Why are Crossover Cords Needed?

Why are Crossover Cords Needed?
Imagine a network card with RJ-45 port, which is connected by a "normal" straight-through patch cord to a RJ-45 port in a hub.

"Straight-through" means that you connect Pin 1 at the one side of the cord to Pin 1 at the other side etc.

At the network card, pins 1 and 2 of the RJ-45 port are used for transmit, and pins 3 and 6 for receive.

At the hub ports (called MDI-X ports), pins 1 and 2 are used for receive, and pins 3 and 6 for transmit. This arrangement is called *internal crossover* and is done to internally enable communication between the network card and hub (Transmit pins of the network card connect to Receive pins of the hub etc.) and allowing using normal, straight-through cords.

But what to do, if you want just to connect two stations with the network cards *without* using a hub?

If you do it using a normal straight-through patch cord, you will connect the Receive pins at the one end with the Receive pins at other end AND the Transmit pins at the one end with the Transmit pins at other end. This way the system will not work.

For doing the system functional, you will have to use the *external cross-over* by using a cross-over cord, which connects the Receive pins of the first network card with the Transmit pins of the second card, and the Transmit pins of the first network card with the Receive pins of the second card.

Additionally, you may need a cross-over cord when connecting your management computer to the management port of a hub or switch (see appropriate manuals for details).

The pin out of the 10Base-T/100Base-T cross-over cord can be find at this link:
http://www.cabling-design.com/references/pinouts/10-100base_t_crossover.shtml

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