Layer 1, the bottom layer, of the seven-layer Open Systems Interconnection (OSI) Reference Model.The Physical Layer defines the electrical and mechanical aspects of the interface between a device and a physical transmission medium, such as twisted pair, coaxial cable, optical fiber, or radio link. Communications hardware and software drivers are found at this layer, as are electrical and optical specifications. E/J/T-Carrier runs at the Physical Layer. Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH) run at the Physical Layer and Data Link Layer. See also coaxial cable, Data Link Layer, driver, E-carrier, electrical, hardware, J-carrier, layer, link, mechanical, network architecture, optical fiber, OSI Reference Model, physical, protocol, radio, SDH, software, SONET, T-carrier, and twisted pair.