PHY (often "fye") is a common abbreviation for the physical layer of the OSI model.

A PHY connects a link layer device (often called a MAC) to a physical medium such as an optical fibre or copper cable. A PHY typically includes a PCS (Physical Coding Sublayer) and a PMD (Physical Medium Dependent) layer. The PCS encodes and decodes the data that is transmitted and received. The purpose of the encoding is to make it easier for the receiver to recover the signal.

Example uses

  • Ethernet: A PHY chip (called PHYceiver) is commonly found on Ethernet devices. Its purpose is digital access of the modulated link. Usually used together with an MII-chip or interfaced to a Microcontroller that takes care of the higher layer functions.
  • USB: A PHY chip is integrated into most Universal Serial Bus (USB) controllers in hosts or embedded systems and provides the bridge between the digital and modulated parts of the interface.
  • IrDA: The Infrared Data Associations IrDA specification includes an IrPHY specification for the physical layer of the data transport.
  • S-ATA: Serial ATA controllers like the VIA6421 use a PHY.

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