Phy

PHY stands for physical layer, the lowest layer in the OSI model and in many network architectures. It covers the transmission and reception of raw bit streams over a physical medium and is distinct from the MAC (media access control) layer above it, which handles framing, addressing, and error detection.

In wired networks, a PHY transceiver converts digital data from a MAC into analog or optical signals suitable for the medium, and performs modulation, line coding, equalization, clock recovery, and signal integrity functions. The MAC and PHY communicate over interfaces such as MII, RMII, GMII, or RGMII, allowing the MAC to be implemented separately from the PHY. PHY devices can be external chips or integrated into a system-on-chip. Modern Ethernet PHYs commonly support speeds of 10/100/1000 Mbps and beyond, implement auto-negotiation, and may include features such as energy-efficient Ethernet and power-down modes.

In wireless and other non-wired systems, the term PHY likewise denotes the physical layer that defines how bits are translated into radio signals, including modulation schemes, coding, bandwidth, and front-end operations. Wireless PHY standards include IEEE 802.11 (Wi‑Fi), cellular technologies (3G/4G/5G), and Bluetooth, each with its own specification for modulation, coding, and channel usage.

In software and systems design, PHY is also used to describe the software and drivers that implement and manage physical-layer devices, such as the Linux PHY subsystem that exposes MII/MMD interfaces to the MAC layer.

Overall, PHY is a foundational concept describing how raw data becomes usable signals over a physical medium, and how the hardware interfaces with higher network layers.