Management Data Input/Output (MDIO), also known as Serial Management Interface (SMI) or Media Independent Interface Management (MIIM), is a serial bus defined for the Ethernet family of IEEE 802.3 standards for the Media Independent Interface, or MII. The MII connects Media Access Control (MAC) devices with Ethernet physical layer (PHY) circuits. The MAC device controlling the MDIO is called the Station Management Entity (SME). (source: Wikipedia)