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Media-independent interface
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<p>The media-independent interface (MII) was originally defined as a standard interface to connect a <a href="/facts/Fast_Ethernet/YM5HjX0C">Fast Ethernet</a> (i.e., 100 Mbit/s) <a href="/facts/Medium_access_control/hFpNzFFD">medium access control</a> (MAC) block to a <a href="/facts/PHY/tkEaBnt9">PHY chip</a>. The MII is standardized by <a href="/facts/IEEE_802.3u/YM5HjX0C">IEEE 802.3u</a> and connects different types of PHYs to MACs. Being <i>media independent</i> means that different types of PHY devices for connecting to different media (i.e. <a href="/facts/Ethernet_over_twisted_pair/2jRlgJb4">twisted pair</a>, <a href="/facts/Fiber_optic/5ruuIFbj">fiber optic</a>, 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 medium. </p><p>The MII can be used to connect a MAC to an external PHY using a pluggable connector, or directly to a PHY chip on the same <a href="/facts/Printed_circuit_board/muSR2p43">PCB</a>. On older PCs the <a href="/facts/Communications_and_networking_riser/JlRehm8Q">CNR connector</a> Type B carried MII signals. </p><p>Network data on the interface is <a href="/facts/Ethernet_frame/LC4BXFhI">framed</a> using the IEEE <a href="/facts/Ethernet/nMfhU8tU">Ethernet</a> standard. As such it consists of a preamble, start frame delimiter, Ethernet headers, protocol-specific data and a <a href="/facts/Cyclic_redundancy_check/u2exo4sv">cyclic redundancy check</a> (CRC). The original MII transfers network data using 4-bit <a href="/facts/Nibble/PUaqCEVw">nibbles</a> 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: </p> <ul><li>Reduced media-independent interface (RMII)</li> <li>Gigabit media-independent interface (GMII)</li> <li>Reduced gigabit media-independent interface (RGMII)</li> <li>Serial media-independent interface (SMII)</li> <li>Serial gigabit media-independent interface (serial GMII, SGMII)</li> <li>High serial gigabit media-independent interface (HSGMII)</li> <li>Quad serial gigabit media-independent interface (QSGMII)</li> <li>Penta serial gigabit media-independent interface (PSGMII)</li> <li>10-gigabit media-independent interface (XGMII)</li></ul> <p>The <a href="/facts/Management_Data_Input/Output/sl8lAs2n">Management Data Input/Output</a> (MDIO) serial bus is a subset of the MII that is used to transfer management information between MAC and PHY. At power up, using <a href="/facts/Autonegotiation/dHFQJDEs">autonegotiation</a>, the PHY usually adapts to whatever it is connected to unless settings are altered via the MDIO interface. </p>