What is the Physical Layer?

The Physical Layer is layer one of the OSI model.

The Physical Layer resides immediately below the Data Link Layer.

The Physical Layer is responsible for bit-level transmission between network nodes.

In copper networks, the Physical Layer is responsible for defining specifications for electrical signals. In fiber optic networks, the Physical Layer is responsible for defining the characteristics of light signals.

The Physical Layer defines items such as: connector types, cable types, voltages, and pin-outs.
Physical Layer Standards

Standards which operate at the Physical Layer include: RS-232, T1, and 10Base-T.