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I'm trying to understand how crossover cables work, and am looking at this image: enter image description here

I can understand from the image that the transmission wires are swapping with the receive wires, and I know why that is important, but I'm struggling to see why the right hand side shouldn't be vertically flipped from what it is in the image above.

My thoughts are this: imagine a laptop with an ethernet port on its right side, and on the left hand side of the port is the TX+ pin. Now imagine another laptop on the right of the first, with an ethernet port on its left side and, again, having the TX+ pin on the left side of the port. The crossover in the image above would connect the RX+ to the right side of the rightmost laptop's port, so that rather than connecting to the TX+ it is connecting it to one of the unused wires.

I'm not sure how confusing that came across, but I've (badly) drawn a diagram to try and show what I'm trying to explain (L meaning left relative to the arrow, and R meaning right relative to the arrow, with the arrow just being the way the port is facing):

enter image description here

Which part am I getting wrong?

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It would be better if the image showed the pin numbers:

enter image description here

When you are looking at the laptop port, pin 1 is on the left, pin 8 on the right. It wouldn't matter if the port was on the left or right side of the laptop, as the pins would always be in the correct order if you are facing the port.

  • 1
    It does depend on which way up the socket is! With tab-hole downwards, the sockets connectors have pin 1 on the left. – jonathanjo Nov 4 '17 at 13:47
  • Yes, that is true, hopefully the laptop will be the right way up – Karl Billington Nov 4 '17 at 13:49
  • My point is that you never know whether the sockets are mounted tab-up or tab-down on the equipment, and many switches have both. – jonathanjo Nov 4 '17 at 14:06
  • True, definitely something to watch out for on switches and as you say some switches have the top row of ports one way and the bottom row the other way – Karl Billington Nov 4 '17 at 14:09
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It doesn't make any sense until it's clear there are two types of twisted-pair ethernet socket, and they are almost never labelled. One is found on laptops/desktops/etc and is called "MDI"; the other is found on hubs/switches/etc and is called "MDI-X", for "crossover".

The following is how it is for 10baseT:

  • On the computer (MDI) it transmits on 1+2 and listens on 3+6.
  • On the switch (MDI-X) it listens on 1+2 and transmits of 3+6.

Thus each transmitter is connected to the other's receiver.

You can see that if you plug computer-to-computer (or switch to switch) you need either:

  • A crossover cable
  • Computers with "auto-crossover" (now extremely common)

100baseT is usually actually "100base-TX" and works the same, but more complexity applies to less-common variants and to 1000baseT.

Obviously the same is the case for switch-to-switch connections, except that before auto-crossover was common, there was usually an "uplink" socket, which was MDI, for plugging into the upstream switch.

  • Also watch out for alternative wiring for ethernet crossovers, which are wired with pins 4+5, 7+8 straight through; these are okay for 2-pair ethernet but not full-duplex 4-pair ethernet. – jonathanjo Nov 4 '17 at 14:16
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All Ethernet ports count the contacts in the same way, so with a straight, 1:1 cable you connect port A's contact 1 to port B's contact 1, A's contact 2 to B's contact 2 and so on. Crossover cables connect two ports in such a way that transmitter contacts on the one side are connected to receiver contacts on the other side and vice versa.

This is only necessary when connecting two ports of the same type to each other (MDI to MDI, or MDI-X to MDI-X). Most often, MDI ports are connected to MDI-X port which already have swapped contacts, so you use a 1:1 cable.

Note that due to today's Auto MDI-X and Gigabit+ Ethernet, crossover cables aren't usually required anywhere any more.

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