Electronic – Reasoning behind Ethernet pair order


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

Best Answer

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.