Page "Fast Ethernet" Paragraph 3

A fast Ethernet adapter can be logically divided into a Media Access Controller ( MAC ) which deals with the higher level issues of medium availability and a Physical Layer Interface ( PHY ).

The MAC may be linked to the PHY by a 4 bit 25 MHz synchronous parallel interface known as a Media Independent Interface ( MII ) or a 2 bit 50 MHz variant Reduced Media Independent Interface ( RMII ).

Repeaters ( hubs ) are also allowed and connect to multiple PHYs for their different interfaces.