The crossover (or crossover) CAT5 UTP cable has to be one of the most used cables after the classic straight-thru...
cable. The 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 thru cable from the PC to the hub. Since now we don't have a hub, we need to manually do the crossover.
Why do we need a crossover?
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 a 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 CAT5 crossover cable and it's pretty simple. Those who read the "wiring UTP" section know a 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 I'll be giving you enough information to understand what we are talking about.
As mentioned previously, a crossover 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 crossover CAT5 cable:
As you can see, only four pins are needed for a crossover cable. When you buy a crossover cable, you might find that all eight 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 crossover cable which has all eight pins connected:
Where else can I use a crossover?
Crossover 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.
What happens though if you haven't got 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 crossover cable to connect the two hubs:
Cabling tips for network professionals series
Lesson 1: Network history and fundamentals
Lesson 2: Straight-through UTP cables
Lesson 3: CAT5 UTP crossover cable
Lesson 4: 10Base-T/2/5/F/35 - Ethernet
Lesson 5: 100Base-(T) TX/T4/FX - Ethernet
Lesson 6: Fiber cable
Lesson 7: Direct cable connection
Lesson 8: Serial direct cable connection
Lesson 9: Parallel direct cable connection
Lesson 10: USB direct cable connection
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