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I have seen the abbreviation PHY beeing used for a handful of different things within the context of Ethernet:

  • a PHY is a type of Ethernet physical layer (eg. 100BASE-TX, 10BASE-T)
  • a PHY is an Ethernet transceiver IC (eg. an IC that converts 100BASE-TX to MII/RMII)
  • a PHY is a physical layer device (more than just the transceiver IC)

Is PHY ambiguous and can refer to all of these or did I understand something wrong?

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  • \$\begingroup\$ MAC is logical, while PHY actually converts logical signals into a physical form in the form of voltage transitions to send through a medium like a coaxial cable or so. \$\endgroup\$
    – Mitu Raj
    Commented Oct 20, 2021 at 8:48
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    \$\begingroup\$ Re, "...type of Ethernet physical layer..." I have never heard "PHY" used in that way. The only word other than "cable" that I recall hearing to describe the physical wire that ran from station to station was "medium." 10BASE-5, 10BASE-2, 10BASE-T,... all were different media. Of course, any given PHY (transceiver) would work only with one specific medium: You couldn't physically connect a 10BASE-T PHY to a 10BASE-2 coax cable. \$\endgroup\$
    – Solomon Slow
    Commented Oct 20, 2021 at 17:45

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The terminology comes from what’s known as the OSI networking model. PHY is short for “physical layer”, the protocol that actually drives and receives communication to and from the physical medium.

More here: https://www.educative.io/blog/osi-model-layers

A circuit or a block that implements the physical layer coding and decoding is often also called a PHY. In OSI working, this PHY in turn, connects to the layer above it. In Ethernet this layer is Media Access Control (MAC), and its interface is the Media Independent Interface (MII) in one of its variants (MII, RMII, RGMII, SGMII, etc.)

The OSI model is not only useful in understanding networking protocols like Ethernet, it can be applied to practically any interface. USB2 for example has its PHY and connects to its controller via UTMI+ or ULPI; PCI Express has the PIPE interface between its PHY and controller. Both interfaces can (and are) used to carry network information of some kind; with a proper networking stack the difference is transparent to the upper layers.

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