The first layer of the seven layers of Open Systems Interconnection (OSI) network model is called the Physical layer. Physical circuits are created on the physical layer of Open Systems Interconnection (OSI) model. Physical layers describe the electrical or optical signals used for communication.
Physical layer of the Open Systems Interconnection (OSI) model is only concerned with the physical characteristics of electrical or optical signaling techniques which includes the voltage of the electrical current used to transport the signal, the media type (Twisted Pair, Coaxial Cable, Optical Fiber etc), impedance characteristics, physical shape of the connector, Synchronization etc. The Physical Layer is limited to the processes needed to place the communication signals over the media, and to receive signals coming from that media. The lower boundary of the physical layer of the Open Systems Interconnection (OSI) model is the physical connector attached to the transmission media. Physical layer of the Open Systems Interconnection (OSI) model does not include the transmission media. Transmission media stays outside the scope of the Physical Layer and are also referred to as Layer 0 of the Open Systems Interconnection (OSI) Model.

Layer 2. Datalink Layer