Ethernet physical layer or PHY, as an abstraction layer, transmits and receives data. The PHY encodes data frames for transmission and decodes received frames with a specific modulation speed of operation, transmission media type and supported link length.
The Synopsys article, Understanding the Ethernet Nomenclature – Data Rates, Interconnect Mediums and Physical Layer, describes in more detail the Ethernet PHY nomenclature, which is based on data rate, modulation, and media type. Speed-specific Media Independent Interfaces (MIIs) allow use of various PHY devices for operation over different media types such as twin axial copper (BASE-C), twisted pair (BASE-T), electrical backplanes (BASE-K), or fiber optic cables (BASE-L/R).
For example, most personal computer users are familiar with “Ethernet cables” in their laptops/PCs. Figure 1 shows a simplified block diagram describing how data is transferred to and from the processor over an Ethernet cable. In this use case, Ethernet data frames (packets of data), assembled by Ethernet MAC in the CPU, travel across the mother board (a printed circuit board) through MII/GMII, defined by the IEEE802.3 standard, before reaching an Ethernet PHY which transmits electrical signals over twisted pair cables through RJ 45 connectors.