Medium Dependent Interface (MDI)

Medium dependent interface, or MDI, describes the assignment of transmit and receive lines on an Ethernet port. End devices such as PCs and servers use the MDI assignment, while switches and hubs use the crossed MDI-X variant, in which transmit and receive pairs are swapped. This allows an end device and a switch to be connected with a straight-through patch cable, whereas connecting two identical device types previously required a crossover cable. Modern network components support auto MDI-X and detect the required assignment automatically, making the cable type irrelevant. From Gigabit Ethernet onwards, this function is part of the standard. MDI remains relevant mainly with legacy equipment, in industrial environments and when troubleshooting links that fail to establish.