MII & RMII (ethernet)

RMII means reduced MII interface.
The interface clock is 50Mhz instead of 25Mhz. Due to this higher clock speed you need instead of 4 data signals (tx+rx) only 2.
Some control signals are also merged together.

For single Ethernet PHY/MAc I would recommend to use MII. MII is more popular and it is cheaper.

Bernie
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i think RMII is much more efficient than MII as concerned with no.of pins used.
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MII comprised of 16 pins for data and control is defined. and frequency is 25 mhz . but RMII is 8 pin interface and a single reference clock with 50 mhz
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No, you don't loose nothing. You still have 100Mbps in the RMII (2 data lines x 50MHz = 100MBps) and the control signals are still there as in the MII.
For 1Gbps you need RGMII - 4 DDR data lines and 1 control line plus clock (125MHz). So 4x125=500Gbps but because the data lines are DDR signals you get 2x4x125=1Gbps.
Also, there the serial equivalent for the MII signals, It's the SMII, 1 data line and one 125Mhz clock. It's 125Mhz clock because 2 of the bits are used control. And for 1Gbps you have SGMII with 1 data line and one 1.25GHz clock.
Have a look at the IEEE 802.3 standard for better explanation.
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