In networking, MDI stands for Medium Dependent Interface. It refers to the physical and electrical interface on a network device (like a computer's network interface card (NIC) or a router port) that connects to the network transmission medium, typically a twisted-pair Ethernet cable (like Cat5e or Cat6).
Key points about MDI:
- Pinout: An MDI port uses a specific pinout where pins 1 and 2 are typically used for transmitting (TX) data, and pins 3 and 6 are used for receiving (RX) data (for 10/100 Mbps Ethernet).
- Connection: End devices like computers and routers usually have MDI ports.
- MDI vs. MDI-X: Network infrastructure devices like hubs and switches traditionally used MDI-X (MDI Crossover) ports. MDI-X ports swap the transmit and receive pairs internally (pins 1 & 2 are RX, pins 3 & 6 are TX). This allows a standard "straight-through" Ethernet cable to connect an MDI device (computer) to an MDI-X device (switch).
- Cabling: Connecting two MDI ports (e.g., computer to computer directly) or two MDI-X ports (switch to switch) traditionally required a "crossover" cable, which swaps the TX and RX pairs within the cable itself.
- Auto MDI-X: Modern network interfaces often feature Auto MDI-X. This technology automatically detects the type of port (MDI or MDI-X) on the other end of the cable and configures its own port accordingly. This eliminates the need for specific straight-through or crossover cables, as the interface handles the necessary pin swapping electronically. Most modern devices support Auto MDI-X.