In the Open Systems Interconnection (OSI) communications model, the physical layer supports the electrical or mechanical interface to the physical medium. For example, this layer determines how to put a stream of bits from the upper (data link) layer on to the pins for a parallel printer interface, an optical fiber transmitter, or a radio carrier.

The physical layer is usually a combination of software and hardware programming and may include electromechanical devices. It does not include the physical media as such.