AUTO-MDIX (Automatic Medium-Dependent Interface Crossover)

To connect two ports of the same configuration (MDI to MDI or MDI-X to MDI-X) with a 10 or 100 Mbit/s connection (10BASE-T or 100BASE-TX), an Ethernet crossover cable is needed to cross over the transmit and receive signals in the cable, so that they are matched at the connector level. The confusion of needing two different kinds of cables for anything but hierarchical star network topologies prompted a more automatic solution.

Auto MDI-X automatically detects the required cable connection type and configures the connection appropriately, removing the need for crossover cables to interconnect switches or connecting PCs peer-to-peer. As long as it is enabled on either end of a link, either type of cable can be used. For auto MDI-X to operate correctly, the data rate on the interface and duplex setting must be set to “auto”. Auto MDI-X was developed by Hewlett-Packard engineers Daniel Joseph Dove and Bruce W. Melvin. A pseudo-random number generator decides whether or not a network port will attach its transmitter, or its receiver to each of the twisted pairs used to auto-negotiate the link.

When two auto MDI-X ports are connected together, which is normal for modern products, the algorithm resolution time is typically < 500 ms. However, a ~1.4 second asynchronous timer is used to resolve the extremely rare case (with a probability of less than 1 in 5×1021) of a loop where each end keeps switching.

Subsequently, Dove promoted auto MDI-X within the 1000BASE-T standard and also developed patented algorithms for “forced mode auto MDI-X” which allow a link to be automatically established even if the port does not auto-negotiate. This may or may not be implemented on a given device, so occasionally a crossover cable may still be necessary when connecting auto MDI-X to MDI-X (hub or switch), especially when autonegotiation is deactivated.

Newer routers, hubs and switches (including some 10/100, and all 1 gigabit or 10 gigabit devices in practice) use auto MDI-X for 10/100 Mbit connections to automatically switch to the proper configuration once a cable is connected.

Gigabit and faster Ethernet links over twisted pair cable use all four cable pairs for simultaneous transmission in both directions. For this reason, there are no dedicated transmit and receive pairs, and consequently, crossover cables are never required for 1000BASE-T communication. The physical medium attachment sublayer (PMA) provides identification of each pair and usually continues to work even over cables where the pairs are unusually swapped or crossed.

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