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The CAT5 UTP crossover cable has to be one of the most used cables, after the classic straight-through cable. The unshielded twisted pair, or UTP, crossover cable allows us to connect two computers without needing a hub or switch. If you recall, the hub does the crossover for you internally, so you only need to use a straight-through cable from the PC to the hub. Since we don't have a hub now, we need to manually do the crossover.
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Why do we need a crossover?
When sending or receiving data between two devices -- for example, computers -- one device sends, 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. This is exactly what we take into account when creating a UTP crossover 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.
There is only one way to make a CAT5e crossover cable, and it's pretty simple. Those who read lesson two on straight-through UTP cables know a UTP crossover cable is a 568A on one end and a 568B on the other. If you haven't read the wiring section, don't worry, because we'll provide enough information here to help you understand the concept.
As mentioned previously, the purpose of a crossover cable is to connect the transmitting side from one end to the receiving side at the other end, and vice versa.
Let's now have a look at the pinouts of a typical crossover CAT5e cable:
As you can see, only four pins are needed for a crossover cable. When you purchase a crossover cable, you might find that all eight pins are used. These cables aren't any different from the above, because the rest of the pins are not used. Whether your UTP crossover cable has four or eight pins connected, it won't make any difference in performance.
Here are the pinouts for a UTP crossover cable, with all eight pins connected.
Where else can I use a crossover?
UTP crossover cables are not just used to connect computers, but a variety of other devices. Prime examples are switches and hubs. If you have two hubs and you need to connect them, you would usually use the special uplink port that, when activated through a little switch -- in most cases -- prevents the particular port from crossing the TX and RX cables, but leaves them as if they were straight through.
What happens if you haven't any uplink ports or they are already used? The crossover 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 crossover 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 UTP crossover cable to connect the two hubs:
Almost all switches on the market today support the Auto-MDIX feature, allowing the connection between two switches with the use of a straight-through network cable.
Catalog of all 10 cabling lessons
Cabling series, lesson 4: 10Base-T/2/5/F/35 -- Ethernet
Cabling series, lesson 5: 100Base-(T) TX/T4/FX -- Ethernet
Cabling series, lesson 5b: Gigabit Ethernet