Connecting MDI to MDI (no crossover) or MDI-X to MDI-X (two crossovers) requires a(nother) crossover in the cable to get an odd number. Thus, connecting MDI to MDI-X requires a straight-through cable (one crossover in total). The requirement of connecting the transmitter of one side to the receiver on the other side and vice versa makes it necessary to always have an odd number of crossovers between two devices, with an MDI-X port containing an internal crossover. Some routers and other devices had an uplink/normal switch to go back and forth between MDI and MDI-X on a specific port. The general convention is for network hubs, bridges and switches to use the MDI-X configuration, while all other nodes such as personal computers, workstations, servers and routers use an MDI interface.

Similarly, pins 3 and 6 are receive pins on an MDI device and transmit pins on an MDI-X device. Straight through cables connect pins 1 and 2 (transmit) on an MDI device to pins 1 and 2 (receive) on an MDI-X device. The X refers to the fact that transmit wires on an MDI device must be connected to receive wires on an MDI-X device. The terminology generally refers to variants of the Ethernet over twisted pair technology that use a female 8P8C port connection on a computer, or other network device. Some hubs and switches have an MDI uplink port (often switchable) to connect to other hubs or switches without a crossover cable. Conventionally, MDI is used on end devices and routers while MDI-X is used on hubs and switches. When connecting an MDI port to an MDI-X port, a straight-through cable is used, while to connect two MDI ports or two MDI-X ports, a crossover cable must be used. These are referred to as MDI and MDI-X (medium-dependent interface crossover). Since twisted pair cables are conventionally wired pin to pin (straight-through) there are two different pinouts used for the medium-dependent interface. With 10BASE-T and 100BASE-TX, separate twisted pairs are used for the two directions of communication. For fiber, a variety of connectors are used depending on manufacturer and physical space availability.

For twisted-pair cabling 8P8C, modular connectors are used (often incorrectly called 'RJ45' in this context). For 10BASE2, the connection to the coaxial cable was typically made with a single BNC connector to which a T-piece was attached. For 10BASE5, connection to the coaxial cable was made with either a vampire tap or a pair of N connectors. The popular Ethernet family defines common medium-dependent interfaces. Auto MDI-X ports on newer network interfaces detect if the connection would require a crossover, and automatically chooses the MDI or MDI-X configuration to properly match the other end of the link.Įthernet Straight-through MDI to MDI-X connection for 10BASE-T Ethernet over twisted pair also defines a medium dependent interface crossover (MDI-X) interface. Interface between a network device and the data link it communicates over Hub with three MDI-X ports and one switchable port, circa 1998 Switch showing a port in MDI-X (16x) and MDI (Uplink) pinoutsĪ medium dependent interface ( MDI) describes the interface (both physical and electrical/optical) in a computer network from a physical layer implementation to the physical medium used to carry the transmission.

Crossover definition networking