Media Independent Interface

The media-independent interface (MII) was originally defined as a standard interface to connect a Fast Ethernet (i.e., 100 Mbit/s) media access control (MAC) block to a PHY chip. The MII is standardized by IEEE 802.3u and connects different types of PHYs to MACs. Being media independent means that different types of PHY devices for connecting to different media (i.e. Twisted pair copper, fiber optic, etc.) can be used without redesigning or replacing the MAC hardware. Thus any MAC may be used with any PHY, independent of the network signal transmission media.

The MII can be used to connect a MAC to an external PHY using a pluggable connector, or directly to a PHY chip which is on the same PCB. On a PC the CNR connector Type B carries MII bus interface signals.

The Management Data Input/Output (MDIO) serial bus is a subset of the MII that is used to transfer management information between MAC and PHY. At powerup the PHY usually adapts to whatever it is connected to (autonegotiation) unless settings are altered via the MDIO interface.

The original MII transfers data using 4-bit words (nibbles) in each direction (4 transmit data bits, 4 receive data bits). The data is clocked at 25 MHz to achieve 100 Mbit/s throughput. The original MII design has been extended to support reduced signals and increased speeds. Current variants include, reduced media-independent interface (RMII), gigabit media-independent interface (GMII), reduced gigabit media-independent interface (RGMII), 10-gigabit media-independent interface (XGMII) and serial gigabit media-independent interface (SGMII).

The standard MII features a small set of registers:

The MII Status Word is the most useful datum, since it may be used to detect whether an Ethernet NIC is connected to a network. It contains a bitmask with the following meaning:

A more detailed reference on registers exported by MII-compatible PHY's can be found looking at the Linux MII interface definition include/linux/mii.h

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