Media Independent Interface (MII)

Media Independent Interface (MII)
FastEthernet requires a different interface than 10BaseT Ethernet. 10Mbps Ethernet used the
attachment unit interface (AUI) to connect Ethernet segments. This provided a decoupling of the
MAC layer from the different requirements of the various Physical layer topologies, which
allowed the MAC to remain constant but meant the Physical layer could support any existing and
new technologies. However, the AUI interface could not support 100Mbps Ethernet because of
the high frequencies involved. 100BaseT needed a new interface, and the media independent interface
(MII) provides it.
Using Ethernet Media in Your Network
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100BaseT actually created a new subinterface between the Physical layer and the Data Link
layer, called the Reconciliation Sublayer (RS). The RS maps the 1s and 0s to the MII interface.
The MII uses a nibble, which is defined as 4 bits. AUI used only 1 bit at a time. Data transfers
across the MII at one nibble per clock cycle, which is 25MHz. 10Mbps uses a 2.5MHz clock.