The Media Independent Interface (MII) is a chip commonly found on Ethernet devices, together with the 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

For more info, see http://www.scyld.com/mii_status.html