[General] What is the difference between MDI and MDI-X connectors?

In 10Base-T and100Base-TX specifications, two pairs of wires are used to propagate a signal between nodes. When two nodes are connected, the transmitting line of one node must be connected to the receiving line of the other, and vice-versa. This is called a "cross." Introducing another cross creates a straight-through connection. As a rule of thumb, we say, "An odd number of crosses are needed in a connection between two nodes." A standard UTP/RJ45 cable has straight-through wiring, i.e., pin to pin between the end of plugs. The jacks on a typical hub have crossed wiring, allowing the use of standard cabling. Hub devices are typically configured as crossed rather than straight, as NIC cards are configured. This crossing allows connecting a NIC to a hub device using a standard cable. If you want to connect a NIC card directly to another NIC or hub to another hub, you will have to use a special cable in which two pairs of wires are crossed. This cable is called a "crossover" cable. Some hubs provide a special port that allows you to connect the hub to another hub's MDI-X port without using a crossover cable. This port is configured as MDI. It is often called an up0link port or MDI-II port.