDI/MDX was quite proliffic long before I used a gigabit for a first time. That is if I remember it correctly, ofcourse. Also admitedly I wasn't at the bleeding edge of technologies. But it is part of Gb standart and just very convinient feauture implemented at large by manufacturers for 100Mb at the time, AFAIK. \$\endgroup\$
    – zzz
    Commented May 12, 2014 at 8:07

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