Media Independent Interface
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. The MII bus (standardized by IEEE 802.3u) connects different types of PHYs (Physical Transceivers) to Media Access Controllers (MAC). Thus any MAC may be used with any PHY, independent of the network signal transmission media. The MII bus transfers data using 4-bit words (nibble) in each direction (4 transmit data bits, 4 receive data bits). The data is clocked at 25 MHz to achieve 100 Mbit/s speed.
The MII can be used to connect a MAC to an external PHY using a pluggable connector (shown in the figure (extreme right)), or direct to a PHY chip which is on the same printed circuit board. On a PC the CNR connector Type B carries MII bus interface signals.
The MDIO Serial Management Interface (SMI) (see MDIO) is used to transfer management information between MAC and PHY.
The standard MII features a small set of registers:
- Basic Mode Configuration (#0)
- Status Word (#1)
- PHY Identification (#2, #3)
- Ability Advertisement (#4)
- Link Partner Ability (#5)
- Auto Negotiation Expansion (#6)
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:
0x8000 Capable of 100baseT4 0x7800 Capable of 10/100 HD/FD (most common) 0x0040 Preamble suppression permitted 0x0020 Autonegotiation complete 0x0010 Remote fault 0x0008 Capable of Autonegotiation 0x0004 Link established 0x0002 Jabber detected 0x0001 Extended MII register exist.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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