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Chapter 6. Ethernet > Media Independent Interface

6.7. Media Independent Interface

All Ethernet transmission systems defined by the 802.3 standards share the same MAC protocol. The 802.3-defined MAC basically needs to know if the medium is free before the transmission and if there is a collision during the transmission. Therefore, different physical layers can be presented to the MAC through a common interface encompassing these sensing and transmission capabilities. A Media Independent Interface is defined in the Ethernet standards to separate the common MAC function from different physical layer transmission systems. This separation is very valuable to a multiple physical layer capable Ethernet transceiver as well as to the organization of technology development efforts. Besides these media-sensing and data-transmitting capabilities, a management interface is also defined as a part of the MII to enable functions such as autonegotiation.

The data transmitting capabilities of the MII include transmit clock, transmit enable, transmit error, and 4 bits of transmit data, in the transmit direction, and receive clock, receive enable, receive error, and 4 bits of receive data in the receive direction. Because the transmit and receive paths are separate, MII is capable of full-duplex operation. There are also a carrier sense pin and a collision detection pin defined for media sensing. For management purposes, a Management Data Clock (MDC) pin and a Management Data I/O (MDIO) pin are defined, but a 40-pin connector is also defined in case that physical layer and the MAC circuits are implemented separately. On this 40-pin connector, 4 power supply pins and 18 ground pins are also defined in addition to these 18 media sensing, data transmission, and management clock and I/O pins.


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