I am reading on Ethernet and I understand that for copper ports, the MDI is the physical connections from the PHY to the medium's connector, including the physical port itself, where the cable is plugged. But a question pops into my mind:
What exactly is the MDI when we are talking about Ethernet SFP interfaces?
Does the SFP represent an extension to the MDI ? Is MDI just the 'port' in which the SFP is plugged?

1 Answer 1


By definition, the MDI is the "lower end" of the Physical Medium Dependent (PMD) sublayer - the exact point where the medium attaches. SFP allows for a field-replaceable transceiver which includes the MDI at its front connector. The transceiver module pretty much forms the PMD below the PMA (Physical Medium Attachment) sublayer.

A special case is when you use a direct-attach copper (DAC) or an active optical cable (AOC) with SFP+ upwards. Here, the actual medium is permanently fixed inside the SFP+ "plug". Therefore, IEEE moves the MDI to the SFP+/QSFP/... port itself (see 25GBASE-CR, clause 110.11 for an example).

  • All right, I understand. However, I get more and more confused. en.wikipedia.org/wiki/Physical_layer#PHY says that "Modular transceivers for fiber-optic communication (like the SFP family) complement a PHY chip and form the PMA sublayer. ". What does this mean? Because, to me, it sounds like PMD is the sublayer placed between PCS and PMA if you're using SFPs
    – Marcus
    Sep 13, 2022 at 17:07
  • 1
    @Marcus, remember that Wikipedia is not the standards keeper (anyone can edit it), and you often get exactly what you paid for its use. It does usually include valuable references at the bottom, but always defer to the actual standards body for the real answer.
    – Ron Maupin
    Sep 13, 2022 at 17:33
  • @Marcus Yes, that's correct - the SFP socket inside the cage is very much an additional connector between PMA and PMD (except for special cases where there's PCS transcoding or similar). The "PHY" is a bit larger, but the exact extent may vary with the chipset at hand (with or without PCS).
    – Zac67
    Sep 13, 2022 at 17:39

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