physicallayer

The physical layer, often abbreviated as PHY, is the lowest layer of the OSI model. It is responsible for the transmission and reception of the raw bit stream over a physical medium, converting digital bits into electrical, optical, or radio signals and back again.

Key functions include signaling, encoding, and modulation schemes, line coding, shaping, clock recovery, bit and symbol timing, and the interface to the data link layer. It also defines the mechanical, electrical, and functional characteristics of the physical medium and hardware, such as cables, connectors, transceivers, switches, repeaters, and antennas.

The PHY operates under the control of higher layers via standardized interfaces; it is designed to transmit unstructured bits, leaving framing, addressing, and error control to higher layers. It is subject to noise, attenuation, and interference, which can cause bit errors; the data link layer typically handles error detection and retransmission strategies.

Standards and specifications for the physical layer are defined by standards bodies such as IEEE and ITU. Examples include Ethernet physical layer specifications (IEEE 802.3) for copper and fiber media, and wireless PHY specifications (IEEE 802.11, 4G/5G radio interfaces) which define modulation, channel bandwidth, and multiple-input multiple-output techniques.

In wireless and mobile networks, the PHY describes frequency bands, modulation schemes, coding rates, and antenna techniques. The term PHY is also used in the context of modern communications to distinguish the physical layer from the MAC layer and higher layers, emphasizing the signal transmission domain rather than software protocol handling.