The physical layer also known as PHY is the lowest layer in the OSI model of computer networking. This layer is responsible for transmitting raw data bits over a physical medium such as copper wire, optical fiber, or radio waves.
The physical layer defines the electrical, mechanical, and procedural characteristics of the interface between a network and its components. For example, the physical layer of an Ethernet network defines how signals are encoded onto cables and how those cables are physically connected to each other and to devices.
In contrast to lower levels (e.g., the data link layer), which generally define protocols that govern the communication between adjacent nodes on a single link (e.g., two nodes connected by an Ethernet cable), the physical layer protocols often define communication between non-adjacent nodes across multiple links (e.g., two nodes connected by multiple Ethernet cables).