Physical Layer
The Physical Layer defines how bits are passed over a medium. They can be passed electronically, mechanically, optically or by radio signals. It consists of various network components such as power plugs, connectors, receivers, cable types, etc.
The physical layer sends data bits from one device(s) to another device(s). It also defines the types of encoding (that is how the 0’s and 1’s are encoded in a signal). This layer is responsible for the communication of the unstructured raw data streams over a physical medium.