Medium Dependent Interface (MDI)

Medium Dependent Interface: Configuration of transmit and receive lines

The Medium Dependent Interface (MDI) refers to the configuration of transmit and receive lines at an Ethernet port. It defines which wires within a network cable are used for transmitting data and which are used for receiving it.

End devices - such as PCs, servers, or printers - typically use the MDI configuration. Switches and hubs, by contrast, use a crossed configuration known as MDI-X (Medium Dependent Interface Crossover). In MDI-X, the transmit and receive pairs are swapped. This allows an end device to be connected to a switch using a straight-through patch cable: the switch ensures that the output of one device is connected to the input of the other.

In the past, connecting two similar devices directly - for example, two PCs or two switches - required a "crossover cable." In such a cable, the relevant wire pairs are intentionally swapped so that the transmit and receive lines align correctly. Alternatively, some network devices feature fixed MDI-X ports, where the line configuration is already crossed.

Most modern network components support "Auto-MDI-X." This feature automatically detects how the transmit and receive lines should be configured, allowing the use of straight-through patch cables regardless of the device type. From the Gigabit Ethernet standard onwards, Auto-MDI-X has been an integral part of the technical specification.

Consequently, MDI and MDI-X are generally transparent in today’s network environments. They remain relevant, however, when dealing with older equipment, in industrial settings, or during troubleshooting - specifically when an Ethernet connection fails to establish a link despite the cable being correctly plugged in.