UTP network cables have eight wires. Some of these are used for sending information, while others are used for receiving.
When one device uses a wire to transmit information, the other device must use that same wire to receive it. If they both used the same wire to transmit, the signal wouldn’t get through.
In times past, some devices would be called MDI devices, and others would be called MDI-X devices. There is only one difference: which network card pins are used for transmitting and receiving.
Suppose you wanted to connect two identical devices, like a workstation and another workstation, or a switch and another switch, then you would need a crossover cable. Some wires within the cable are crossed over to align the transmitter and receiver.
In order to connect an MDI device to an MDI-X device, such as connecting a workstation to a switch, you would use a straight-through cable.
In a modern network, this is much less of a hassle. We don’t need crossover cables anymore, as we have Auto-MDIX. This is a feature built into network cards and switches.
This automatically detects which pins are connected, and automatically adjusts which pins are used for sending and receiving. These days, we only need to use straight-through cables.