Is my understanding about Ethernet - MII & MDI correct?

  • MII - Media Independent Interface is about the connectivity between MAC and PHY. In other words, it is about TXD and RXD signals from MAC to PHY.

  • MDI - Media Dependent Interface is about the connectivity between PHY and Connector/Jack(RJ45). In other words, it is about TX+/- and RX+/- signals from PHY to RJ45.

Also, I noticed that for 10/100Mbps Ethernet, only 2 differential pairs are needed between PHY transceiver and RJ45 whereas for 10/100/1000Mbps Ethernet, 4 differential pairs are needed between PHY transceiver and RJ45. Is that correct?

It would be really nice if you could post a link to a resource or app note that illustrates the basics of Ethernet connectivity that discusses the concepts above.

  • \$\begingroup\$ Consider 1000 Base T. I think MII is pretty much the '1000'-part describing speed and frame contents (possibly including duplex-setting), whereas 'T' is the MDI part describing the physical part of the interface such as connector, voltages and impedances. \$\endgroup\$
    – jippie
    Mar 30, 2014 at 15:46

2 Answers 2


MDI is a type of Ethernet port connection using twisted pair cabling. The MDI is the component of the media attachment unit that provides the physical and electrical connection to the cabling medium which is used to connect network devices (hubs and switches) with other hubs and switches without the use of a crossover cable or null modem.

Where MII is Media Independent Interface which is responsible for the connectivity between MAC and PHY as you mentioned.


Do you mean links such as the Wikipedia articles for Fast Ethernet (100 Mb/s) and Gigabit Ethernet (1 Gb/s)? They seem to be rather complete to me as far as the kinds of questions you're asking.

Yes, Gigabit Ethernet (1000base-T) requires all four pairs in each link.

  • \$\begingroup\$ I don't see how this answers the question.... \$\endgroup\$
    – Pacerier
    Oct 19, 2017 at 9:09
  • 1
    \$\begingroup\$ @Pacerier: Did you read the very last paragraph of the question? \$\endgroup\$
    – Dave Tweed
    Oct 19, 2017 at 12:15

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