Medium dependent interface
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A medium dependent interface is also referred to as an MDI port or an uplink port. Abbreviated MDI, an Ethernet port connection typically used on the Network Interface Card (NIC) or Integrated NIC port on a PC. Since inputs on a NIC must go to outputs on the router, hub or switch, these latter devices have their inputs and outputs (transmit and receive signals) reversed in a configuration known as MDIX or MDI-X. Some network hubs or switches have an MDI port (often switchable) in order to connect to other hubs or switches without an Ethernet crossover cable.
Auto MDI/MDIX ports on newer hubs or switches are designed to detect if the connection is backwards and automatically chooses MDI or MDIX to properly match the connection.
For a diagram of non-automatic connections between MDI and MDIX devices, see donbot.com
[edit] Medium dependent interface crossover
Medium dependent interface crossover or MDIX (the “X” representing “crossover”) is a female RJ-45 port connection on a computer, router, hub, or switch. Straight through cables connect pins 1 & 2 (transmit) on an MDI device to pins 1 & 2 (receive) on an MDIX device. Similarly pins 3 & 6 are receive on an MDI device and transmit on an MDIX device. The X refers to the fact that transmit wires on an MDI device must be connected to receive wires on an MDIX device. As a rule, end stations (i.e., PCs or workstations) and routers use an MDI interface, hubs and switches use MDIX interfaces. Newer routers, hubs and switches (including some 10/100, and all 1 Gigabit or 10 Gigabit devices) use Auto MDI/MDIX to automatically switch to the proper configuration once a cable is connected. The other four wires are used but are not crossed since Auto MDI/MDIX is mandatory at the higher speeds. See Ethernet over twisted pair for more details on cables and standards. Some Routers and other devices have an uplink switch to go back and forth between MDI and MDIX on a specific port.
