The cross-over (x-over) CAT5 UTP cable has to be one of the most used cables after the classic straight-thru cable. The x-over cable allows us to connect two computers without needing a hub or switch. If you recall, the hub does the x-over for you internally, so you only need to use a straight thru cable from the PC to the hub. Since now we don't have a hub, we need to manually do the x-over.
Why do we need an x-over?
When sending or receiving data between two devices (I.E. computers) one will be sending while the other receives. All this is done via the network cable and if you look at a network cable you will notice that it contains multiple cables. Some of these cables are used to send data, while others are used to receive data and this is exactly what we take into account when creating an x-over cable. We basically connect the TX (transmit) of one end to the RX (receive) of the other! The diagram below shows this in the simplest way possible:
As mentioned previously, an x-over cable is as simple as connecting the TX from one end to the RX of the other and vice versa.
Let's now have a look at the pinouts of a typical x-over CAT5 cable:
As you can see, only 4 pins are needed for a x-over cable. When you buy an x-over cable, you might find that all 8 pins are used, these cables aren't any different from the above, it's just that there are cables running to the unused pins. This won't make any difference in performance, but is just a habit some people follow.
Here are the pinouts for a x-over cable which has all 8 pins connected:
There is only one way to make a CAT5 x-over cable and it's pretty simple. Those who read the "wiring utp" section know an x-over cable is a a 568A on one end and a 568B on the other. If you haven't read the wiring section, don't worry because I'll be giving you enough information to understand what we are talking about.
Where else can I use an x-over?
What happens though if you haven't got any uplink ports or they are already used? The X-over cable will allow you to connect them and solve your problem. The diagram below shows a few examples to make it simpler:
As you can see in the above diagram, thanks to the uplink port, there is no need for a x-over cable.
Let's now have a look at how to cope when we don't have an uplink to spare, in which case we must make a x-over cable to connect the two hubs:
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X-over cables are not just used to connect computers, but a variety of other devices. Prime example are switches and hubs. If you have two hubs and you need to connect them, you would usually use the special uplink port which, when activated through a little switch (in most cases), makes that particular port not cross the tx and rx, but leave them as if they where straight through.